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WBBSE Chapter 8 Organic Chemistry Organic Compounds Are Compounds Of Carbon

In the past when there was little knowledge about chemistry, scientists classified compounds on the basis of their sources.

They thought that substances like starch, sugar, proteins, lipids, acetic acid, etc. could only be obtained from plants and animals (living organisms) and called them organic compounds.

On the other hand, the compounds like salts (NaCI), sulfates (CaSO₄), nitrates (KNO₃), carbonates (CaCO₃), etc.

Are obtained from minerals and non-living organisms and they are called inorganic compounds. Lavoisier also accepted this type of idea.

Later, in 1809, Berzelius explained in his vital force theory that organic compounds are only synthesized by living organisms by some unknown vital force and they cannot be prepared in the laboratory.

This old belief has been changed in 1828 when a German scientist Friedrich Wohler was successful to prepare the first organic compound urea in the laboratory.

WBBSE Notes For Class 10 Physical Science And Environment

He took an inorganic salt ammonium cyanate which after heating converted to urea. For this conversion, no vital force was required.

Already Lavoisier proved in 1784 that the main constituent of organic compounds is carbon.

In 1845, Kolbe prepared acetic acid and in 1856, Berthelot prepared methane (bio-gas) in the laboratory.

After that, the old concept of chemistry has been changed. Scientists accepted that all organic compounds essentially contain carbon and “organic chemistry is essentially the chemistry of carbon compounds.”

Exceptions: Oxides of carbon (CO, CO₂), metallic carbonates (CaCO₃), carbides (AI₄CI₃), metal cyanides (NaCN), etc. They are inorganic compounds.

Organic compounds: The compounds which contain carbon atoms excluding oxides of carbon, metallic carbonates, bicarbonates, carbides, and cyanides are called organic compounds.

They show some special properties like isomerism and catenation which the inorganic compounds do not show.

Difference between organic and inorganic compounds:

Organic compounds must contain carbon, but inorganic compounds may or may not contain carbon.

Organic compounds do not form ionic bonds.

They contain covalent bonds whereas inorganic compounds (like table salt) mostly contain ionic bonds.

Due to the difference in chemical bonding, certain special/different qualities are seen in organic compounds such as-

Most of organic compounds are volatile in nature,

The m.p. and b.p. are relatively lower than those of inorganic compounds,

Organic compounds are usually insoluble in water but soluble in organic solvents like benzene, alcohol, etc.

They are non-electrolytes as they are not ionized in an aqueous or fused state,

They have the property of catenation and isomerism,

Most of them form covalent bonds, due to which they react slowly in comparison to inorganic compounds.

WBBSE Chapter 8 Organic Chemistry Tetravalency And Catenation Property Of Carbon

Tetravalency of carbon: The electronic configuration of ₆C is = 2 (K) + 4 (L).

It has 4 valence e¯s in the outermost shell. How does C achieve its stable state (duplet/octet state)? Is it either by losing 4e¯s or gaining 4e¯s?

But it doesn’t do any one of this. Because to lose 4e¯s, it would need extra energy and to gain 4e¯s it would be difficult for 6ps to hold 10e¯s.

So for the C atom, transfer of e¯s is not possible. C atom likes to share its e^–s. That’s why, C forms 4 bonds.

C is tetravalent, has valency = 4 i.e. its combining capacity is relatively high. It can combine with 4 other atoms having valency 1 or 2.

In the simplest organic compound methane molecule, a 1C atom is surrounded by 4 bonds.
Scientist Kekule thought that the structure is planer— something looks like where the bond angle (angle between 2 bonds) should be 90°.

But this general model couldn’t explain all properties of organic compounds.

Tetrahedral model: In 1874, Van’t Hoff and Le Bel explained the properties of some organic compounds while trying to rationalize experimental data.

According to this explanation, the structure in methane is tetrahedral (a 3D structure). In case of which all the 4 bonds are not visible.

3 bonds out of 4 are above the plane (towards the observer) and 1 bond is below the plane (away from the observer).

Due to the tetrahedral structure, the bond angle is not 90° it is 109.28′. Later, by X-ray analysis, the model has been proved correct.

Catenation property of carbon: The self-linking property or tendency of a C-atom to join with another C-atom with a single covalent bond to form a long chain-like structure is called catenation.

Different types of chains can be formed like straight/open chains, branched chains, cyclic closed chains,s or a combination of all these.

For example:

C is tetravalent in nature for which other atoms like another C/H/O/N/F/CI/Br/l/S and so on can join with it and thus different organic compounds are formed.

For example:

Remember:

Most of the time, C and H atoms are joined. C can form bonds in different forms. It could be a single bond/double bond/triple bond.,

(Each covalent bond consists of 2 e¯s).

For example:

(Here the 3D tetrahedral model is projected in the 2D plane).

Structure: Trigonal; Bond angle = 120° (All 3 bonds are visible).

Structure: Coplaner; ingle triple bond; Bond angle = 180° (linear)

WBBSE Chapter 8 Organic Chemistry Structures Of C₂H₆, C₂H₄, C₂H₂

Millions of organic compounds are there. About 10 million C-compounds have already been discovered and still new compounds are being found and synthesized.

Hydrocarbons are the simplest organic compounds.

They contain only C and H atoms—no other elements. The open-chain hydrocarbons are divided into three groups —

1. alkanes,
2. alkenes and
3. alkynes.

(Here ‘o’, ‘e‘, and V are in alphabetical order).

Hydrocarbons are of two types—

Saturated (means less reactive) and unsaturated (very reactive).

The saturated hydrocarbons are those in which the C-atoms are connected by single bonds only (like C – C) while in unsaturated hydrocarbons the C-atoms are connected by at least one double bond (C = C) or triple bond (C = C).

Saturated hydrocarbons are also called alkanes, but unsaturated hydrocarbons are of two types alkenes and alkynes.

Structural and molecular Formula:

In alkanes, no. of H-atoms = 2 x no.of C-atoms +2; in alkenes,  no.of H-atoms = twice the no. of C-atoms, and in alkynes, no. of H- atoms = 2 x no. of C-atoms – 2. For example hex and (n = 6) = I
C6H14 Pent ene (n = 5) = C₅H₁₀, hex Yne (n = 6) = C₆H₁₀ etc.

Note:

(1) Methane is the simplest alkane, similarly ethene (or ethylene) is the simplest alkene, and ethyne (or acetylene) is the simplest alkyne.

(2) As a whole, the alkanes, alkenes, and alkynes are collectively known as aliphatic hydrocarbons.

WBBSE Chapter 8 Organic Chemistry Functional Groups

We see a wide variety of properties in C-compounds. Because elements other than C and H like 0, N, S, Cl, Br, F … etc. are present in them.

Definition: Functional groups are the groups of atoms that determine all characteristic properties (both physical and chemical) of the compounds in which they are present.

The presence of functional groups determines the properties of C-chains (irrespective of straight or branched).

There are many different functional groups like — OH (alcohol), — CHO (aldehyde), — COOH (carboxylic acid), — NH₂ (amine), etc.

Note: C = C double bond in alkenes and C = C triple bond in alkynes are usually considered functional groups because of which the alkenes and alkynes participate in an addition reaction.

WBBSE Chapter 8 Organic Chemistry Isomerism

The term Isomerism’ originated from two Greek terms Isos→ meaning same or equal and meros → meaning parts. In organic chemistry, isomerism is a very important concept.

There are many C-compounds that have the same molecular formula but different physical and/or chemical properties they are called isomers of each other.

Isomers are made up of the same type of atoms but the atoms are arranged differently.

Basically, two different types of isomerism are seen —

1. constitutional isomerism and
2. stereoisomerism.

Among these, constitutional isomerism is of different types. Here we will read about two types of constitutional isomerism –

Functional group isomerism and

Positional isomerism.

Functional group isomerism: Same molecular formula but different functional groups having different structures.

Common examples of functional group isomers:

We already got the idea that if functional groups are part of a compound, the entire compound will take the properties of functional groups.

For this reason, functional group isomers show properties different from each other. it is cited here-

Positional isomerism: In positional isomerism, the difference in structure occurs due to the position of the functional groups must having the same parent chain. Example:

Both have a parent chain of 3 C-atoms.

Molecular Formula: C₃H_gO In the 1st compound, the functional group (- OH) is attached to the 1st C-atom, whereas in the 2nd compound, with the 2nd C-atom.

So the position of the functional group is different.

Note: Positional isomerism also occurs due to unsaturation (position of double/triple bond in the compound).

WBBSE Chapter 8 Organic Chemistry Homologous Series Homo→Means Same Functional Groups

It is the series of compounds having the same general formula, the same type of molecular structure, and similar chemical properties which appear in immediate succession of molecular mass/Compounds in homologous series (alkanes/alkenes/alkynes) are called homologs.

Characteristic properties of homologous series:

They basically differ in no. of C and H atoms and also molecular mass.

Successive compounds in this series differ by – CH₂.

Physical properties like m.p., b.p., and solubility of the compounds that exist in the same series vary in succession.

But the chemical properties are definitely similar as they are dependent on the functional groups present.

The compounds can be artificially prepared by the same method.

For examples: 

WBBSE Chapter 8 Organic Chemistry IUPAC Nomenclature Of Simple Organic Compounds

Need for IUPAC nomenclature: Carbon has millions and millions of compounds.

Different common names are used for naming organic compounds over the years in different countries.

Common names are like a nickname (what your family members or friends call you). They are not systematic or random.

In 1967, a group of scientists decided to follow a very systematic scheme for naming the organic compounds which are known as IUPAC nomenclature

(Full form: International Union of Pure and Applied Chemistry). IUPAC naming scheme is based on the number of C-atoms in the longest chain/position of functional groups/nature of bonds/…. etc.

Basic IUPAC naming rules:

Root word: First count the number of C-atoms in the longest continuous C-chain. If the compound contains lC-atom, the name starts with meth, for 2C-atoms → eth, 3C -atoms → prop,…. etc. This gives the 1st part of the IUPAC name.

Suffix: Used after root word,

Primary suffix? Check the type of bond between the C-atoms. This gives the 2nd part of the IUPAC name.

Secondary suffix: The name of functional groups comes after the primary suffix.

While combining the IUPAC suffix remove ‘e’ from the alkane,

Make numbering in C-chain forward and backward and chose the lowest possible number for indicating the position of functional groups, Between the number and the letter always use a dash (-). (d) No. 1 in between two parts of the IUPAC name is not necessary.

let us take a few examples:

WBBSE Chapter 8 Organic Chemistry Industrial Source And Major Uses Of CH₄, C₂H₄, C₂H₂, LPG And CNG

[Methane]→

1. Colourless  Tasteless Odourless Non-toxic
2. Highly flammable
3. Abundantly available in marshy areas like wetlands, and swamps where organic substances are decomposed in the absence of O₂ by micro-organisms
4. Naturally present in natural gas (about 90%)
5. second most abundant greenhouse gas after CO₂.

Industrial source of CH₄:

 By destructive distillation of bituminous coal (continuous heating in the absence of air), a gaseous mixture (called coal gas) comes out.

1. CH₄ occurs in about 30 – 50% of coal gas.
2. Biogas or gobar gas contains about 60% CH₄.
3. CH₄ is a major component ( ~90%) of natural gas.

Uses of CH₄:

1. CH₄ is primarily used as a fuel for domestic uses, industries, automobiles, etc.
2. Refined liquid CH₄ is used as rocket fuel.
3. For the production of methyl alcohol, formal dehyde, and methanoic acid by slow combustion (catalytic oxidation) of methane.
4. In manufacturing carbon black, printing ink, etc.

[Eythlene]→ Colourless

1. Sweat taste and odor
2. Natural source of ethylene is petroleum or crude oil.
3. Naturally occurring hormone for plants.

Industrial source of C₂H₄:

By steam cracking of petroleum hydrocarbons at very high temperatures in the presence of a catalyst.

Fractional distillation of petroleum gives a mixture of gases from which C₂H₄ can be separated.

Uses of C₂H₄:

1. Widely used in the production of polymers like polythene, synthetic rubber, etc.
2. Used as a refrigerant, especially in LNG liquefaction.
3. Artificial ripening of fruits.
4. Oxy-ethylene flame in metal cutting, and welding.

[Eythlene or Acctylene] ⇒

1. Colourless, odorless when pure
2. Industrial acetylene is
3. Extremely explosive
4. Produces oxy-acetylene flame (~ 3100°C)
5. Non-polluting

Industrial source of C₂H₂:

1. By fractional distillation of crude oil.
2. The thermal cracking process by raising the temperature of some suitable hydrocarbons.
3. Hydrolysis of calcium carbide.

Uses of C₂H₂:

1. For welding and cutting.
2. Used in carbide lamp to get light.
3. Production of several inorganic compounds by chemical synthesis of C₂H₂.
4. Used in the production of different variants of plastic like PVC etc.
5. (Stands for Liquefied Petroleum Gas) A mixture of (45%) propane (C₃H₈) and (55%) butane (C₄H₁₀)
6. Colourless
7. Odourless but ethyl mercaptan added to it gives a strong smell for detecting any leakage
8. It’s a liquid under high pressure and very low temperature and turns back into gaseous vapor on releasing pressure
9. Heavier than air.

Source: LPG is extracted from petroleum/crude oil.

Uses:

1.  LPG has a higher calorific value (~ 94 MJ/m³) than other conventional fuels and is widely used as cooking gas.
2. Also in the petroleum Industry.

CNG⇒ Compressed Natural Gas

Compressed at very high pressure in the cylinders

To avoid transportation problems, it is delivered with the help of pipelines

Composition: Natural gas-basically methane (~ 85 – 90%) with (15 – 10%) ethane, propane, and butane.

Source: Natural gas.

Uses: CNG forms less CO₂ and footprints. That’s why it is used as an alternative to petrol/ diesel and can.be used in transportation by bus/taxi.

Advantages of using LPG and CNG: Higher calorific value than conventional fuels like petrol/diesel. Produces no smoke, no soot, and almost no unburnt C-particles.

WBBSE Chapter 8 Organic Chemistry Reactions of CH₄, C₂H₄, C₂H₂

Some reactions of methane (CH₄):

Combustion: Methane is highly flammable but it is not a supporter of combustion.

In a free supply of air (presence of O₂), CH₄ burns with a non-sooty blue flame and produces CO₂ (g) and water vapor.

CH₄ + 2O₂ CO₂ + 2H₂O + Heat

The reaction is exothermic (produces a large amount of heat). No C-particles are produced (i.e. pollution-free). That’s why, CH₄ can be used as a good fuel.

Substitution reaction with chlorine: CH₄ is a saturated hydrocarbon for which it is less reactive. It shows only substitution reactions.

In diffused sunlight (not in direct sunlight), CH₄ reacts with Cl₂ slowly in which 4 H-atoms of CH₄ are^ubstituted by Cl-atom and produce different chloro compounds step by step. \(\underset{\text { sunlight }}{\stackrel{\text { Diffused }}{\longrightarrow}}\)

 Some reactions of ethylene or ethene (C₂H₄): It is an unsaturated hydrocarbon (i.e. very much reactive) because of a double bond in it. It takes part in additional reactions.

(1) Addition of H₂ or hydrogeneration: At 200°C, in presence of Ni-catalyst, C₂H₄ reacts with H₂ to produce ethane (unsaturated hydrocarbon)

In this reaction, the double (=) bond of C₂H₄ is broken into a single (-) bond i.e. unsaturated compound is converted into the saturated compound.

Addition reaction with bromine: In the presence of a non-polar solvent like carbon tetrachloride (CCI₄), Br₂ combines with ethylene to produce ethylene di bromide.

In this reaction, the red color of the bromine solution is decolorized. This reaction can be used as a test for the unsaturated nature of ethene.

Some reactions of acetylene or ethyne (C₂H₂): It is an unsaturated compound having a triple bond (=). It is also very reactive and does additional reactions.

Addition of H₂/hydrogenation: In the presence of Ni-catalyst, at 200°C temperature, C₂H₂ is added with 2 mol H-, and produces ethane (a saturated compound).

Addition of Br₂in the presence of CCI₄solvent: This reaction is called

Polymerization of ethylene: Ethene or ethylene is an alkene where 2 C-atoms are joined by a double In this reaction, the red color of the bromine solution is decolorized.

This reaction can be used as a surer test for unsaturated compounds like ethene and ethyne bonds (=).

Polythene or polyethylene is formed when this double bond breaks at a temperature of about 200°C and very high pressure of around 2000 atm in the presence of an organic peroxide (not H₂O₂) catalyst.

When the double bond breaks, adjacent other molecules link together to form a very large (solid) molecule (macro-molecule) having many small repeating units with very high molar mass.

The repeating unit is called the monomer unit and the long molecular chain is called the polymer.

In this reaction, ethylene is the monomer and polyethylene is the polymer.

Homopolymer is made up of only one type of monomer unit, while copolymer is made from two or more types of monomer units.

The process of formation of polymers is called polymerization.

General Difference Between Ethylene and polythylene:

WBBSE Chapter 8 Organic Chemistry Synthetic organic polymers

On the basis of origin, polymers are of two types:

Natural polymers like starch, cellulose, DNA, protein, silk, etc. which are obtained from natural sources,

Synthetic polymers are artificially prepared in the laboratory. Examples of synthetic polymers are plastic, nylon, Teflon, PVC, polystyrene, etc. In our daily life, we use many types of polymers.

WBBSE Chapter 8 Organic Chemistry Biodegradable Polymers

Environmental impact of using synthetic polymers: The biggest problem in using synthetic polymers is that they are not degradable biologically through micro-organisms so the soft drink bottles or the plastic wrappers what we throw away are not biodegradable.

They are not destroyed for hundreds of years. In addition to this, when the rejected plastic materials are burnt toxic fumes directly mix with the atmosphere, the CO₂(g) also contributes to global warming.

As a whole, they cause a devastating impact on the environment.

Biodegradable polymers: Materials that are designed such that they get decomposed wholly when exposed to micro-organisms (fungi and bacteria) through aerobic and/or anaerobic processes.

Many natural biodegradable polymers are formed naturally by plants.

Examples: Cellulose, starch, silk, wool, and natural rubber.

Examples of synthetic biodegradable polymers: are bioplastic or green plastic (plant-derived materials), polylactic acid, bio pol resin (PHB + PHV), polyglycolide (polyglycolic and lactic acid,

PHB- Polyhydroxy butyrate; PHV – polyhydroxy venerate).

Long polymer chains are broken down by bacterial action when disposed of without producing toxic substances in the environment.

This is the main advantage of using biodegradable polymers. There are many applications of biodegradable polymers.

Examples: Polyglycolic and lactic acid→heart repair, Dextron→post-operative stitches, Polylactic acid, and lactic acid→drug delivery. –

To avoid non-biodegradable polymers, we can use jute and paper in packing. The use of jute and paper is safe and eco-friendly.

WBBSE Chapter 8 Organic Chemistry Uses And Properties Of Ethyl Alcohol And Acetic Acid

Properties and uses of ethyl alcohol or ethanol (H₃C – H₂C – OH):

Physical properties:

1. Colourless
2. Pleasant sweety
3. Volatile liquid
4. Highly soluble in water

Chemical properties: Ethanol is slightly acidic in nature. It has a tendency to lose H⁺

Reaction with sodium (a very reactive metal): Ethanol reacts with Na metal vigorously and produces sodium ethoxide (a Na—salt) and releases H₂(g).

In this reaction, Na replaces H from ethanol (substitution) to produce C₂H₅ONa.

Dehydration reaction (removal of water): At about 170°C temperature, ethanol is dehydrated by the cone. H₂SO₄(a dehydrating substance).

During this reaction, ethanol is converted into ethene.

Uses: Ethanol is used

1. As a solvent for gum,
2. As antifreeze liquid (due to very low freezing temperature),
3. In thermometers,
4. In car radiators,
5. Mostly for making wine/ beer/whisky.

Properties and uses of acetic acid or ethanoic acid (CH₃ – II – OH):

Physical properties: Colourless liquid Characteristic unpleasant pungent odor Soluble in water 5 – 8% solution of acetic acid is called vinegar.

Chemical properties: Less acidic in nature than mineral acids. It has ‘carboxylic acid’ as a functional group.

Reaction with sodium bicarbonate (a weak base):

Reaction with sodium hydroxide (strong base): Ethanoic acid reacts with NaOH to form salt and water (neutralization reaction).

Reaction with ethanol (esterification reaction): The reaction of an alcohol with carboxylic acid is called esterification which produces a new compound called ester.

In this reaction, OH from ethanoic acid and H from ethanol are removed to form water, and ethyl acetate an ester, having sweet fruity odour is formed.

This reaction establishes the identification of carboxylic group.

Uses: Used

1. In manufacturing vinegar
2. Esters
3. Many polymeric materials like cellulose acetate (used in photographic film)
4. As solvent.

Important uses of synthetic polymers:

WBBSE Chapter 8 Organic Chemistry Harmful Effects Of Methanol And Ethanol

Methanol (CH₃– OH) is highly poisonous/highjy toxic : It makes alcohol unfit for drinking. If methanol is taken with wine, it oxidises to form toxic formic acid which can cause blind ness, madness or even death.

Ethanol: (CH₃ – CH₂ – OH) is mainly used in making wine/beer/whisky. In low proportion, it affects cerebral cortex temporarily.

But for excessive intake of ethanol affects the nervous system and harmful for liver/kidney.

WBBSE Chapter 8 Organic Chemistry Denatured Spirit Or Methylated Spirit

In industry, ethanol is used as a solvent. 95% ethyl alcohol is called rectified spirit.

Rectified spirit is denatured by adding toxic substances like methyl alcohol (nearly poisonous), pyride (highly poisonous) and naptha.

This mixture is commercially known as denatured spirit. It is sold excise duty free for industrial use.

In market two types of spirits are available:

Industrial denatured spirit which contains 95% rectified spirit and 5% methyl alcohol,

Mineralised denatured spirit which contains 90% rectified spirit, 9% methyl alcohol and 1% pyride and naptha.;

Uses: Denatured spirit is used as a solvent for varnish.

WBBSE Chapter 8 Metallurgy Uses Of Fe, Cu, Zn, And Al And Their Alloys

Advantages of using alloys:

Alloys are harder than their constituent metals but less ductile and malleable. Example: Aluminium is lightweight, but its alloy duralumin (Al + Cu + Mg + Mn) is light but strong. Copper is soft, but it is alloy brass and bell metal are strong.

WBBSE Notes For Class 10 Physical Science And Environment

Alloys are resistant to corrosion: For example, Iron rusts, but stainless steel (Fe + C) does not. The melting point of an alloy may be lower than any of its original constituents.

Example: For soldering, an alloy of Pb and Sn is used whose m.p. is lower than that of Pb or Sn.  Using alloys the electrical conductivity can be changed.

Example: The alloy of Ni + Cr + Fe, called nichrome, resistivity is high.

In making 24-carat pure gold jewellery, 22 parts of gold are mixed with 2 parts of either Cu or Ag. So the jewellery we use is alloys.

Some important alloys:

WBBSE Chapter 8 Metallurgy Ores And Minerals

Most of the metals exist in the earth’s crust in combined states with some impurities such as sand/stones/rocks/limestone.

These naturally occurring substances in the earth’s crust which contain metals are called minerals. For example, iron exists in the earth’s crust as sulphides/carbonates/oxides/nitrates, with some impurities like C, Si, S, Mn etc.

Metallurgy refers to the process of extraction of pure metals from some minerals (ores). But from all minerals, metals cannot be extracted.

Minerals from which pure metals can be extracted cheaply and easily/conveniently are called ores. For example,

Al is most abundant in the earth’s surface /clay. But Al is not extracted from clay. Because the process is very costly. Rather, Al is extracted from bauxite.

So that bauxite is the ore of Al.

Haematite (Fe₂O₃) and iron pyrites (FeS₂) both contain a high percentage of Fe. However the removal of S from FeS₂ is very difficult and costly also. So FeS₂ is not considered as the ore of Fe.

Conclusion: All minerals are not ores, but all ores are minerals.

Major ores of Fe, Cu, Zn and Al :

WBBSE Chapter 8 Metallurgy Electronic Theory And Redox Processes

According to electronic theory, oxidation is loss of e~ and reduction is gain in e¯.

Examples of oxidation: \(\mathrm{Na}-e^{-} \rightarrow \mathrm{Na}^{+} ; \mathrm{Ca}-2 e^{-} \rightarrow \mathrm{Ca}^{2+}\)

Examples of reduction: \(\mathrm{Al}^{3+}+3 \mathrm{e}^{-} \rightarrow \mathrm{Al} ; \quad \mathrm{Fe}^{3+}+3 e^{-} \rightarrow \mathrm{Fe} ; \quad \mathrm{Zn}^{2+}+2 e^{-} \rightarrow \mathrm{Zn}\)

In metallurgy reduction/dressing/removal of oxygen of metal oxide is a very important step.

In order to free metal from its oxide, the reduction is done by excluding the non-metallic part Usually, the reduction is done in two ways:

Oxides of Cu, Pb, and Fe can be reduced by using chemical-reducing agents like C (coke), CO, H₂, and NH₃.

ZnO can only be reduced using coke (C) heated at high temperatures.

⇒ \(\mathrm{ZnO}+\mathrm{C} \rightarrow \mathrm{Zn}+\mathrm{CO}\) (carbon reduction of ZnO) Reduction

Note: During electrolytic extraction, reduction takes place at the cathode. General equation:

Mⁿ⁺+ ne¯→ M (where M = metal, n = number of electrons)

WBBSE Chapter 8 Metallurgy Thermite Reaction

Al has a high affinity to react with O₂ at high temperatures and the reaction is exothermic in nature. Using this principle, Fe is extracted from its oxide using the Goldschmidt thermite process.

Thermite mixture is basically a mixture of fine aluminium oxide (Al₂O₃) and ferric oxide (Fe₂ O₃) in a ratio of 1: 3 by mass.

The mixture also consists of a very minimal amount of igniting material like barium peroxide (BaO₂).

Taking the whole mixture in a crucible, ignite it with an Mg-ribbon. As soon as Mg- ribbon is burnt, a high temperature is created. As a result of which, both Al₂O₃ and Fe₂O₃ react with each other.

⇒ \(2 \mathrm{Al}(s)+\mathrm{Fe}_2 \mathrm{O}_3(s) \rightarrow 2 \mathrm{Fe}(s)+\mathrm{Al}_2 \mathrm{O}_3(s)+\text { Heat }\)

The reduction of Fe₂O₃ by Al is highly exothermic (2500 – 3000°C), which takes place within a maximum time of 30 seconds.

This produces molten Al₂O₃ and molten Fe. Molten Fe goes at the bottom since it is heavier than molten Al.

Application: Molten Fe is used for welding ferrous metals like the joining of rails, pipes, and broken parts of large gears.

Activity series of metals: It is a list of metals in ascending order of their activity. From top to bottom, the tendency of gaining e“s decreases i.e. reducing ing property gradually decreases.

Hence, the most reactive metal (K) exists at the top and the least reactive metal (Au) at the bottom. Any metal in this series can displace any other metal below it from its salt solution.

Example: Fe + ZnSO₄ does not take place, but \(\mathrm{Zn}+\mathrm{FeSO}_4 \rightarrow \mathrm{ZnSO}_4+\mathrm{Fe}\) Fe can take place as the position of Zn is above Fe in the series.]

Reactive metals like Au and Pt exist in a free state in nature. So, for these metals reduction is not needed.

Oxides/sulphides of Ag, and Hg are less stable. They can be reduced by heat. ng.

Oxides/sulphides of Zn and Fe can only be reduced by using Coke or any other suitable substances.

Oxides of most reactive metals (K, Na, Ca, Mg, Al) are stable and ionic compounds. They can not be reduced using coke or any other reducing substance.

Electrolysis is the only process (applying an electric field or potential difference) -in which reduction takes place at the cathode.

Activity: Carry out the following activities and note down your observations :

Add Cu-wire in ferrous sulphate solution (FeS0₄) → No reaction.

Add iron nails in copper sulphate solution (CuSO₄) → The Blue colour of CuSO₄ fades away and brown-coloured FeSO₄ deposits on Fe-nails. (Explain the reason)

WBBSE Chapter 8 Metallurgy Metal Corrosion

Usually, metals are lustrous/shining. But with time, metals start appearing dull, less shining. This is because of metal corrosion.

You might have noticed the difference in the appearance of a gold ring, silver spoon, copper coin, iron nail, and aluminium toy left in the air for a long time.

Silver spoon will appear less lustrous having a black coloured layer [Ag + H₂S (from the air) → Silver spoon Copper coin Ag₂S (black silver sulphide)]

On the copper coin, a green-coloured layer will form, and the iron nail will appear in the worst condition having a reddish-brown layer (rust),

The gold ring will remain in the best condition (as gold is the least reactive), and the aluminium toy will also be preserved very well.

Metals are considered to be corroded when they react with air/water. Corrosion happens very slowly.

Corrosion Definition: Corrosion is described as the formation of compounds on the surface of a metal when it is exposed to air/water or moisture/acid or electrolyte like salt water.

Rusting (Corrosion of iron): If impure iron is left in moist air, it gets corroded with a reddish brown coating on its surface.

This coating is called rust which is a hydrated ferric oxide (Fe₂O₃ . x H₂O where x represents an unknown quantity of water used in rusting and it is a variable) and the entire process is called rusting.

Rust is porous and can be removed. Iron corrodes very easily as compared to other metals like Cu/Ag/AI.

Because Fe is more reactive than Cu/Ag/AI. In this process of rusting, the upper surface of Fe is eaten up gradually.

Rusting weakens Fe objects and cuts short their lives. So, it’s an economic loss also. Pure Fe does not rust.

Mechanism of rusting of iron: For rusting to occur -Fe, water and O₂ are required. Here we will study the roles of water and O₂ in rusting.

The overall change “Fe→ Fe²⁺(unstable state) → Fe³⁺ (very stable state) → Fe₂O₃ • x H₂O¯ is a redox reaction i.e. an irreversible process once the rust is formed, it is not possible to get back the original Fe from rust.

When Fe-surface is exposed to water droplets, the Fe^– at- oms at the centre of water droplets give up 2 e¯sto form Fe²⁺ Fe (s) → Fe²⁺ (aq.) + 2e^– (oxidation takes place – here Fe is oxidised)

The e^–s move from the centre of the water droplet towards the edge all around. At the edge of the water droplet, there is an abundance of O₂.

O₂ is reduced to OH^– ions in the presence of water.

⇒ \(\mathrm{O}_2(g)+2 \mathrm{H}_2 \mathrm{O}(\mathrm{I})+4 e^{-} \rightarrow 4 \mathrm{OH}^{-} \text {(aq.) (reduction) }\)

We know that the electrode anode always donates e^– s and the cathode gains e^– s. 

So, on Fe^– surface plenty of small local chemical cells are formed, where each Fe atom at the centre of the water droplet acts as an anode,

The edge of the water droplet where C (impurity)-atoms are present acts as the cathode and the water molecule as an electrolyte.

Fe²⁺ ions react with OH^– ions to form solid ferrous hydroxide [Fe (OH)₂

⇒ \(\begin{aligned}
& 2\left[\mathrm{Fe} \rightarrow \mathrm{Fe}^{2+}+2 \mathrm{e}^{-}\right] \\
& \frac{\mathrm{O}_2+2 \mathrm{H}_2 \mathrm{O}+4 \mathrm{e}^{-} \rightarrow 4 \mathrm{OH}^{-}}{2 \mathrm{Fe}+2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2 \rightarrow 2 \mathrm{Fe}^{2+}+4 \mathrm{OH}^{-}} \rightarrow 2\left[\mathrm{Fe}^{2+}+2 \mathrm{OH}^{-}\right] \rightarrow 2 \mathrm{Fe}(\mathrm{OH})_2 \text { (s) (unstable) } \\
&
\end{aligned}\)

Fe (OH)₂ is further oxidised by O₂ and forms ferric hydroxide [Fe (OH)₃].

⇒ \(4 \mathrm{Fe}(\mathrm{OH})_2(s)+2 \mathrm{H}_2 \mathrm{O}(\mathrm{I})+\mathrm{O}_2(g) \rightarrow 4 \mathrm{Fe}(\mathrm{OH})_3(s) \text { (a stable compound) }\)

Fe (OH)₃ decomposes into hydrated ferric oxide.

Fe (OH)₃ (s)→Fe₂O₃ • x H₂O (s)

Overall electro-chemical rusting reaction: 4Fe (s) + 2 x H₂O (l) + 3O₂ (g) 2Fe₂O₃ • x H₂O(s)

Note: Rusting is faster in the presence of chloride (Cl^–) ions. It is a serious problem in sea-going ships or submerged parts of pipelines.

Prevention of rusting:

Barrier protection: The simplest way of preventing rusting is by painting/oiling/greasing iron objects because of which iron will not get exposed to moisture.

Metallic coating: Rusting can be prevented by making a coating of another more reactive metal.

For example Galvanization

(Coating of Zn on Fe): Zn is more reactive than Fe. So, Zn reacts more rapidly than Fe forming a thin layer of ZnO on the surface of the Fe-object.

It does not allow more O₂ to react with the inner layer of Fe. Thus, galvanization is a better way to prevent Fe from rusting,

Tin plating, Chromium plating (plating of chromium metal Nickel plating This is done by electrolysis.

Alloying: Fe rusts very easily. But when Fe is mixed with Cr and Ni (an alloy), it becomes strong as well as does not rust. (Cr, Ni give a shiny look to Fe).

Cathodic protection: In an electro-chemical reaction, Fe-atoms (anode) at the surface give up e“s which flow towards the cathode because of this the anode gets corroded slowly.

In the cathodic protection method, the flow of e”s can be stopped by the use of (a) a sacrificial anode.

A more active metal like Mg-block is connected with Fe so that Mg acts as an anode instead of Fe. So, oxidation takes place in Mg.

[Mg (s)→ Mg²⁺ (aq.) + 2e ]. Instead of Fe- Mg gets corroded. This process is used for the protection of underground pipes, tanks etc.

When the Mg-block is corroded completely, it has to be replaced by a fresh one.

Sherardizing is the process of formation-corrosion-resistant Zn-layer by vapour galvanizing on the surface of iron or steel.

The Delhi Pillar of the Gupta Age is a unique metallurgical marvel of high-quality of steel production in ancient India. It was made from 98% corrosion-free wrought iron.

Corrosion of other metals and its health implications:

Al is a good conductor of heat and electricity. It is not affected in dry air. But in contact with moist air, Al reacts with the O₂ from air forming aluminium oxide (Al₂O₃) which is very much unreactive.

Once the Al₂O₃ layer is formed, it binds very tightly with the Al-surface. As a result of which, the thermal and electrical conductivity of Al decreases. Although, the Al₂O₃ layer keeps Al unaffected in moist air.

We have learnt that metals get corroded in moist air for a long time. On Cu or alloys of Cu, left in the open air for many years, green-coloured patches are formed. CO₂ and water vapour reacting with Cu form basic copper carbonate [Cu + O₂ + CO₂ + H₂O →Cu (OH)₂ • CuCO₃].

Al, Zn react with weak organic acidic substances like vinegar or lemon juice, and they react to form soluble metallic compounds (harmful to our health).

Cu also forms soluble metallic compounds by reacting with acidic pickles or fruits. For this reason, acidic foods should not be kept or processed in Al, Zn or Cu containers.

Also, tarnished metallic utensils should be cleaned well before use to avoid metal poisoning.

WBBSE Chapter 8 Inorganic Chemistry In The Laboratory And In Industry Laboratory Preparation Of Ammonia(NH₃)

Principle: Ammonium salt + Alkali → Salt + Ammonia Gas+ H₂O.

When ammonium salts are heated with less volatile strong alkalis, then comparatively more volatile ammonia gas is produced. For example:

⇒ \(\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4(s)+\mathrm{CaO}(s) \stackrel{\Delta}{\longrightarrow} \mathrm{CaSO}_4(s)+2 \mathrm{NH}_3(g)+\mathrm{H}_2 \mathrm{O}(I)\)

⇒ \(\mathrm{NH}_4 \mathrm{Cl}(s)+\mathrm{NaOH}(\text { aq. }) \stackrel{\Delta}{\longrightarrow} \mathrm{NaCl}(\mathrm{s})+\mathrm{NH}_3(\mathrm{~g})+\mathrm{H}_2 \mathrm{O}(I)\)

⇒ \(\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4(s)+2 \mathrm{KOH}(\text { aq. }) \stackrel{\Delta}{\longrightarrow} \mathrm{K}_2 \mathrm{SO}_4(s)+2 \mathrm{NH}_3(g)+2 \mathrm{H}_2 \mathrm{O}(I)\)

Caustic alkalis like NaOH/KOH are not used because they are deliquescent i.e. from an atmosphere they absorb moisture and dissolve.

In the laboratory, slaked lime [Ca (OH)₂] is used as an alkali.

Chemicals required: Dry ammonium chloride (NH₄CI) and dry Ca (OH)₂in 1: 3 mass ratio (Both are finely ground).

Condition:

1. Direct contact of the reactants and
2. Application of heat.

Chemical equation: \(2 \mathrm{NH}_4 \mathrm{Cl}(s)+\mathrm{Ca}\begin{aligned}
& (\mathrm{OH})_2(s)-\stackrel{\Delta}{\longrightarrow} \mathrm{CaCl}_2(s)+2 \mathrm{NH}_3(g)+ \\& 2 \mathrm{H}_2 \mathrm{O}(g)\end{aligned}\)

Laboratory arrangement: The round bottom flask is kept in a tilting position.

Drying agent: Drying is essential because water vapor is one of the products.

Moist NH₃(g) is dried by passing it through quick lime (CaO). Ammonia gas is basic in nature. Quicklime is also a base. So they never react to one another.

WBBSE Notes For Class 10 Physical Science And Environment

Gas collection: Dry NH₃(g) is collected by downward displacement of air in the inverted gas jar. Because-

1. NH₃(g) is lighter than air and
2. NH₃ (g) is highly soluble in water. A moist red litmus paper held near the mouth of the gas jar turns blue in the presence of NH₃ (g).

Precautions:

The reagents are finely ground in dry condition, otherwise, NH₄CI may sublime on heating,

The round bottom flask is kept tilted so that water may not trickle back and crack the hot flask,

In the laboratory process, we should not use ammonium nitrate (NH₄NO₂).

Because NH₄NO₂ is explosive in nature, it decomposes on heating forming nitrous oxide (N₂O) gas and water vapor.

⇒ \(\mathrm{NH}_4 \mathrm{NO}_3(s) \stackrel{\Delta}{\longrightarrow} \mathrm{N}_2 \mathrm{O}(g)+2 \mathrm{H}_2 \mathrm{O}(g)\)

In drying NH₃(g), common drying agents are like a cones. H₂SO₄, P₂ O_5, and anhydrous CaCl₂ are not used.

Because NH₃ reacts  with these substances: 2NH₃ + H₂SO₄ →(NH₄)₂SO₄; 6NH₃ + P₂O₅ + 3H₂O→ 2 (NH₄)₃ PO₄ (P₂O₅ is acidic in nature); x NH₃ + (anhydrous) CaCI₂ → CaCI₂ . × NH₃ (addition compound)

WBBSE Chapter 8 Inorganic Chemistry In The Laboratory And In Industry Properties Of Ammonia

Physical properties: Ammonia is a colorless gas having a characteristic pungent choking smell.

The pungent smell in the washroom is due to ammonia.

Density: Under standard conditions, the specific density of ammonia is 8-5 whereas the specific density of air is 14-4. This means that NH₃ (g) is lighter than air.

Solubility: At ordinary temperature, NH₃ (g) is highly soluble in water (in 1 volume of water about O₂ volume of NH₃ (g) is soluble).

Aqueous ammonia: Ammonia, being very much soluble, is dissolved in water and forms ammonium hydroxide (NH₄OH) which is a weak base due to the presence of OH¯ ions.

⇒ \(\mathrm{NH}_3+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{NH}_4 \mathrm{OH} ; \mathrm{NH}_4 \mathrm{OH} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{OH}^{-} \text {(Arhenious theory) }\)

This is called aqueous ammonia (or in the aqueous form of ammonia) called ammonium hydroxide. Under high pressure, the volume of NH₃ (g) is compressed, and when the temperature is decreased, the gas gets liquified.

By cooling NH₃(g) to a temperature – 33-4°C under normal pressure, it changes into a colorless liquid called anhydrous or liquid ammonia /moisture-free ammonia.

Liquor ammonia: It is the 35% concentrated/saturated solution of ammonia in water. Its specific gravity is 0.88. That’s why liquor ammonia is treated as a very strong solution of NH₃ (g) in water.

Remember: Liquor ammonia is not liquid ammonia. Liquor ammonia contains NH₄OH (NH₄⁺and OH¯ions) whereas liquid ammonia contains NH₃

Chemical properties:

Reactions due to basic nature: Ammonia is a weak base, so reacting with acids (HCl, H₂S0₄) it forms ammonium salts.

For example:

⇒ \(\mathrm{NH}_3 \text { (aq.) }+\mathrm{HCl} \text { (aq:) } \rightarrow \mathrm{NH}_4 \mathrm{Cl} \text { (aq.) (ammonium chloride salt) }\)

The same reaction happens in the gaseous state also.

⇒ \(\mathrm{NH}_3(g)+\mathrm{HCl}(g) \rightarrow \mathrm{NH}_4 \mathrm{Cl}(s) \text { (dense white fumes) }\)

When a glass rod is dipped in a cone. HCl is brought in contact with NH₃ (g), and it produces dense white fumes (fine dust of solid NH₄CI).

This is a reaction between two gases that produces a solid compound. This reaction is also used for the detection of ammonia gas.

⇒ \(2 \mathrm{NH}_3 \text { (aq.) }+\mathrm{H}_2 \mathrm{SO}_4 \text { (aq.) } \rightarrow\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4 \text { (ammonium sulphate salt) }\)

NH₄OH (aq.) turns red litmus blue and colorless phenolphthalein solution pink.

Reducing property: NH₃ is a good reducing agent. This means NH₃ reduces i.e. removes O₂ from another substance. Example:

When NH₃ (g) is passed over red hot copper (II) oxide (black), then NH₃ reduces CuO to form metal copper [red).

At the same NH₃ gets oxidized in N₂ (g). This reaction proves that NH₃ contains nitrogen.

Catalytic oxidation of NH₃:

⇒ \(4 \mathrm{NH}_3(g)+5 \mathrm{O}_2(g) \frac{\mathrm{Pt}}{800^{\circ} \mathrm{C}} 4 \mathrm{NO}(g)\)+ 6 \(\mathrm{H}_2 \mathrm{O}(g)+\text { Heat (Exothermic reaction). }\)

NH₃ (g) reacts with O₂ in the presence of Pt- catalyst heated at 800°C and colorless nitric oxide gas (NO) is produced (oxidation reaction).

⇒ \(4 \mathrm{NH}_3+3 \mathrm{O}_2 \rightarrow 2 \mathrm{~N}_2+6 \mathrm{H}_2 \mathrm{O}(\mathrm{g})\)

The oxidizing tendency is so high that NO (g) further oxidizes into reddish-brown nitrogen dioxide gas (NO₂).

⇒ \(\mathrm{NO}(g)+\mathrm{O}_2(g) \rightarrow \mathrm{NO}_2(g) \text { (reddish brown gas) }\)

4. Precipitation of metal hydroxides from aq. solution of metal ions (Fe³⁺, Al³⁺) :

Analytical chemistry: Usually, all cations like Na⁺, K⁺, Ca²⁺, Zn²⁺, Mg²⁺, Al³⁺, NH₄⁺ are colourless and all anions like Cl^–, Br^–, CO₃^2-, NO₃^1-, SO₄^z-

Are colorless except for some cations: Fe²⁺ (-ous) → light green, Fe³⁺ (-ic) → yellowish/brown, Cu²⁺ (-ous) → blue and some anions MNO₄^1- (permanganate) →pink, Cr₂O₇^2- (dichromate) → orange.

That’s why Zn (NO₃)₂ → colourless, FeSO₄ → light green, FeCI₃ →yellow, AICI₃ → colourless, CuSO₄ → blue, …. etc.

Here we will study what happens due to the addition of NH₄OH/ NaOH solution into the aq. solution of Fe³⁺ (ferric), and Al³⁺ salts—especially to note down the color of the salt and its solution.

⇒ \(\begin{array}{ll}
\mathrm{FeCl}_3 \text { (aq.) }+3 \mathrm{NH}_4 \mathrm{OH}(\text { aq.) } \longrightarrow & \mathrm{Fe}\left(\mathrm{OH}_3\right)(s) \downarrow+3 \mathrm{NH}_4 \mathrm{Cl} \text { (aq.) } \\
\text { yellow } & \text { Brown ppt. } \\
\mathrm{AlCl}_3 \text { (aq.) }+3 \mathrm{NH}_4 \mathrm{OH} \longrightarrow & \mathrm{Al}(\mathrm{OH})_3(s) \downarrow+3 \mathrm{NH}_4 \mathrm{Cl} \text { (aq.) } \\
\text { Colourless } & \text { Gelatinous } \\
\text { white ppt. }
\end{array}\)

Significance of such reactions: Observing the color of metal hydroxide precipitate, we can easily detect /identify the presence of metallic cation (Fe³⁺, Al³⁺).

Physical observation: In aq. CuSO₄, aq. NH₃ is added :

⇒ \(\mathrm{CuSO}_4 \text { (aq.) }+2 \mathrm{NH}_4 \mathrm{OH} \text { (aq.) } \longrightarrow \mathrm{Cu}(\mathrm{OH})_2(\mathrm{~s}) \downarrow+\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4 \text { (aq.) }\)

But this blue ppt. of Cu(OH)₂ dissolves in excess NH₄OH forming a deep blue soluble com plex salt-tetramine copper (II) sulfate.

The alkaline solution of potassium mercuric iodide (K₂Hgl₄) is called Nessler’s reagent. It is used to identify/test the presence of ammonia.

Ammonia in contact with Nessler’s reagent produces yellowish/brown ppt. A bottle of liquor ammonia is cooled before opening.

We know that a large quantity of NH₃ vapor is kept under high pressure inside the bottle. If the bottle is extremely suddenly opened due to the release of pressure, NH₃ vapor may spurt danger to the eyes, out and come in contact with the eyes.

This is very much harmful to the eyes.

Use of liquid ammonia as a coolant: It is used as a refrigerant in ice plants.

Liquid NH₃ is capable of absorbing heat as its specific heat capacity is very high and cools another substance.

In the refrigerator, tetra fluoro ethane is used. Why not liquid NH₃? Because- NH₃ reacts with Cu (refrigerator pipe metal) a high quantity of NH₃ is poisonous.

Ammonia gas is non-poisonous. If inhaled, affects the respiratory system and brings tears to the eyes.

NH₃ concentration (300 ppm) is immediately dangerous to life. Workers should use personal protective equipment:

In case of accidental leakage of NH₃ from cold storage, the first and foremost duty is to wash out the affected body parts with water as NH₃(g) is highly soluble in water.

In the factory, after NH₃ (g) pipeline leakage inhalation of NH₃ can cause severe irritation of the nose and throat, coughing, and shortness of breath/difficulty in breathing resulting in respiratory failure. People fell unconscious and they need to be immediately hospitalized.

WBBSE Chapter 8 Inorganic Chemistry In The Laboratory And In Industry Major Industrial Uses And Industrial Manufacture Of NH₃ And Urea

Industrial uses of ammonia: Ammonia is used in the manufacture of

Nitrogenous fertilizers: examples – ammonium sulfate [(NH₄)₂SO₄], ammonium nitrate (NH₄NO₃), ammonium phosphate [(NH₄)₃P0₄], urea [CO (NH₂)₂], etc.

Nitric acid by Ostwald’s process, sodium carbonate (Na₂CO₃) by Solvay’s process

Explosives: examples-TNT (tri nitro toluene), RDX, etc.

Polymers: Examples – nylon, rayon, plastics, dyes, etc.

Ammonium chloride is used in dry cells, ammonium carbonate is used as a smelling salt for reviving a fainted person

As a refrigerant in ice-plants

Many pharmaceutical products, and household cleaning products – removing grease/perspiration strains, cleaning tiles/windows, etc.

Industrial manufacture of NH₃ (Haber’s process): (For industrial production on a large scale) From the synthesis of dry and pure N₂ (g) and H₂ (g) in the ratio 1 : 3 by volume under some favorable conditions NH₃ (g) is produced.

Favorable conditions:

High temperature: 450 – 500°C
High pressure: 200 – 900 atm
Catalyst: Finely divided Fe
Promoter: Molybdenum (Mo) or Al₂O₃

Relevant Reaction:

In this reaction, both the reactants and product are gaseous; the reaction is reversible and exothermic; volume decreases in the forward direction.

Collection Of NH₃: The gaseous mixture contains produced NH₃ along with unrelated N₂ and H₂ Then by condensation through a cooling chamber, NH₃ (g) is liquefied easily as compared
to N₂ and H₂ molecules and dissolved in water (because NH₃ is highly soluble in water).

By recirculating unreacted N₂ and H₂, an eventual yield of NH₃ (~ 98%) can be obtained.

Uses of urea [CO (NH₂)₂]: Urea is used both as a nitrogenous fertilizer (about 90%) and animal feed.

In the manufacture of urea-formaldehyde resin/plastic used for lamination/fabrication purposes.

In the production of urea stibine, a medicine for leishmaniasis (kala-azar), and barbiturates, used as sleeping pills.

Manufacture of urea: Raw materials are:  CaCO₃ (indirectly used). \(\mathrm{CaCO}_3 \stackrel{\Delta}{\longrightarrow} \mathrm{CaO}+\mathrm{CO}_2\)

This CO₃ is used in urea production, NH₃ (directly used through Haber’s process).

Condition: Liquid NH₃ and liquid CO₂ react at 200°C temperature Under 200 atm pressure.

Chemical reaction:

WBBSE Chapter 8 Inorganic Chemistry In The Laboratory And In Industry Laboratory Preparation Of Hydrogen Sulphide(H₂S)

Principle: A strong acid can substitute the weak acid part from its salt. Example- (where H from strong acid HCI substitutes Na of Na₂S)

Chemicals required : A few pieces of solid ferrous sulfide (FeS) [dark brown in color] and

H₂SO₄(or dil. HCI).

Condition: Direct contact of FeS and dil. H₂SO₄ at normal temperature.

Chemical equation: FeS (s)+ H₂SO₄(aq.) →FeSO₄+ H₂S (g)

(where H₂SO₄→strong acid, H₂S→ weak acid, FeS→ salt of weak acid)

Laboratory arrangement: H₂S (g) is collected by the upward displacement of air, as it is heavier than air.

How Laboratory make H₂S(g) free from water vapor? this is done by passing H₂S (g) through acidic Phosphorous pentoxide (P₂O_s) (because H₂S is acidic and reducing in nature).

Why drying agents like

conc. H₂SO₄ Quicklime(CaO) or  Anhydrous CaCl₄are not used? Conc. H₂SO₄ is a strong oxidises H₂S to sulfur(s)

⇒ \(\mathrm{H}_2 \mathrm{SO}_4 \text { (aq.) }+\mathrm{H}_2 \mathrm{~S}(g) \longrightarrow 2 \mathrm{H}_2 \mathrm{O}(I)+\mathrm{SO}_2(g)+\mathrm{S}(s)\)

CaO is a base. \(\mathrm{CaO}(s)+\mathrm{H}_2 \mathrm{~S}(g) \longrightarrow \mathrm{CaS}(s)+\mathrm{H}_2 \mathrm{O}\)

(anhydrous s) \(\mathrm{CaCl}_2+\mathrm{H}_2 \mathrm{~S}(g) \rightarrow \mathrm{CaS}(s)+2 \mathrm{HCl}(g)\)

In the laboratory preparation of H₂S(g) from FeS (s)why conc. H₂SO₄ conc. HNO₃ conc. H₂SO₄ is not taken?

Reaction: FeS + H₂SO₄ → FeSO₄ + HS

A, B, and C are respectively bottom, middle, and top chambers or globes of Kipp’s apparatus. Chamber C connects chamber A passing through middle chamber B. A tap is fitted to the middle chamber B.

Take solid FeS in B. Add dil. H₂SO₄ through C, until acid touches FeS in B. At once, the reaction starts and H₂S (g) is produced. By opening the tap, H₂S (g) can be collected.

When the tap remains closed (air-tight), the high pressure of gas in B pushes H₂S0₄ down to bottom chamber A and up into the top chamber C.

This separates FeS and H₂SO₄. Immediately, further production of gas is stopped.

As per need, when a tap is opened, pressure in the middle chamber decreases, which allows H₂SO₄ to enter into B from A making contact with FeS. Again H₂S (g) starts producing.

WBBSE Chapter 8 Inorganic Chemistry In The Laboratory And In Industry Properties Of Hydrogen Sulphide

Physical properties:  Foul odor of rotten egg, colorless, poisonous gas.

G Density: Heavier than air. H₂S has a vapor density of 1-2, it is large as compared to air which is 1 Solubility Moderately soluble in cold water. But insoluble in hot water.

Chemical properties:

Acidic property: Aq. solution of H₂S (g) behaves like a mild acid. It is a di-basic acid.

⇒ \(\mathrm{H}_2 \mathrm{~S}+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{HS}^{-} \text {i.e. } \mathrm{H}_2 \mathrm{~S} \text { (aq.) } \rightleftharpoons \mathrm{H}^{+}+\mathrm{HS}^{-}\)(partial ionization)

⇒ \(\mathrm{HS}^{-}+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{S}^{2-}+\mathrm{H}_3 \mathrm{O}^{+} \text {i.e. } \mathrm{H}_2 \mathrm{~S} \text { (aq.) } \rightleftharpoons 2 \mathrm{H}^{+}+\mathrm{S}^{2-}\)(complete ionization)

Reaction with alkali [ex. NaOH (aq.)]: Since aq. H₂S behaves as a di-basic acid, it reacts with NaOH solution to form both acid salt and normal salt.

⇒ \(\begin{array}{ll}
\mathrm{NaOH}+\mathrm{H}_2 \mathrm{~S} \longrightarrow & \mathrm{NaHS}+\mathrm{H}_2 \mathrm{O} \text { (partial ionization) } \\
& \text { acid salt (sodium hydrogen sulfide) } \\
\mathrm{NaHS}+\mathrm{NaOH} \longrightarrow & \mathrm{Na}_2 \mathrm{~S}+\mathrm{H}_2 \mathrm{O} \text { (complete ionization) } \\
\text { normal salt (sodium sulfide) }
\end{array}\)

Combustibility of H₂S (g): H₂S (g) burns in air (O₂) with blue flame but does not support in burning.

In the excessive supply of O₂, sulfur dioxide gas (another toxic gas) is produced, and in low O₂-level, sulfur deposits.

(excessive supply of O₂) 2H₂S + 3O₂ 2H₂O + 2SO₂ (low supply of O₂) 2H₂S + O₂→2H₂O+ 2S

As a strong reducing agent (reaction with acidified potassium dichromate solution): H₂S (g) is passed through acidified K₂Cr₂O₇ solution (orange).

Result: Orange Coloured Solution Solution is Converted Into a Green Colour

This Colour Change occurs only because of the reducing property of H₂S.

(4) Precipitation of mental sulfides (Cus, Pbs, Ag₂S- black) From Aqueous solution of copper sulfate CuSO₄ (blue), lead nitrate pb (NO₃)₂ (Colourless): H₂S (g) is passed through the Queous solutions separately in each case, Black ppt. of mental sulfide is obtained.

⇒ \(\begin{array}{ll}
\mathrm{CuSO}_4 \text { (aq.) }+\mathrm{H}_2 \mathrm{~S}(g) \longrightarrow & \mathrm{CuS}(s) \downarrow+\mathrm{H}_2 \mathrm{SO}_4 \text { (aq.) } \\
\text { (Blue) } & \text { (Black) } \\
\mathrm{Pb}\left(\mathrm{NO}_3\right)_2 \text { (aq.) }+\mathrm{H}_2 \mathrm{~S}(g) \longrightarrow & \mathrm{PbS}(s) \downarrow+2 \mathrm{HNO}_3 \text { (aq.) } \\
\text { (Colourless) } & \text { (Black) } \\
2 \mathrm{AgNO}_3 \text { (aq.) }+\mathrm{H}_2 \mathrm{~S}(g) \longrightarrow & \mathrm{Ag}_2 \mathrm{~S}(s) \downarrow+2 \mathrm{HNO}_3 \text { (aq.) } \\
\text { (White) } & \text { (Black) }
\end{array}\)

 Identification test for H₂S: Dip a filter paper into lead acetate solution, which in contact with H₂S (g) immediately turns black due to the formation of insoluble black lead (II) sulfide.

This method is important to determine the presence and amount of H₂S (g).

Reaction with alkaline sodium nitroprusside solution: When H₂S {g) is passed through freshly prepared alkaline sodium nitroprusside solution (colorless), the solution becomes violet.

Toxicity of H₂S: It is a highly toxic gas.

Rapidly affects the central nervous system and respiratory system. Inhaling high concentrations (over 500 -1000 ppm) can cause instant death.

There is no proven antidote for H₂S poisoning. Wear a nose mask and avoid inhaling H₂S.

WBBSE Chapter 8 Inorganic Chemistry In The Laboratory And In Industry Laboratory Preparation And Major Uses Of Nitrogen (N₂)

Laboratory preparation: Chemical required: A concentrated aqueous solution of ammonium chloride (NH₄CI) and sodium nitrite (NaNO₂) in 1:1 molar proportion.

Condition:  In the laboratory N₂(g) is prepared by (gently) heating the mixture of NH₄CI and NaNO₂.

Chemical reaction: The reactants initially undergo double decomposition to form ammonium nitrite (NH₄NO₂) and sodium chloride (NaCI). NH₄NO₂ (aq.) + NaCI (aq.)

⇒ \(\mathrm{NH}_4 \mathrm{Cl} \text { (aq.) }+\mathrm{NaNO}_2 \text { (aq.) } \stackrel{\Delta}{\longrightarrow}\)

⇒ \(\mathrm{NH}_4 \mathrm{NO}_2 \text { (aq.) }+\mathrm{NaCl} \text { (aq.) }\)

Thereafter the NH₄NO₂ formed here decomposes to produce N₂ (g).

⇒ \(\mathrm{NH}_4 \mathrm{NO}_2 \text { (aq.) } \stackrel{\Delta}{\longrightarrow} \mathrm{N}_2(g)+2 \mathrm{H}_2 \mathrm{O}(g)\)

Gas collection: N₂(g) is collected by the downward displacement of water.

Drying of laboratory-prepared N₂ (g): By passing through

Cone. H₂S0₄, existing moisture can be removed

Cone. NaOH/KOH, Cl₂ (g) can be removed

Finally passing over red hot Cu, oxides of nitrogen are removed (reduced) to form fresh N₂ (g).

Main uses: N₂(g) is used in the industrial manufacture of NH₃ (by Haber’s process.

NH₃ is an important starting material in the production of HNO₃ (by Ostwald’s process), nitrogenous fertilizers such as (NH₄)₂SO₄, NH₄NO₂, (NH₄)₃PO_4, etc, explosives, and pharmaceutical products.

Being an inert gas creates an unreactive atmosphere inside an electric bulb, and thermometer to prevent oxidation.

In many industrial pros, N₉ boils at —196 C. So, liquid N₂ is used cesses it is used to create an unreactive atmo- ² ………. ² to cool down the exothermic reaction, sphere, as N₂ is cheaper than He/Ar.

Packets of chips/pop-corn/ other food packets are made puffy using N₂ (g) so that oxidation and growth of bacteria are stopped.

The food products also remain fresh. That’s why N₂ is very important in food packaging.

Liquid N₂ has many real-life applications such as the preservation of biological specimens example: blood, cornea, eye, tissue samples, etc.

WBBSE Chapter 8 Inorganic Chemistry In The Laboratory And In Industry Properties Of N₂

Physical properties: N₂a colorless, odorless, and tasteless gas.

Density: Slightly less dense than air (vapor density of N₂ and air are respectively 14 and 14-4).

Solubility: N₂ (g) is sparingly soluble in water (1L of water dissolves 22 mL of N₂ at 0°C). 0 N₂ (g) freezes at – 210-1°C.

At high pressure, N₂ (g) can be liquified. N₂ boils at – 196°C.

Chemical properties: The molecular structure of nitrogen is However each N₂ molecule is very difficult to break and that’s why N₂ molecules have a very strong triple bond shared between two N atoms which behave like an inert/unreactive gas at normal temperature.

But at extremely high temperatures N₂ can react with other metals/non-metals. For example—

Reaction with hydrogen: Mixture of pure and dry N₂ (g) and H, (g) in the volume ratio 1 : 3 when passed over hot (450 – 500°C) finely divided Fe-catalyst and Mo-promoter under high pressure (> 200 atm) then NH₃ (g) is produced through an exothermic reaction.

This is the principle of industrial production of NH₃ by Haber’s process.

⇒ \(\mathrm{N}_2+3 \mathrm{H}_2 \underset{\substack{450-500^{\circ} \mathrm{C} \\ \text { above } 200 \mathrm{~atm}}}{\stackrel{\mathrm{Fe} \text { and } \mathrm{Mo}}{\rightleftharpoons}} 2 \mathrm{NH}_3+\text { Heat }\)

Reaction with magnesium: N₂ (g) puts off a burning candle as it is neither combustible nor a supporter of combustion (inert gas).

But a burning element can react with N₂ exactly what happens in the case of a burning magnesium ribbon placed in an Infilled gas jar.

Mg-ribbon burns and forms a yellowish powder called magnesium nitride (Mg₃N₂).

Chemical reaction:  \(3 \mathrm{Mg}(s)+\mathrm{N}_2(g) \stackrel{\text { Heat }}{\longrightarrow} \mathrm{Mg}_3 \mathrm{~N}_2(s)\)

Reaction with calcium carbide: Formation of nitrolim: When N₂(g) is passed over hot (~ 1100°C) calcium carbide dust (CaC₂), a brownish grey solid mixture is produced which contains calcium cyanamide (CaNCN) and carbon (C)…

noo°c nitro slim Commercially, this solid mixture (CaNCN + C) is called nitrolim- an inorganic compound used as a chemical fertilizer.

Nitrogen fixation: Nitrogen is present in abundance (~ 78%) in the atmosphere. Plants and animals cannot take nitrogen directly from the atmosphere.

So, anyhow N₂ needs to be converted into its usable forms (usually nitrogenous compounds). This process of conversion of atmospheric N₂ into useable forms is referred to as “nitrogen fixation.”

N₂ combines with O₂ in the presence of lightning at a temperature of 3000 – 5000°C to form nitric oxide (NO), and NO further oxides into NO₂ (nitrogen dioxide).

⇒ \(\mathrm{N}_2(g)+\mathrm{O}_2(g) \stackrel{\text { lightning }}{\longrightarrow} 2 \mathrm{NO}(g) ; 2 \mathrm{NO}(g)+\mathrm{O}_2(g) \stackrel{\text { lightning }}{\longrightarrow} 2 \mathrm{NO}_2(g)\)

During rainfall, NO₂ (g) reacts with water and produces both aq. nitric acid (HN0₃) and unstable aq. nitrous acid (HNO₂) and fall down to the soil.

⇒ \(2 \mathrm{NO}_2(g)+\mathrm{H}_2 \mathrm{O}(l) \rightarrow \mathrm{HNO}_3 \text { (aq.) }+\mathrm{HNO}_3 \text { (aq.) }\)

Alkaline substances present in soil react with aq. HNO₃ forms nitrate (NO₃) and nitrite (NO₂) salts (soluble in water).

This is how atmospheric nitrogen is fixed/ converted into a usable form. Plants use these nitrates to make proteins.

Animals also eat plants to get proteins. Animal excreta and dead plants/animals again get converted to nitrates.

Denitrifying bacteria convert these nitrates back to atmospheric nitrogen. As a whole, % of N₂ in the air remains constant.

WBBSE Chapter 8 Inorganic Chemistry In The Laboratory And In Industry Industrial Manufacture Of HCl, HNO_3, And H₂SO₄

Manufacture of HCI (by Synthetic method): In the Castner-Kellner (commercial) process, caustic soda (NaOH) is manufactured by the electrolysis of aqueous sodium chloride (NaCI) solution. The by-products obtained in this process are H₂ (g) and Cl₂ (g).

Industrially hydrogen chloride gas is manufactured by burning /direct combustion of equal volumes of H₂ (g) and Cl₂ (g) in a combustion chamber made of silica (SiO₂).

⇒ \(\mathrm{H}_2(g)+\mathrm{Cl}_2(g) \rightarrow 2 \mathrm{HCl}(g)\)

The HCI (g) is passed through a cooling chamber. Then the cooled gas is allowed to absorb water. Finally, a saturated solution of hydrochloric acid is prepared.

⇒ \(\mathrm{HCl}(g) \stackrel{\mathrm{H}_2 \mathrm{O}}{\longrightarrow} \mathrm{HCl} \text { (aq.) }\left[\mathrm{H}^{+} \text {(aq.) }+\mathrm{Cl}^{-} \text {(aq.) }\right]\)

2. Manufacture of HNO₃ (by Ostwald’s process):

The reaction\(\mathrm{NH}_3(g) \rightarrow \mathrm{HNO}_3 \text { (aq.) }\) takes place in 3 steps:

Step 1: Oxidation of NH₃(g): A mixture of NH₃ (g) and dry pure air in a 1:10 volume ratio is passed very fast over catalyst platinum-rhodium (90:10) gauze heated at about 800°C.

The time of contact is ~ 0-0014 seconds and NH₃ (g) gets oxidized by O₂ of air to produce nitric oxide vapor (NO).

The reaction is exothermic and reversible. Once the reaction starts, the heat evolves and continuous heating is not required.

⇒ \(4 \mathrm{NH}_3+5 \mathrm{O}_2 \stackrel{\mathrm{Pt}-\mathrm{Rh}}{\underset{800^{\circ} \mathrm{C}}{\rightleftharpoons}} 4 \mathrm{NO}(g)+6 \mathrm{H}_2 \mathrm{O}(g)+\text { Heat }\)

Step 2: Oxidation of NO vapor: Hot gaseous mixture (NO vapor + water vapor + excess O₂) is cooled to 50°C and NO vapor is further oxidized by O₂ sent in the oxidizing chamber to produce nitrogen dioxide gas (NO₂).

⇒ \(2 \mathrm{NO}(g)+\mathrm{O}_2(g) \longrightarrow 2 \mathrm{NO}_2(g)\)

Step 3: Absorption of NO₂(g): The absorption of NO₂ (g) by water sprayed from the top of the absorption tower produces nitric acid (HNO₃).

Initially, very dilute HNO₃ is produced which is recycled to absorb, more and more NO (g) till 68% HNO₃ is obtained.

Further concentration is done by distillation with a cone. H₂SO₄ at temperature ~120°C. This HN0₃ is almost 98% concentrated.

⇒ \(3 \mathrm{NO}_2(g)+\mathrm{H}_2 \mathrm{O}(I) \longrightarrow 2 \mathrm{HNO}_3 \text { (aq.) }+\mathrm{NO}(g)\)

⇒ \(\mathrm{HNO}_3(68 \%) \stackrel{+ \text { conc. } \mathrm{H}_2 \mathrm{SO}_4}{\underset{\text { Distillation }}{\longrightarrow}} \mathrm{HNO}_3(98 \%)\)

(3) Manufacture of H₂SO₄ (by Contact Process):

Step 1: Production of sulfur dioxide (SO₂):

⇒ \(\mathrm{S}(\mathrm{s})+\mathrm{O}_2(g) \rightarrow \mathrm{SO}_2(g)\)

Or, \(4 \mathrm{FeS}_2(s)+11 \mathrm{O}_2(g) \longrightarrow 2 \mathrm{Fe}_2 \mathrm{O}_3(s)+8 \mathrm{SO}_2(g)\)

By heating sulfur or iron pyrites in the presence of air (O₂) SO₂ (g) is prepared [heating in the presence of excess air is called Roasting].

Step 2:  Oxidation of sulfur dioxide(SO₂) to sulphur trioxidfe (SO₃):  (most important step:)

In the presence of a solid vanadium pentoxide catalyst (V₂O₅) at the optimum temperature of 450^°C and high pressure -2 atm, pure and dust-free SO₂ (g) is oxidized by dry pure O₂ (g) to form SO₃ (g).

Step 3: Absorption of SO₃ (g): [SO₃ is not diluted directly by adding water into \(\mathrm{SO}_3 \mathrm{O}\mathrm{SO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{SO}_4\)  (not done).

Because too much heat is evolved for SO₃ and H₂SO₄ vapors form an acid mist.]

After cooling, SO₃ (g) is dissolved in 98% concentrated H₂SO₄ sprayed from the top of the absorption chamber.

Fuming sulphuric acid or pyro-sulphuric acid (H₂S₂O₇) is formed. Its commercial name is Oleum,

⇒ \(\left.\mathrm{SO}_3+\mathrm{H}_2 \mathrm{SO}_4 \text { ( } 98 \% \text { conc. }\right) \rightarrow \mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_7(l)\)

Then, after dilution with water, sulphuric acid of any desired concentration can be produced. \(\mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_7(I)+\mathrm{H}_2 \mathrm{O}(I) \rightarrow 2 \mathrm{H}_2 \mathrm{SO}_4 \text { (aq.) }\)

Note:

Oleum is a more powerful oxidizing agent than a con. H₂SO₄.

Solid crystals used in industrial manufacturing processes (Ostwald’s process/Contact Process) are usually taken in dust form not in bigger granules in order to get a greater surface area.

For it, the activity of catalysts/ rate of reaction increases because of greater adsorption.

WBBSE Chapter 8 Ionic And Covalent Bonding What Is Chemical Bond? Why Chemical Bonds Are Formed?

By natural process, atoms of the same element or different elements have a tendency to combine with each other to form different molecules.

So, there exists a force of attraction between the atoms that holds them together in the molecule in order to get stability.

A chemical bond means a chemical force of attraction by which two or more ions/atoms/molecules/elements are held together to form a stable state.

We know that valency is the combining capacity of an atom of a particular element with other elements.

WBBSE Notes For Class 10 Physical Science And Environment

On the basis of the electronic concept of valency, we will learn about two types of chemical bonds, which are—

1. Ionic or electrovalent bonds and
2. covalent or molecular bonds.

Ionic bonds are formed through the complete transfer of electrons from the outermost shells between a metal and a non-metal.

For example solid NaCI (Na⁺cr), MgCI₂ (Mg²⁺2Cr), CaO (Ca²⁺ O^2—), etc. Here (Na⁺cr) is an ionic compound.

On the other hand, covalent bonds are formed through the mutual sharing of one or more electron pairs between two or more non-metals.

For example: H₂(H-H), O₂(O=O), N₂(N=N), etc. H₂ and O₂ are covalent compounds.

1. Metals have a tendency to lose electrons.
2. Non-metals have a tendency to accept electrons.
3. Metals get oxidized, while non-metals get reduced.
4. As ionization potential is inversely proportional to the tendency to lose electrons, metals should have low ionization potential. As electron affinity is directly proportional to the tendency of accepting electrons, so non-metals should have high electron affinity.

WBBSE Chapter 8 Ionic And Covalent Bonding Properties Of Ionic Compounds

The crystalline structure of sodium chloride where Na⁺ (cations) and Cl¯ (anion) ions are held together by a very strong electrostatic force of attraction, mainly because of which different bulk properties in ionic solids are observed, such as:

Physical state: Ionic compounds are generally hard and brittle solids.

In ionic compounds, particles are only ions (both cations & anions) due to the transfer of electrons.

Melting point (m.p.) and boiling point (b.p.): Because of the very strong electrostatic force of attraction, ionic compounds have high m.p. & high b.p. Lot of temperature (energy) is required for melting & boiling of ionic solids.

For example: for solid NaCI m. p. is = 820 e and b. p. = 1600°C.

Electrical conductivity: They conduct electricity in a molten state or aqueous state but not in a solid state.

Reason: In solid-state (Na⁺CP) — the Na⁺ & Cl^– ions are tightly bound with each other and the ions are not free to move to conduct electricity.

But when Na⁺CP is molten (which means heated enough) or in the aqueous state (which means NaCI in water), the Na⁺ and Cl” ions break down and become mobile.

Know: Water molecules have a property to lessen the force between two charges and this is known as dielectric property.

Exception: CaF₂, Ba₃(PO₄)₂—for these compounds, the electrostatic force of attraction is so huge that water cannot separate out the ions.

Solubility: Usually highly soluble in water. You know from your daily experience that NaCI is such a compound.

But they are insoluble in organic solvents like alcohol/acetone/benzene/toluene/CCI₄/CS₂ etc. because water is itself a polar solvent.

WBBSE Chapter 8 Ionic And Covalent Bonding Discovery Of Noble Gases Ionic Bonding

Discovery of noble gases: We know that Earth’s atmosphere contains very little amount of noble gases mainly argon.

In the last decade of the 19th century, English scientist Rayleigh and Scottish scientist Ramsay were able to separate out the noble gases or inert gases (He, Ne, Ar, Kr, Xe, and Rn) which are chemically inactive.

In the first half of the 20th century, Lewis and Kossel separately predicted that except He atoms (which have 2 electrons), other noble gas atoms have 8 electrons in their outermost / valence shell.

Electronic configuration: ₂He (2), ₁₀Ne (2, 8), _lgAr (2, 8, 8), ₃₆Kr (2, 8, 18, 8), ₅₄X(2, 8, 18, 18, 8), ₈₆Rn(2, 8, 18, 32, 18, 8).

According to Gilbert Newton Lewis, an American physical chemist, noble gas atoms have complete outermost shells, which made them stable in comparison to rest atoms.

So we can say that for this special type of electronic configuration, the noble-gas atoms are especially stable.

Thus, the stability of an atom is decided by its electronic configuration.

There are two stability rules-

Octet Rule: If an atom has 8 electrons in an outermost shell then the atom becomes stable,

Duplet Rule: If an atom has only one shell and 2 electrons in the outermost shell then also the atom becomes stable.

₃Li, ₁H follows the duplet rule to become stable. All the atoms other than noble-gas atoms are trying to follow the octet rule to acquire stability.

An atom can achieve an octet or a duplet structure in two ways:

1. Either by transfer of electrons or
2. By mutual sharing of electron pairs.

The respective type of chemical bonding is

1. ionic bonding and
2. covalent bonding.

Let us now see the electronic configuration of elements of the third period and their corresponding ions

Note:

Na, Mg, and Al try to acquire electronic configurations like their nearest inert gas Ne

S (Group-14) and Cl (Group-17) try to acquire electronic configuration like their nearest inert gas Ar.

Si (Group-14), and P (Group-15) do not form ions.

Ionic bonding: Kossel established the existence of Na⁺ & CP ions in ionic compounds. NaCI from the idea of conductivity and other experiments.

It has also been established that ionic bonding takes place when a metal and a non-metal try to acquire an electronic configuration like their nearest inert gas.

Let us take some examples:

Bonding in NaCI (Na⁺CP): Electronic configuration: _nNa = 2 + 8 + 1 and ₁₇CI = 2 + 8 + 7

Neither Na nor Cl has stable electronic configurations. To avail stability, the Na atom needs either to give away le^_ or to accept 7 more e¯s in the outermost shell.

Similarly, the C1 atom needs either to accept le¯or give away all the 7 e¯s from the outermost shell. For the Na atom, it is easier to give away le¯ than to accept 7 e¯s (because less energy is utilized).

A similar reason is applicable to the Cl atom. So for attaining a stable electronic configuration, the Na atom will lose le¯ from its valence cell to become the Na⁺ ion & Cl atom will gain that single e^– to become the CP ion. Basically, this is a complete transfer of electrons.

These Na⁺ and CP ions attract each other by strong electrostatic Schematic diagram force and they form ionic compound NaCI.

the relative position of Na⁺ and CP ions in the schematic diagram of the NaCI crystal lattice)

Here, the no. of electrons lost or gained by the atoms at the time of formation of the ionic bond is called electrovalency.

In this example, the electrovalency of Na⁺= 1 and that of Cl¯ = -1.

Note :

When metals of Group 1 and 2 react with non-metals of Group 16 and 17, ideal ionic bonding is formed,

Due to the strong electrostatic force of attraction between cations and anions, the ionic lattice structure of NaCI is very stable.

Definition: To acquire the nearest noble-gas-like stable electronic configuration, cations, and anions are formed through the transfer of electrons from neutral metal and non-metal atoms.

The electrostatic force of attraction which holds oppositely charged ions together is called the ionic bond. The new compound so-formed is called an ionic compound.

This type of combining capacity is called ionic valency or electrovalency.

Representation of ionic bonding in terms of electron dot (•) / cross (x) structure:

Note: Li⁺ & H ions in the LiH compound fulfill the duplet rule in their outermost shells with no question of making an octet.

Why for ionic compounds, the concept of formula mass is more appropriate than molecular mass?

Ionic compounds are made up of huge no. of cations and anions arranged in a 3-dimensional stable ionic structure (strong electrostatic force of attraction).

They contain ions only no molecules are there.

So the concept of molecular mass is not appropriate in the case of ionic compounds-rather, formula mass is more appropriate.

For example, not saying molecular mass of NaCl = 23 + 35-5 = 58-5, we will say, the formula mass of NaCI = 58-5 or molar mass of NaCI = 58-5 g/mol.

Solid NaCI contains Na⁺and Cl” ions which are not free to move due to the existence of a strong electrostatic force of attraction between the ions even when an external potential difference is applied across a NaCI crystal.

But when NaCI is in a molten or aqueous state, then the ions become free to move and can conduct electricity.

That’s why we say that ionic solids have low intrinsic electrical conductivity in contrast to that in their molten or solution states.

WBBSE Chapter 8 Ionic And Covalent Bonding Properties Of Covalent Compounds

In order to achieve a stable electronic configuration like the nearest noble gases, atoms of two or more non-metals come together.

This coming together and mutual sharing of one or more electron pairs leads to the formation of a chemical bond known as covalent bonding.

The new compound so formed is called covalent compound and this type of combining capacity is called covalency.

Some common compounds in which covalent bonding is found to exist are naphthalene, sugar, water, ethanol, methane, chloroform, carbon dioxide, carbon monoxide, hydrogen chloride, ammonia, etc.

Let us see some of their physical properties:

From this table, we come to know about some common properties of covalent compounds, which are—

Physical state: Covalent compounds exist as gases, liquids, or soft solids because of the very weak intermolecular force of attraction (weak Van der Waals force) existing between the molecules.

Due to this fact, a covalent bond is weak in comparison to an ionic bond. In covalent compounds, particles are molecules with no existence of ions.

Melting point (m.p.) and boiling point (b.p): Covalent compounds have low m.p. and low b.p.

Because of the very weak intermolecular force of attraction, solid covalent compounds can easily melt and boil, a liquid covalent compound is volatile in nature and a gaseous covalent compound exists in vapor form.

Solubility: Generally, covalent compounds are insoluble in polar solvents like water, but soluble in non-polar solvents like alcohol, and ether.

Exception: Cane sugar, glucose, H₂S, NH₃ HCI (g), ethanol, etc are soluble in water.

Electrical conductivity: They are non-conductors in a solid, molten, or aqueous state. Because covalent compounds do not contain ions to conduct electricity.

Exception: Aqueous conduct electricity in very little proportion.

WBBSE Chapter 8 Ionic And Covalent Bonding Covalent Bonding

According to the proposal of G. N. Lewis (1916) about electron pairs, the concept of covalent/ chemical bonding has grown up.

Covalent bonding is said to exist between non-metal atoms and covalent compounds are formed through mutual sharing of electrons.

Here we will discuss Lewis dot diagrams (where only the valence electrons are represented by a dot (•) / cross (x) (2D structure). Let us understand covalent bonding with the help of examples:

Bonding in F₂molecule: Electronic configuration: _gF = 2 + 7.

Each F atom has 7 e^_s in its valence shells. They need 1 more e¯ to attain stable e-configuration like the nearest noble gas Ne (complete octet).

Two F atoms come together and they share le each in a valence shell to complete the octet. Ultimately a molecule of F₂ is formed.

Since a single pair of e” is shared between two F atoms, the bond is known as a single covalent bond and is represented by a single dash (—) between two F atoms.

Bonding in H₂0 (water molecule): Electronic configuration: _XH = 1 & ₈0 = 2 + 6.

Each H atom needs 7 e¯ in its valence shell to complete the duplet and the 0 atom needs 2 more e¯s to complete the octet. So, in a water molecule (H₂0) two single covalent bonds are formed.

Bonding in CO₂ molecule: Electronic configuration : ₆C = 2 + 4 and _gO = 2 + 6. Each  O atom needs 2 more e¯s to complete the octet and the C atom needs 4 more e¯s to complete the octet.

So, when two O atoms and one C atom come together, each O atom will share 2 e¯s from the valance shell with 2 e¯s of the C atom in the valance shell so that all atoms acquire stable electronic configuration.

Since each O atom shares 2e¯s with the C atom, so two double covalent bonds are produced keeping C at the center and O atoms at two ends.

Bonding in N₂ molecule: Electronic configuration: ₇N = 2 + 5.

N atom has 5 e¯s in its valence shell. It needs 3 more e¯s to complete its octet.

So, when two N atoms come together, each N atom will share its 3 e¯s with another N atom and becomes stable.

So, each N atom shares 3 e¯ s with each other. Since the two N atoms share 3 pairs of e¯s, there is a triple covalent bond between two N atoms.

Few more examples of covalent bonding:

Note: (1)  The no.of Electrons shared by an atom for the formation of shared electron pairs to covalent compounds is called the covalency of that particular atom.

For example: As H or Cl Or F Atom Shares One explain Pair so the covalency of H or Cl or F is 1. O atom shares two pairs of electrons,

So its Covalency =2 n atom Shares 3 Pairs Of electrons, So its Covalency = 2 N atom dot structure of O-does does not support the experimental result of the paramagnetic property of O₂ molecules paramagnetic property refers to the ability to get attracted to the magnetic field).

WBBSE Chapter 8 Electricity And Chemical Reactions Electrolytes

Electrolytes Definition: Electrolytes are the chemical compounds that ionize or dissociate into their ions [cations (+) and anions (-)] in a molten/aqueous state and the solutions so-produced have the ability to conduct electricity (d.c.).

Take some table salt (NaCI, an ionic compound) and heat over 801°C, it melts and the ions Na⁺ and CP already present in it, get released.

Similarly, in its aqueous solution, H₂O molecules occupy some positions of NaCI molecules and because of the dielectric property of water, the electrostatic force of attraction between Na⁺ and Cl” ions is decreased so the ions get free.

Ionic compounds (NaCI, KCI, CuSO₄, CuCI₂, NaOH, etc.) in a molten/aqueous state can conduct electricity through these ions.

They behave like conductors but are different from metals. But solid NaCI is not an electrolyte. Because in the solid state, a strong electrostatic force of attraction exists between the Na⁺ and CP ions the ions are not free to move.

WBBSE Notes For Class 10 Physical Science And Environment

In the case of covalent compounds like HCI (g), no ions exist but in an aqueous solution, it forms HCI (strong acid) which can be dissociated into H⁺ and CP ions. This is called ionization. Other such examples are HF, HBr, HI, NH_3, etc.

Also, there are some covalent compounds like (pure) distilled water, sugar solution, alcohol, ether, benzene, kerosene, molten sulfur, ethanol, urea, and glycerine which do not dissociate into ions and cannot conduct electricity even by little amount.

They are called nonelectrolytes. They behave like non-conductors of electricity because they do not belong to free electrons (like metals) and ions (like electrolytes).

WBBSE Chapter 8 Electricity And Chemical Reactions Strong And Weak Electrolytes

This classification is based on the degree of ionization or the efficiency of dissociation that an electrolyte exhibits in its aqueous solution.

A greater degree of ionization corresponds with a stronger electrolyte. We can classify strong electrolytes, weak electrolytes, and non-electrolyte by measuring the electrical conductivity of their aq. solutions.

See the experimental- 

Observation of the experiment: Aq. solution of NaCI or KCI or H₂SO₄ or NaOH can conduct electricity very well so they are strong electrolytes.

On the other hand, acetic acid CH₃COOH aq.) or ammonium hydroxide (aq.) can also conduct electricity but not as well as a strong electrolyte they are weak electrolytes.

The non-electrolytes like ethanol/ benzene/ glycerine show zero conductivity.

Strong electrolyte: If a substance is molten/aq. the state completely dissociates into ions such that the solution consists entirely of ions, and molecules left, and can conduct electricity very well is called a strong electrolyte.

Strong acids (H₂S0₄, HN0₃, HCI), strong soluble bases (NaOH, KOH), and soluble salts (NaCI, K₂S0₄, CuS0₄, CuCI₂) are strong electrolytes.

Ionization reaction:

⇒ \(\mathrm{NaCl} \text { (aq.) } \rightarrow \mathrm{Na}^{+}+\mathrm{Cl}^{-} \text {. } \mathrm{CuSO}_4 \text { (aq.) } \rightarrow \mathrm{Cu}^{2+}+\mathrm{SO}_4^{2-} . \mathrm{H}_2 \mathrm{SO}_4 \text { (aq.) } \rightarrow 2 \mathrm{H}^{+}+\mathrm{SO}_4{ }^{2-} \text {. }\)

Since every molecule of a strong electrolyte dissociates into ions, so a single arrow (→) is used in the dissociation reaction. It’s a chemical change.

Weak electrolyte: If a substance in aq. solution partially ionizes (which means only a few molecules break into ions and most stay as neutral molecules) which still can conduct electricity but not as well as a strong electrolyte then it is called a weak electrolyte.

Weak acids (CH₃COOH, H₂C0₃), weak bases (NH₄OH), and some salt (AgCI) are examples.

Ionization reaction:

⇒ \(\mathrm{CH}_3 \mathrm{COOH} \text { (aq.) } \rightleftharpoons \mathrm{H}^{+}+\mathrm{CH}_3 \mathrm{COO}^{-} . \mathrm{NH}_4 \mathrm{OH} \text { (aq.) } \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{OH}^{-} .\)

In such solutions, an equilibrium is established between the ions and the non-ionized neutral molecules.

This is represented by a double arrow () between the ions and the non-ionized molecules of weak electrolytes.

WBBSE Chapter 8 Electricity And Chemical Reactions Mechanism Of Electrical Conduction In Molten Solution State

Take molten/aq. solution of table salt (NaCI) in a glass bowl. In the solution state, NaCI contains plenty of Na⁺ (cation) and Cl^– (anion) ions. \(\left[\mathrm{NaCl} \text { (aq.) } \rightarrow \mathrm{Na}^{+}+\mathrm{Cl}^{-}\right]\)

Two conducting rods (electrodes) are inserted into the solution.

The rod connected with the + ve terminal of the battery is the anode (A) and the rod connected with the – ve terminal of the battery is the cathode (C).

[The electrodes are usually made of inert material (platinum/graphite) to ensure that the electrodes itself do not involve in the electrolytic reactions.] Now let us see what happens when the electrodes (say 9V or 12V).

⇒ \(2 \mathrm{NaCl} \text { (aq.) } \stackrel{\text { electricity }}{\longrightarrow} 2 \mathrm{Na}^{+}+2 \mathrm{Cl}^{-} \text {. }\)

The Na⁺ ions get attracted towards the – ve electrode (cathode) and Cu ions are attracted by the +ve electrode (anode).

Thus, the cations and anions move in opposite directions for the purpose of electrical discharge.

At the cathode, each Na⁺ ion captures le^_ to produce metal Na (it is a reduction reaction) and at the anode, each Cl¯ ion gives up le¯ and becomes neutral Cl.

Atom (it is an oxidation reaction). The single Cl atoms then pair up and form Cl₂ gas. Electron flows from the anode to the cathode in the external circuit.

At the Cathode:

⇒ \(\begin{array}{r}
2 \mathrm{Na}^{+}+2 e^{-} \rightarrow 2 \mathrm{Na}(s) \ldots . . \\
\quad \text { (reduction reaction) }
\end{array}\)

At the anode:

⇒ \(2 \mathrm{Cl}^{-} \rightarrow 2 \mathrm{Cl}+2 e^{-}, 2 \mathrm{Cl} \rightarrow \mathrm{Cl}_2(g)….. (oxidation reaction)\)

So, solid sodium is formed at the cathode, and chlorine gas being given released at the anode (chemical change).

Here, the coefficients used in the equation (for overall electrical neutrality of the entire system) indicate mol numbers.

In this reaction, at the anode 2 mol Cl^– ion removes 2 mol e¯, and finally, 1 mol Cl₂ gas is produced. Similarly, at the cathode 2 mol Na⁺ ion gain 2 mol e¯, and finally, 2 mol Na (s) is produced.

That’s why, the mechanism of electrical conduction metals and electrolytes are not exactly the same.

In electrical conduction through molten/aq. electrolytes only the anions and cations (not electrons like conductors) take part, and during electrolysis, they move in opposite directions.

The difference in electrical conductivity between metals and electrolytes:

WBBSE Chapter 8 Electricity And Chemical Reactions Electrolysis

Electrolysis Definition: Electrolysis is a chemical dissociation/ionization of an electrolyte (chemical change) in a molten/aqueous state by passing direct current (d.c.)-

Electrolysis Example: 

⇒ \(\mathrm{CuSO}_4 \text { (aq.) } \stackrel{\text { electricity }}{\longrightarrow} \mathrm{Cu}^{2+}+\mathrm{SO}_4^{2-} ; \quad \mathrm{HCl} \text { (aq.) } \stackrel{\text { eléctricity }}{\longrightarrow} \mathrm{H}^{+}+\mathrm{Cl}^{-}\)

The ionic compounds in their solution state, ionize into cations and anions which conduct electricity.

In the case of covalent compounds like HCI (g)- no free ions are there, which in an aqueous state dissociates into H⁺ and Cl¯ ions.

In the case of electrolysis, electrical energy → is chemical energy. Just the opposite for an electric cell where chemical energy → electrical energy.

In the case of electrolysis, the source of electricity must be d.c. (obtained from a cell/battery of low voltage). No heavy current is to be used. No. a.c.

Electrolysis is a type of Redox reaction. Always oxidation (loss in e¯ anion) takes part at the anode and reduction (gain in e¯ by cation) at the cathode.

With respect to electron transfer, the cathode can be defined as the electrode towards which cations move and the anode is the electrode towards which anions move.

Electrolysis of acidified water using Pt-electrode: Already learned that pure water has no free ions to conduct electricity. Few drops of oil. H₂SO₄ can ionize pure water.

Then pure water becomes to be an electrolyte.

Electrolytic cell: Electrolysis of acidified water forms a cell, called Hoffman Voltameter.

The arrangement in which two Pt-electrodes and a 12 V d.c. supply batteries are used. (Pt is inert and cannot react with the solution).

Ionization:

⇒ \(\mathrm{H}_2 \mathrm{O}(\mathrm{H}-\mathrm{OH}) \rightarrow \mathrm{H}^{+}+\mathrm{OH}^{-}, \mathrm{H}_2 \mathrm{SO}_4 \rightarrow 2 \mathrm{H}^{+}+\mathrm{SO}_4{ }^{2-}\)

When more than one anions/cations come into the electrolytic solution, then only one of them gets preferentially discharged at the electrode.

This factor depends on the position of anion/cation in the electrochemical series, from which we can determine the tendency to lose/ gain e¯s.

This tendency is as follows:

At Cathode:

⇒ \(\begin{aligned}
& \mathrm{Ag}^{+}>\mathrm{Cu}^{2+}>\mathrm{H}^{+}>\mathrm{Fe}^{2+}>\mathrm{Zn}^{2+}>\mathrm{Al}^{3+}>\mathrm{Mg}^{2+} \\
& >\mathrm{Na}^{+}>\mathrm{Ca}^{2+}
\end{aligned}\)

At anode:

⇒\(\mathrm{I}^{-}>\mathrm{Br}^{-}>\mathrm{Cl}^{-}>\mathrm{OH}^{-}>\mathrm{NO}_3^{-}>\mathrm{SO}_4^{2-}>\mathrm{F} .\)

As soon as the switch is on, a lot of bubbles are seen to be formed in two tubes.

This happens because – both the an-ions OH and SO₄^2¯ get attracted towards the anode while cations H⁺ are attracted by the cathode.

According to the electrochemical series, OH¯ has a greater tendency to lose e” than SO₄^2¯. Thus, at the anode, each OH¯ ion loses le¯ (oxidation), and at the cathode, each H⁺ ion gains le¯ (reduction).

So electron flow continues from anode to cathode in the external circuit and constitutes a flow of electric current from anode to cathode in the electrolyte solution.

At anode:

⇒ \(\begin{aligned}
& \mathrm{OH}^{-} \rightarrow 1 e^{-}+\mathrm{OH} \text { (unstable); } \mathrm{OH}+\mathrm{OH} \rightarrow \mathrm{H}_2 \mathrm{O}+\mathrm{O} \text { (nascent); } \mathrm{O}+\mathrm{O} \rightarrow \mathrm{O}_2 \\
& 4 \mathrm{OH} \rightarrow 4 e^{-}+4 \mathrm{OH} ; 4 \mathrm{OH} \rightarrow 2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2 \uparrow \text { (oxidation) }
\end{aligned}\)

At Cathode: 

⇒ \(\begin{aligned}
& \mathrm{H}^{+}+1 e^{-} \rightarrow \mathrm{H} ; \mathrm{H}+\mathrm{H} \rightarrow \mathrm{H}_2 \\
& 4 \mathrm{H}^{+}+4 e^{-} \rightarrow 2 \mathrm{H}_2 \uparrow \text { (reduction) }
\end{aligned}\)

Observation: 

At the – ve and + ve electrodes, the gases collected are detected as H₂ and O₂ respectively. The ratio of the volume of H₂ and O₂ is 2 :1.

Electrolysis of CuSO₄ (aq.) using inert (Pt/graphite) electrode: Two Pt-electrodes are used as anode (+ ve) and cathode (- ve).

In place of Pt, graphite can also be used. Pt. is nonreactive but there is a little chance of oxidation of graphite.

Ionization: \(\mathrm{CuSO}_4 \text { (blue solution) } \rightarrow \mathrm{Cu}^{2+}+\mathrm{SO}_4^{2-}, \quad \mathrm{H}_2 \mathrm{O}(\mathrm{H}-\mathrm{OH}) \rightarrow \mathrm{H}^{+}+\mathrm{OH}^{-}\)

As soon as the switch is on, the -vely charged OH¯ ions move towards the anode (+ ve), and + very charged Cu²⁺ ions move towards the cathode (- ve).

Because according to the electrochemical series, OH¯ has a greater tendency to lose e¯ then SO₄^2+, and Cu²⁺ has a greater tendency to gain e¯ than H.

Then OH¯ and Cu²⁺ ions are discharged at the anode and cathode respectively.

At Anode:  \(\mathrm{OH}^{-} \rightarrow 1 e^{-}+\mathrm{OH} ; 4 \mathrm{OH}^{-} \rightarrow 4 e^{-}+4 \mathrm{OH} ; 4 \mathrm{OH} \rightarrow 2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}, \uparrow \text { (oxidation) }\)

At Cathode: \(\mathrm{Cu}^{2+}+2 e^{-} \rightarrow \mathrm{Cu} \downarrow ; 2 \mathrm{Cu}^{2+}+4 e^{-} \rightarrow 2 \mathrm{Cu} \downarrow \text { (reduction) }\)

Observations :

Products are O₂ gas at the anode and Cu deposited at the cathode.

Pink/reddish brown Cu metal gets deposited at the cathode. So, cathode mass increases.

The presence of Cu²⁺ ions makes CuSO₄ While electrolysis, these Cu²⁺ ions from the solution get deposited slowly at the cathode, so the blue color of CuSO₄ solution fades away.

Electrolysis of CuSO₄ (aq.) using Cu electrodes: 

In the Previous case, we used Pt-electric rods in the case, and both the electrodes are of Cu.

In this case, both electrodes are of Cu. Now let’s see what difference is observed in the electrolysis of CuSO₄ solution.

Now let’s see what difference is observed in the electrolysis of CuSO₄ solution.

The ions present in the solution are

⇒\(\mathrm{Cu}^{2+}+\mathrm{SO}_4^{2-}\mathrm{H}^{+}, \mathrm{OH}^{-}\)

At cathode: \(\mathrm{Cu}^{2+} \text { (aq.) }+2 e^{-} \rightarrow \mathrm{Cu}(s) \text { [Reduction reaction] }\)

So Cu²⁺ ions on being discharged are deposited at the cathode as Cu-atoms.

At anode: \(\mathrm{Cu}(\mathrm{s}) \rightarrow \mathrm{Cu}^{2+}+2 e^{-} \text {[Oxidation reaction] }\)

Within the solution, Cu²⁺ ions exist and the anode itself is made up of Cu-atoms. In such a case, neither OH+ nor SO₄²⁺ will be discharged – rather, at the anode each Cu-atom losing 2e¯s goes into the solution as a Cu²⁺ ion.

As a whole, when a Cu² ion is deposited at the cathode, simultaneously from the anode another Cu²⁺ ion goes into the solution.

Ultimately, the amount by which anode mass decreases, and cathode mass increases by the same amount.

Also, the concentration of CuSO₄ solution effectively remains constant. So, the blue color of the CuSO₄ solution doesn’t fade away. (This process is used as electroplating).

WBBSE Chapter 8 Electricity And Chemical Reactions Applications Of Electrolysis

Electrolysis is mostly used in industry in many ways such as

In metal extraction,

In electrorefining,

In electroplating.

Electrolysis in the extraction of metals from their ores: Highly electropositive (active) metals like Na, Ca, and Al are extracted from their ores using electrolysis.

Al-Extraction: The principal ore of Al is bauxite (Al₂O₃-2H₂O).

It is purified to yield aluminum oxide (Al₂O₃) [white powder – its m.p. is very high (over 2000°C)].

Al₂O₃ is dissolved in molten cryolite (Na₃AlF₆) [an Al-compound with much lower m.p. than Al₂O₃ and the temperature is maintained at ~ 925°C.

Molten Al₂O₃ contains Al-ions (Al³⁺) and Oxide-ions (O^2- ).

Both the electrodes are made up of graphite. More than one graphite rod is used as the anode.

At the Anode: \(\mathrm{O}^{2-}-2 e^{-} \underset{\text { oxidation }}{\longrightarrow} 0 ; 2 \mathrm{O}^{2-} \rightarrow \mathrm{O}_2(g)+4 e^{-}\)

At the Cathode: \(\mathrm{Al}^{3+}+3 e^{-} \underset{\text { Reduction }}{\longrightarrow} \mathrm{Al}(\mathrm{s})\)

The O₂ molecules formed at the anode react with the graphite (carbon atoms) rods (Anode) and form  \(\mathrm{CO}_2 \text { gas. }\left[\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2(g)\right] \text {. }\)Due to this reason, the anode rods are regularly replaced.

Electro-refining of copper: Electro-refining is a process of purifying an impure metal to a high purity level to get pure metal using electrolysis.

Electrolytic solution: Aq. CuSO₄ + dil. H₂SO₄.

Cathode: A thin sheet of pure Cu.

Anode: Impure sheet of Cu. The ions present in the solution are Cu²⁺, SO₄^2-.

At the anode: From impure sheet \(\mathrm{Cu}(\mathrm{s}) \underset{\text { oxidation }}{\longrightarrow} \mathrm{Cu}^{2+}(\mathrm{aq} .)+2 e^{-}\)

At the cathode:

⇒ \(\mathrm{Cu}^{2+}(\text { aq. })+2 e^{-} \underset{\text { Reduction }}{\longrightarrow} \mathrm{Cu}(\mathrm{s})\)

The Cu²⁺ ions move between the anode and the cathode. When a Cu²⁺ ion is deposited cathode, at the same time, another Cu ion from an impure Cu sheet goes into the solution.

Thus, the concentration of CuSO₄ solution effectively remains the same.

As this happens, the size of the cathode sheet gets larger while the size of the anode sheet gradually decays, a sludge is left at the bottom of the anode which is called anode mud.

The anode als mud (Ag/Au) may contain many valuable met- Cu obtained from electro-refining and is about 99-99% pure.

Electroplating: Electroplating is an electrolytic process by which a thin layer of superior metal or noble metal like gold, silver, nickel, or chromium is deposited/coated on the surface of a base metal either for protection against corrosion or making attractive.

Basic principles of electroplating:

The article to be electroplated is cleaned first,

Metal on which electroplating is to be done is used as the cathode (- ve electrode). Because deposition always takes place at the cathode,

A pure block of the metal to be electroplated is used as the anode (+ ve electrode),

A solution of a salt of the metal is used as the electrolyte (the solution contains metal ions).

For example, in Cu-plating the electrolyte is CuS0₄(aq.); in silver plating, silver nitrate (AgNO₃)/potassium argento cyanide K [Ag (CN)₂J solution;

In gold-plating, potassium auro cyanide K [Au (CN)₂] solution; in nickel plating, a mixed solution of

⇒ \(\mathrm{NiSO}_4+\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4+\text { slight boric acio }\)

For getting uniform deposition, a low direct current (3A or 4A) for a longer time is passed through the solution. (No heavy current to be used or no a.c. to be used).

WBBSE Chapter 8 Physical And Chemical Properties Of Matter Periodic Table And Periodicity Of The Properties Of Elements

Brief History Of The Periodic Table

In the 18th century, only 31 elements were known. Up to 1865, more elements (63) were identified. At that time, the number of known elements was increasing, and separate studies of each element were becoming difficult.

Scientists were trying to classify the elements having identical properties in a single class/group.
The first attempt was made in 1829 by German chemist Johann Dobereiner.

He made groups of 3 elements in the order of increasing atomic mass such that the atomic mass of the middle element is equal to the average value of the atomic mass of the corner elements.

This group of elements was named Dobereiner’s traits He could identify only 3 triads for the elements known at that time.

For the first time, Dobereiner’s model gave an idea of the classification of elements. But all known elements could not be arranged in triads.

WBBSE Notes For Class 10 Physical Science And Environment

In some cases, even in the same group, this model was not obeyed.

For example F-19, CI-35.5, Br-80, the average atomic mass \(=\frac{19+80}{\cdot 2}=49 \cdot 5\) (it did not match). So, ultimately this model was rejected.

After a long time, in 1864, English scientist Newlands, making use of the octave concept of the music system was successful in arranging 8 elements in the order of increasing atomic mass, to make an octave.

Newlands found that every 8th element has chemical properties similar to the 1st element, the same for the 2nd and 9th elements, etc.

This model was successful up to Ca (only for lighter elements not for all elements known at that time). For the first time, Newlands gave the idea of periodicity/repetition of properties.

In 1869-70, German chemist Lothar Meyer and Russian chemist Dmitri Mendeleev were studying to find out the similarity in properties of the arrangement of elements.

Both scientists reached almost the same level of conclusion but in different ways.

Meyer’s investigation was based on a graphical analysis of atomic volume vs atomic mass

According to Meyer, elements at similar positions on the graph have similar properties.

Example: at the peaks he found alkali metals of Gr. 1 (Na, K, Rb), in descending slopes→ alkaline earth metals (Mg, Ca, Sr), in ascending slopes → halogens (Cl, Br, I).

Meyer classified only 28 elements. But from the graph, exact information about elements still remained unexplained.

In the same year, Mendeleev was studying to find out the relation between the atomic mass of

elements and their physical properties (density, LotherMeyersgraph melting point, boiling point) and chemical properties (how an element reacts with other elements).

Mendeleev’s Periodic Law: The physical and chemical properties of the elements are the periodic function of their atomic masses.

Based upon this law, Mendeleev arranged only 63 elements known at that time in order of increasing atomic mass in tabular form which he named Periodic While arranging so, he made 7 horizontal rows called Periods and represented these as periods 1, 2, 3 . . . 7.

He told in the same period moving from left →right, properties shift from metal-non-metal. Mendeleev’s original periodic table has 8 vertical columns which he named Groups

and numbered as Gr. I, II, III, VIII. After the discovery of noble gases (1891 – 1902), a separate Gr. (0) has been added.

He also left some gaps. The revised version of Mendeleev’s Periodic Table has 7 periods and 9 groups. He divided Grs. I → VII into sub-groups A, B.

But Gr. VIII has no sub-group it has 3 elements in a single period. He suggested that elements in the same sub-group have the same valency/identical properties.

Actually, Mendeleev’s analysis was based on the formulae of oxides and hydrides that the elements can form because 0₂ and H₂ are very much reactive and form compounds with most elements

Mendeleev’s Periodic Table completely changed old ideas. Although it was different from the modern one, it was a huge step in the classification of elements.

Mendeleev is considered the ‘Father of the Periodic Table.’

Few achievements of Mendeleev: In his periodic table, he left gaps below Al (26-98) and named it eka-aluminum (later it was discovered Ga) and below Si (28-09) and named it eka-silicon (later discovered element was Ge), etc.

In this way, he made a strong prediction of some undiscovered elements. Also, successfully predicted some of their physical and chemical properties.

In 4 misfit positions, he placed an element with a slightly greater atomic mass before an element with a slightly lower atomic mass.

For example Ar (39-95) and K (39-10); Co (58-93) and Ni (58-71); Te (127 60) and I (126-90); Th (232-04); and Pa (231).

Perhaps, this was done so that elements with similar properties could be grouped together.

After the discovery of noble gases, they could be placed in a new group without disturbing the existing order.

Improvements needed In Mendeleev’s Periodic Table: O In some cases, dissimilar elements were placed in the same group. For example : (Na, K, and Rb) were placed in the same Gr. I with (Cu, Ag).

In some cases, similar elements were placed in separate groups. Example : (Pt and Au).

Isotopes of the same element should have different positions in the periodic table.

No fixed position could be given to hydrogen because of its similarities with Gr IA elements (alkali metals) and Gr. VII B elements (halogens).

WBBSE Chapter 8 Physical And Chemical Properties Of Matter Modern Periodic Table

In 1911 Henry Moseley, while experimenting with X-rays taking 38 different elements as target material proved that the Frequency Characteristic To an Element Depends On the Atomic

a number of the element \((\sqrt{f} \propto Z)\), where Z = atomic number = a number of p’, present in the nucleus of an atom.

For a neutral atom, the number of ps = the number of e s. According to this concept, the chemical properties of the elements are more related to their atomic numbers than their atomic masses.

Modern Periodic Law: The physical and chemical properties of the elements are the periodic function of their atomic numbers.

Modern Periodic Table (Long Form of Periodic Table): Soon after the discovery/ of Moseley, all elements increased in the order of their atomic numbers instead of atomic masses. A group of scientists contributed to this work.

In 1920 Niels Bohr prepared the Modem Periodic Table, which is popularly known as the Long Form of the Periodic it is prepared on the basis of the electronic configuration of the elements.

The modem periodic table has 7 horizontal periods (1, 2, 3, 4, 5, 6, 7) and 18 vertical columns (1 – 18). As per the proposal of IUPAC (1984), the groups are numbered 1, 2, 3, 4, and 18 in place of Mendeleev’s sub-groups A, and B.

The elements mentioned in serial no. 1, 2, and 3, are the atomic number of different elements with every element the atomic number increasing by 1 means 1 extra p or 1 extra e¯ is added to the chemical structure.

Significance of Period: The number of shells (orbits) present in an atom is the period number. As in every next period, 1 more shell gets added.

Significance of Group: Elements having similar chemical properties or more specifically having similar electronic configurations are grouped together. Examples:

(1) Gr. I. elements (₁H – ₇₈Fr) have similar electronic configurations: \({ }_1 \mathrm{H}(\underline{1}),{ }_3 \mathrm{Li}(2, \underline{1}),{ }_{11} \mathrm{Na}(2,8, \underline{1}),{ }_{19} \mathrm{~K}(2,8,8, \underline{1})\)

.. All have 1 valence e¯ in the outermost shell. So they have the same valency = 1 and the same other chemical properties.

All Gr. 1 elements except are called alkali metals, because they form strong alkalis with water.

(2) Gr. 2 elements (₄Be – ₈₈Ra) have 2 valence e¯s: ₄Be = (2, 2), ₁₂Mg (2, 8, 2), ₂₀ Ca (2, 8, 8, 2) . . . So their valency = 2.

All Gr. 2 elements are called alkaline earth metals because they form weaker alkalis as compared to Gr. 1. elements. Their oxides are available on Earth.

(3) Gr. 17 elements (_gF – _g5 Al) have 7 valence e¯s and can accept le^_ in the outermost shell to complete the octet. ₉F (2, 7), ₁₇CI (2, 8, 7).

So they have the same valency = 1. These elements are called halogens (which means salt-producing).

(4) Gr. 18 elements (₂He –₈₆Rn) have fulfilled the outermost shell, for which they have zero (0) valency. ₂He(2), ₁₀Ne(2, 8), _lgAr (2, 8, 8).

These elements are called noble gases/inert gases because of their non-reacting power with other elements.

(5) Gr. 13 elements are elements of the B (Boron) family as B is the 1st member. Similarly, Gr. 14→ C (Carbon) family, Gr. 15→ N family,

Gr. 16→ O family or chalcogens (means ore forming).

Gr. 3-12 elements are called transition elements because the properties shift/transit from metal to non-metal.

Special feature:

Elements in Periodic 6 with atomic number 57 – 71 are called Lanthanides, because they start with ₅₇La (Lanthanum). They are called rare earth elements.

Elements in Period 7 with atomic number 89 – 103 are called Actinides, because they start with ₈₉Ac(Actinium). These are radioactive elements.

The success of the Modern Periodic Table: It rectified the anomalies in Mendeleev’s Periodic Table.

On the basis of the arrangement. of elements with increasing atomic number positions of 1 Co and Ni were corrected.

Isotopes have similar chemical properties. So, they belong to the same position as that of the element in the periodic table.

The position of hydrogen in the periodic, table is controversial. It belongs to some properties like Gr. 1 alkali metals and some properties like Gr. 17 halogens.

Example:

Because of these peculiarities, Mendeleev named hydrogen a ‘rogue element’. In general, hydrogen is considered as a non-metallic gas.

Some trans-uranic elements: These are elements having an atomic number greater than that of uranium i.e. Z > 92 in the periodic table.

Examples are Neptunium (Z = 93), Plutonium (Z = 94), Curium (Z = 96), Einsteinium (Z = 99), Mendeleevium (Z = 101), etc.

Except for Neptunium and Plutonium, others are man-made. The discovery of the element Copernicium (Z = 112) completes the transition element series.

WBBSE Chapter 8 Physical And Chemical Properties Of Matter Periodic Trends In The Periodic Table

When properties of elements in a period/group in the periodic table are compared, certain regularity is observed in their variations which are called periodic trends.

The properties repeat themselves (either increase or decrease) after regular intervals in the periodic table. This is a regular trend or pattern.

This trend is measured in two ways: Across a period (from left to right) and down a group (from top to bottom).

In our syllabus, we will study some periodic trends such as atomic radii, ionization energies, and oxidizing-reducing properties of elements of Gr.1-2 and 13-18.

1. Atomic radii/Atomic size: Atomic radius is the distance of the outermost from the nucleus of the atom (i.e. atom’s boundary).

It is so little that we cannot directly measure the radius of an isolated atom. But from experimental data collected from other experiments, the atomic radius can be derived indirectly.

It is expressed in a picometer (pm) unit which is even smaller than a nanometer (1 pm = 1CT¹² m).

Trends in atomic radii (along a period from left to right): Along a period, the number of shells remains the same.

A number of ps increasing i.e. nuclear charge increasing i.e. e¯s are attracted towards the nucleus by a greater force means atomic radii decrease.

Down the group (Top-Bottom): With, as we move down the group, the number of shells Number of valence e¯s remain the same.

This means outermost e¯s are going away from the nucleus, which means atomic radii increase.

The trend in atomic radii: Along a period from left to right decreases and down the group increases.

Ionization energy (I.E.):

Definition: Ionization (I.E.) energy is defined as the minimum amount of energy required to remove the outermost e¯ of an isolated neutral gaseous atom to form a cation.

i.E. is always + Ve because removal of e requires energy. It is an endothermic reaction.

I.E. is measured in eV / atom unit for an isolated atom. It means how much eV energy is required to remove e¯ from an atom.

Explanation: We know that e¯s are bound with the nucleus with a strong electrostatic force of attraction.

To remove the outermost e¯, energy is needed to be applied externally/ work to be done – which is called i.E.

Example: \(\underset{\text { (neutral) }}{\mathrm{M}^{\circ}} \rightarrow e^{-}+\underset{\text { (cation) }}{\mathrm{M}^{+}}(+\Delta \mathrm{H})\)

Another famous unit is KJ/mol for 1 mol gas is also used (leV/atom = 96 KJ / mol).

Trends in I.E. along a period (Left →Right): As we move along a period, atomic size increases meaning nuclear charge increases means attraction of e¯ increases means removal e¯ becomes tougher. So more I.E…….. is required for ₉F as compared to ₃Li.

Down the group (Top→Bottom): As we move down the group, the number of shells increases means atomic size increases means e¯ is going away from the nucleus means removal of e¯ becomes harder.

So less I.E. is required to produce K⁺ion than to produce Fl⁺ ion.

The general trend in I.E.: Along a period from left to right increases and down the group decreases. (There are some exceptional cases also. You will learn more in higher classes).

Electronegativity (E.N.):

Definition: The ability of an atom to attract shared pair of e^–s towards itself in a covalent bond within a molecule is called electronegativity.

Explanation: Let us take an example: Here, the shared pair of e¯s are attracted by both the atoms X and Y.

The atom which has more E.N. has a greater ability to attract the e pair. On the other hand, the atom which has less E.N. has a lesser ability to attract the e -pair.

So E.N. is a relative property as its idea is not related to an individual atom. So, it has no unit. It is measured in Pawling’s scale.

In the HCI molecule, H-nucleus weakly attracts the e¯pair, which is stronger for Cl-nucleus.
So, the E.N. of Cl is more than the E.N. of H.

As a result of this, both H and Cl atoms become partially charged – Cl gets a slight – ve charge (5-) and H gets a slight +i/e charge (8+). The chemical bond between H and Cl is called a polar bond.

In the F² molecule, the F atoms equally attract the e¯pair. So no partial +ve/-ve charge appears on either atom of F. So F² molecule has a non-polar bond.

Trends of E.N. in the Periodic Table along a period: As we move from left to right, atomic size decreases, and nuclear charge increases means the ability to attract shared e pair also increases. So, _gF is more E.N. than ₅B and ₅B is more E.N. than ₁H.

Note: F is the most electronegative element of all the elements.

Metallic and Non-metallic character: Elements that have more electronegativity have a tendency to gain e¯s.

They have non-metallic On the other hand, elements that have less electronegativity have a tendency to lose e¯s.

They have a metallic character. So in the periodic table, along a period metallic character decreases, and down the group metallic character increases.

Factors on which metallic and non-metallic character of elements depend are:

1. Atomic size
2. Nuclear charge.

Oxidizing / Reducing Properties: Oxidation is the loss of e¯ s and Reduction is the gain of e¯s.

Metals have a tendency to lose e¯s known as reducing property. Non-metals have a tendency to gain e¯s_r known as oxidizing property.

Within a compound, metal atoms have a relatively low attraction for the e¯s because of their low I.E., and low electronegativity.

Elements present on the right of the periodic table are strong oxidizing agents. Elements present on the left side of the periodic table are strong reducing agents.

General trend: Along a period, as we move from left to a right tendency to gain e¯ increases, and the tendency to lose e¯ decreases.

This means that oxidizing property gradually increases and the reducing property gradually decreases.

That is, properties shift from metallic to non-metallic. For the same reason, alkali metals of Gr. 1 are good reducing agents.

Halogens of Gr. 17 have a greater tendency to gain e¯s. Again down a group, reducing property increases, and oxidizing property decrease.

WBBSE Chapter 7 Atomic Nucleus Radioactivity

Radioactivity Definition: Radioactivity is a nuclear phenomenon of spontaneous emission of α or β and γ-radiations from the nuclei of heavier atoms with atomic number more than 82 (i.e., after 82Pb in the periodic table in order to attain a stable state from an unstable state.)

1. In 1896, French scientist Henry Becquerel, while conducting experiments with newly discovered.
2. X-rays to investigate how uranium salts are affected by this light and quite accidentally.
3. He discovered that the uranium salts spontaneously emit highly penetrating radiations (similar to X-rays) from within themselves.
4. The radiations can be registered on a photographic plate. Shortly afterward, Madam Curie and her husband Pierre Curie found that ‘pitchblende’ (an ore of uranium) emits a similar type of radiation.
5. They named this phenomenon natural radioactivity and commented that it is a property of the nucleus.
6. The phenomenon is ‘spontaneous’ and any physical or chemical condition cannot change the emission of radiation.
7. The process of such radiation is known as radioactive decay or disintegration. A nucleus can undergo radioactive -decay through the emission of α or β and γ rays.
8. Examples of some radioactive elements are uranium, radium, polonium, thorium, actinium, etc.

Read And Learn Also WBBSE Notes For Class 10 Physical Science And Environment

WBBSE Chapter 7 Atomic Nucleus Nature α,β And γ-Rays

Rutherford and his co-workers discovered that the radioactive rays are a combination of α, β, and γ-radiations.

Comparison of a, p, and y-radiations with reference to some highlighting properties:

How to know the charge of radioactive radiation? Radioactive radiations are affected by an electric field. It is observed that α-particles bent slightly towards the -ve plate.

So they are positively charged heavier particles, β-particles bent more towards -i-ve plate.

So they are negatively charged lighter particles, and γ-radiations remain undeflected so they are uncharged.

Effect on atomic number and mass number due to α, β, and γ-emi-ssion. In any nuclear reaction, total atomic no. and total mass no. always remain the same.

1. Due to the emission of an a-particle, the atomic number decreases by 2 units and the mass number decreases by 4 units. That is,

For example, 

2. Due to the emission of a (3-particle, the atomic number increases by 1 unit, but the mass number remains the same. That is,

For example,

3. Due to y-radiation, no change in mass and charge occurs.

Note:

The S.l. unit of radioactivity is Becquerel (Bq). lBq = one disintegration per second or one event per second.

Bigger units: 1 megabecqural or MBq = 10⁶ Bq and 1 gigabecqueral or GBq = 10⁹ Bq The common unit for measuring the activity of radioactive substances is curie.

1 curie = 3.70 x 10¹⁰ disintegration per second or 3.70 x10¹⁰ event per second.

Smaller units: 1 milli-curie = 10^-3 curie and 1 micro-curie = 10^-6curie. A unit Rutherford is equal to 10⁶ disintegrations per second.

Smaller units: 1 milli-rutherford = 10³ disintegrations per second and 1 micro-rutherford = 10⁶ disintegrations per second.

Note: Radioactivity is considered a nuclear phenomenon (not a chemical phenomenon).

Inside the nucleus of an atom, the repulsive electrostatic force between +vely charged protons (ps) tends to push them away from each other.

For lighter elements whose Z V 20(like \(\left.{ }_2^4 \mathrm{He},{ }_8^{16} \mathrm{O},{ }_7^{14} \mathrm{~N},{ }_{11}^{23} \mathrm{Na},{ }_{12}^{24} \mathrm{Mg},{ }_{20}^{40} \mathrm{Ca}\right)\)

(like for which these nucleons are held together by a strong force (other than electrostatic force) called nuclear force.

This nuclear force cancels out the electrostatic force of repulsion between Ps and keeps the nucleus stable.

The nuclear force is charge-independent, stronger than electrostatic force (~ 100 times), and has a very very short range.

But when the no. of ns exceeds the no. of ps for atoms with \(\mathrm{Z}>20 \text { (like }{ }_{26}^{56} \mathrm{Fe},{ }_{47}^{108} \mathrm{Ag} \text { etc.) }\) for which the ratio → 1.

Then the strong nuclear force cannot hold the ps and ns together and the nucleus becomes unstable.

To attain stability, the unstable nuclei fling out different particles or some energy. This is the reason why some elements are radioactive.

It has been observed that the rate of decay of the radioactive elements remains unaffected:

By any physical change, such as a change in pressure, or temperature,

By any chemical change, such as excessive heating, cooling, oxidation, the action of strong electric and magnetic fields, etc.

3. How β-particles (fast-moving\({ }_{-1}^0 \text { es }\)) are emitted from the nucleus?

β particles are emitted from the nuclei which have too many ns and ps. In order to, maintain the conservation of electric charge, one of the ns Is transformed into ps and vice versa.

This is the reason for (β-decay from the nucleus. In the case of [β-decay, an n is changed into a p and an electron, accompanied by a charge less, massless particle antineutrino.

Although particles and cathode rays both are fast-moving electrons, they differ from one another.

Because; [β-particles are emitted from the nuclei of radioactive elements, whereas cathode rays originate from the orbital electrons.

4. After the emission of α or (γ-particles, the nuclei of radioactive elements acquire a state of excitation where they possess excess energy.

The excess energy is released in the form of electromagnetic radiation, known as the y-radiation so that the excited nuclei come to their ground state.

The y-radiation can be represented as follows:

Although X-rays and y-rays are similar types of electromagnetic radiation, they differ from one another in their origin.

X-rays are emitted due to the transition of electrons in their inner orbits, whereas y-radiations are emitted from the excited nuclei to come to their ground state.

Uses of radioactivity: Radioactivity is used in

(1) Medical field: In the treatment of thyroid, radioactive iodine\(\left({ }_{53}^{131} \mathrm{l}\right)\); in the treatment of blood cancer or leucemia or to detect brain tumor, radioactive phosphorus \(\left({ }_{15}^{32} \mathrm{P}\right)\) in the treatment of malignant cancer, radioactive cobalt \(\left({ }_{27}^{60} \mathrm{Co}\right)\); in the detection of blood circulation in heart, radioactive sodium \(\left({ }_{11}^{24} \mathrm{Co}\right)\) is used.

(2) To determine the age of dead microorganisms (plants, animals), radiocarbon \(\left({ }_{6}^{14} \mathrm{C}\right)\)

(3) The age of older rocks (dating) can be calculated using ²³⁸U, ⁴⁰K radioisotopes.

As rocks often contain traces of uranium, which eventually decays to lead).

(4) In agriculture, radioactive samples are used as tracers and also used in improving food production, improvement of soil fertility, and others.

WBBSE Chapter 7 Atomic Nucleus Mass Defect, Binding Energy, And Nuclear Fission

In the theory of relativity, Albert Einstein explained the equivalence of mass and energy by the equation: E = mc².

According to this law, when an amount of mass ‘m’-disappears, an equivalent amount of energy ‘E’ is released from the system to the surroundings.

Again, if the energy of a system is increased by the amount ‘E’ its mass will be increased by an amount \(m=\frac{E}{c^2}.\)

In the case of high-energy reactions such as nuclear fission and fusion, mass is converted into energy.

In general, the mass of a nucleus is slightly lower than the total mass of the nucleons (protons + neutrons) in the nucleus.

This difference in mass is termed the mass defect (Δm). Some mass disappears. during the formation of the nucleus (exothermic).

Binding energy is the amount of energy released when the nucleons bind together to form the nucleus of an atom.

Suppose that a nucleus has a rest massp\({ }_z^A X\).

If mp and mn are the mass of a proton and a neutron respectively, then the mass defect, \(\Delta m=\left[\mathrm{Z} \cdot m_p+(\mathrm{A}-\mathrm{Z}) m_n\right]-\mathrm{M}\)

Then according to Einstein’s mass-energy equivalence formula, the binding energy will be \(=\Delta m \cdot c^2=\left\{\left[Z \cdot m_p+\right.\right.\left.\left.(\mathrm{A}-\mathrm{Z}) m_n\right]-\mathrm{M}\right\} c^2\)

Nuclear fission: The process of splitting up the nucleus of a heavy atom after interacting with the projectile (mainly thermal neutron having energy ~ 0.025 eV) into two or lighter nuclei of comparable size with the release of energy is known as nuclear fission.

A typical example of a nuclear fission reaction is a single neutron \(\left({ }_0^1 n\right)\) in a uranium nucleus \(\left({ }_{92}^{235} \mathrm{U}\right)\) produces three neutrons accompanied by two comparatively lighter nuclei of barium and krypton.

The three neutrons produced in the reaction disintegrate three more \(\left({ }_{92}^{235} \mathrm{U}\right)\) nuclei. Then, nine neutrons are produced, which in turn can produce fission of nine more\(\left({ }_{92}^{235} \mathrm{U}\right)\) nuclei and so on.

Thus a chain reaction would start and within a few milliseconds, a tremendous amount of energy would be liberated. The daughter nuclei are also radioactive and they further emit [β-rays, and γ-rays.

Calculation of energy released: For nuclear fission reaction, all the masses are taken in a.m.u. or u (where 1 u= 1.66 x 10^-27Kg).

Total mass before reaction = 234.993 u + 1.00 87 u = 236.0022 u

Total mass after reaction = 140.8834 u + 91.9020 u + 3 x 1.0087 u = 235.8115 u

∴ Difference in mass (i.e., mass defect) = 236.0022 u – 235.8115 u = 0.1907 u

According to the equation E = me², 1 u mass is equivalent to 932-6 MeV energy.

Thus, one nucleus of ²³⁵U releases the energy = 0-1907 x 932-6 MeV = 177-85 MeV.

Can you imagine the fission of only 50 g of ²³⁵U? 3-55 x 10¹² J (very large amount) of energy is released  by which one 100 W bulb could run for about 1125 years

Atom bomb: The principle of nuclear fission (using U-235 or Pu-239) is used in the construction of an atom bomb.

In it, the fission reaction takes place in an uncontrolled manner so that a single fission causes more than fission and an enormous amount of energy (as I energy) is liberated within a few milliseconds. The how an atomic bomb is made.

During the final stage of World War II, two a-bombs were dropped on the Japanese cities Hiroshima (on Aug 6, 1945, at 8:11 am, named “Li Boy’) and Nagasaki (on Aug 9, 1945, at 11: 02 am, explosion and it’s named ‘Fat Man’).

The two bombings killed at least 129000 people, and destroyed a lot like burning effects, radiation sickness, genetic defects, loss of fertility of lands, and many others.

Nuclear reactors as peaceful use of nuclear energy: A nuclear reactor is a device in which a controlled nuclear chain reaction can be initiated to produce energy.

In a controlled nuclear reaction, a single fission causes one fission, and energy is liberated at a constant rate.

This energy is used at nuclear power plants for the generation of electricity. Today, about 450 or more nuclear reactors are used in about 30 nations around the world.

Physics Class 10 Wbbse

Nuclear accidents :

1. The Chornobyl accident was the worst nuclear power plant accident in history. It happened on 26 April 1986 in Ukraine (part of the former Soviet Union).

Operating the plant at very low power, the reactors became highly unstable, and hot (about 2000°C) radioactive fuel substances came out as a steam explosion.

It lifted the cover off the top of the reactor. A large area was contaminated within 36 hours of the accident.

The Fukushima accident (located in the northern part of Japan) occurred on 11 March 2011 following a massive earthquake (recorded magnitude 9-0) and tsunami (about 15 m high).

The plant’s emergency power generators in the basement were flooded, causing the leakage of radioactive substances into the environment.

The fuel rods were also melted with the leakage of deadly radiation. It is the worst nuclear disaster since the 1986 Chornobyl accident along with the release of a large amount of energy is called nuclear fusion.

The process of the formation of a heavy nucleus by the combination of two or lighter nuclei \(\left({ }_1^1 H,{ }_1^2 H,{ }_1^3 \mathrm{H},{ }_3^6\right. \text { Li, etc.) }\)

A typical example of a fusion reaction is:

⇒ \(4{ }_1^1 \mathrm{H} \longrightarrow{ }_2^4 \mathrm{He}+2{ }_{+1}^0 e+\text { Energy or, }{ }_1^2 \mathrm{H}+{ }_1^3 \mathrm{H} \longrightarrow{ }_2^4 \mathrm{He}+{ }_0^1 \mathrm{n}+\text { Energy }\)

The difference in mass between the parent nuclei and the daughter nuclei is released in the form of energy (approximately 17-6 MeV energy perfusion).

Physics Class 10 Wbbse

The nuclear fusion reactions take place at a very high temperature (about 107°C). Due to this fact, nuclear fusion is also called thermonuclear reaction.

Such a high temperature can result only from a nuclear fission reaction. But, once the nuclear fusion reaction starts, the energy release is so high enough to maintain a high temperature to fuse more nuclei and the reaction continues.

That’s why, we say nuclear fission initiates nuclear fusion. Mathematically, it can be proved that in fusion much more amount of energy is released than in fission reaction per unit mass of fuel.

Nuclear fusion is not a chain reaction.

Physics Class 10 Wbbse

Nuclear fusion takes place in the interior of the sun and other stars. Sun and other stars are composed of about 90% hydrogen and helium gas. Only 10% is other elements…

In a hydrogen bomb, the synthesis of a heavier atom from hydrogen results in the release of an enormous amount of energy.

WBBSE Chapter 6 Current Electricity Electric Current, Potential Difference and EMF Concept Of Electric Charge

About 600 B.C., in ancient Greece, it was observed that rubbing of two substances acquired some special property of attracting light objects like small pieces of dry paper, light feathers, dry grass, etc. The reason was not known at that time.

A glass rod, rubbing with silk cloth, an ebonite rod rubbing with flannel (cat’s skin), or rubbing a plastic comb against dry hair or a balloon against woolen material, acquire the same kind of attractive properties.

The reason was explained by considering that the substances (comb, wool, balloon, glass, silk, etc.) become electrically charged on rubbing, as they have acquired electric charges.

Then the question arises—What is an electric charge?
Like mass, the charge is a fundamental (intrinsic) part of matter.

WBBSE Notes For Class 10 Physical Science And Environment

Charges exist in two forms— ‘positive’ and ‘negative’.

‘Positive’ and ‘negative’ are just names—used to indicate the existence of two types of charges. This charge convention was decided by Benjamin Franklin.

Each substance contains an equal amount of positive and negative charge. According to modern electronic theory, an atom has an equal number of protons and electrons (showing that an atom is electrically neutral). Charge of \(1 p=+1.6 \times 10^{-19} \mathrm{C} \text { and } 1 \mathrm{e}^{-}=-1.6 \times 10^{-19} \mathrm{C}\)

The transfer of electrons is the only cause responsible for the charging of the bodies.

If an atom gains electrons, it becomes negatively charged; and if an atom loses electrons, it becomes positively charged.

While rubbing two substances, the generated thermal energy initiates to transfer of electrons from one substance to another substance.

Wbbse Class 10 Physical Science Notes

This type of electricity is known as static electricity because there is no continuous motion of charges.

According to the law of conservation of charge, the charge can neither be created nor destroyed.

Coulomb’s law: Two like charges (either positive or negative) repel each other and two unlike charges (one positive charge and one negative charge) attract each other with a force known as an electric force.

Scientist Coulomb calculated the magnitude of the electric force acting between two charges. According to Coulomb, this force acts along the line joining two points charges.

Here ‘point charge’ refers to the charge of an object seen from a long distance.

Statement: In a particular medium, the force of attraction or repulsion between two point charges acting along the line joining the charges is directly proportional to the product of the charges and is inversely proportional to the square of the distance between them.

Referring to suppose two point charges and Q₂ are separated by a distance. The force acting between the charges is

⇒ \(F \propto Q_1 \cdot Q_2\) (when r is constant) and

⇒ \(F \propto \frac{1}{r^2}\) (When Q₁,Q₂ are Constant)

Or, F= \(k \cdot \frac{Q_1 Q_2}{r^2}\) (k is a constant of proportionality)

k is not a universal constant. The value of k depends on the nature of the medium. It is different for air, water, oil……..etc.

So the electric force between two point charges Q_1,Q₂ kept in different media would be \(F_{\text {air }}=\frac{k Q_1 Q_2}{r^2}, \quad F_{\text {water }}=\frac{k^{\prime} Q_1 Q_2}{r^2}, \quad F_{\text {oil }}=\frac{k^{\prime \prime} \mathrm{Q}_1 Q_2}{r^2} \ldots \ldots\)

Unit of charge: In the SI system, a unit of charge is the coulomb (C) and in the CGS system it is an electrostatic unit (esu) or statcoulomb (state).

Relation: 1 coulomb = 3 × 10⁹ esu Charge.

WBBSE Chapter 6 Current Electricity Electric Potential Difference

An electric field is said to exist around a charged particle within which it can exert an attractive or repulsive force.

Thus, the electric field is associated with each point in space around itself. If a second charge is brought within this electric field, an electric force acts on the charge.

Wbbse Class 10 Physical Science Notes

If the second charge is similar to that of the former, then work is to be done by an external agency against electric force.

And, if the charges are dissimilar, then electric force (i.e., attractive force) ownself does the work.

Definition: The amount of work done by an external agency in moving a unit positive charge from infinity to a point within an electric field, without any change of its K.E. is called the electric potential at that point.

if W amount of work is done in bringing a charge Q.

from infinity to a point P in the vicinity of a positively charged body, then the potential at P is defined as \(v=\frac{W}{Q}.\) [where potential at infinity is assumed to be zero]

This QV amount of work is stored as the electrostatic potential energy within the charge Q.

Both works are done (W) and charge (Q) are scalar quantities so electric potential (V) is also a scalar quantity.

S.l unit of electric potential is volt or V.

From the relation \(\mathrm{V}=\frac{\mathrm{W}}{\mathrm{Q}}, 1 \mathrm{~V}=\frac{1 \mathrm{~J}}{1 \mathrm{C}}=1 \mathrm{~J} / \mathrm{C}\)

What is meant by the electric Potential of a plant being 1 volt?

1 joule of work is done by an external agency in bringing 1 coulomb of charge from infinity to that point.

Potential difference or Voltage: The potential difference (p.d.) between two points is defined as the amount of work done by an external agency in moving a unit positive charge from one point to the other point, without any charge of its K.E.

If W joule of work is done in moving a test charge Q. from point A to point B, then the p.d. between the points is, \(V_B-V_A=\frac{W}{Q}\)

The S.l. unit of potential difference (p.d.) between any two points is a volt.

From the relation,\(\text { p.d. }=\frac{\text { work done }}{\text { charge moved }}\) we obtain, 1 volt = \(=\frac{1 \text { joule }}{1 \text { coulomb }}\)

Thus, if 1 joule of work is done by an external agency in moving 1 coulomb of charge between two points in an electric field, the p.d. between these two points is 1 volt.

Wbbse Class 10 Physical Science Notes

The bigger units of p.d. are kilovolts (KV) and Megavolts (MV). 1KV = 10³V and 1MV = 10⁶V.

We are familiar with the term P.E. or potential at a height. For the motion of the ball, a slope or p.d. is required. Similarly, for the flow of charge, a p.d. or voltage is needed.

When two charged conductors are either placed in contact or connected by a metallic wire, free electrons flow from the conductor with a higher concentration of electrons (at lower potential) to the conductor with a lower concentration of electrons (at higher potential) till both the conductors have an equal concentration of electrons.

In fact, this gives rise to electric current flow in the opposite direction of electron flow. If there is no p.d. there will be no current.

This is similar to the flow of heat in two bodies, and the flow of water in two vessels Flow of heat is due to the difference in temperature,

The flow of water is due to the difference in the level of water or hydrostatic pressure.

WBBSE Chapter 6 Current Electricity EMF And Electrical Cell As Sources Of EMF

We know that for the flow of electric charges through a conductor, a p.d. across its two ends is always needed.

A force is required to set a body in motion. In a similar manner, we can think of a force associated with the motion of charges through an electrical circuit.

Now the question arises -who is pushing/driving the electric charges? This requires a non-electrostatic agency, (like a cell/collection of cells called battery/mains/generator) which is called a source of emf.

Such a source of EMF simply converts some other form of energy into electrical energy.

A cell (or any other electrostatic agency) does not produce charge but maintains a p.d. across its two terminals.

Simply cell is a source of p.d. The word ‘force’ in emf is not used to mean mechanical force (which is measured in Newton) but emf is a potential or energy per unit charge (measured in volts).

That’s why emf can be defined in terms of work done per unit charge.

Definition: Electromotive force (emf) is defined as the amount of work done in establishing the flow of unit positive charge in a closed circuit.

⇒ \(\text { emf }(\varepsilon)=\frac{\text { Work done }}{\text { charge }}\)

Unit of emf \(=\frac{\text { Unit of work done }}{\text { Unit of charge }}=\frac{\mathrm{J}}{\mathrm{C}}=\mathrm{J} / \mathrm{C}=\mathrm{V}\)

This means that the s.l. unit of emf And P.d. is the same volt.

What does the statement ‘The emf of a cell is 1.5 Volt’ mean?

The emf of the cell = 1.5 volt \(=\frac{1.5 \text { joule }}{1 \text { coulomb }}\)

That is, 1-5 joule work is done in moving a charge of 1 coulomb from the positive electrode to the negative electrode of the cell.

WBBSE Chapter 6 Current Electricity Electric Current

The flow of charges (mainly electrons; because electrons are lighter than protons; also protons are tightly bound with the nucleus) from a body at lower potential (- ve potential) to a body at higher potential (+ ve potential) gives rise to an electric current.

Conventional current flows in the opposite direction of the flow of electrons. In the discussion of current electricity, there is a continuous motion of electrons in the wire.

Electric current is defined as the rate of flow of electric charges through a certain cross-section of conductors.

If Q charge flows through a conductor in time t, then the current is, \(I=\frac{\mathbf{Q}}{\boldsymbol{t}}\)

The S.l. unit of current is coulomb/sec and called ampere (A).

The path along which electric current flows is called an electric circuit.

One ampere is the current that flows through a conductor when one coulomb of charge passes through it in one second.

If n electrons flow through the conductor in time t, then Q = n x e, and the current would be \(1=\frac{n e}{t}\) where e = charge on an electron = \(=-1.6 \times 10^{-19} \mathrm{C}\)

∴1C charge is carried by \(\frac{1}{1.6 \times 10^{-19}}\)  electrons\(=6.25 \times 10^{18}\) electrons.

By the statement “1A current flows through a conductor” we mean that 6.25 × 10¹⁸ electrons flow in 1 second across the conductor.

To express weak current, smaller units of current are used. The smaller units are milli-ampere (mA) and micro-ampere (μA).

They are related to ampere (A) as: \(1 \mathrm{~mA}=10^{-3} \mathrm{~A} \text { and } 1 \mu \mathrm{A}=10^{-6} \mathrm{~A} \text {. }\)

Although electric current flows in a particular direction, it is not a vector quantity-as current and does not obey vector addition rules. It adds up like a scalar quantity.

Wbbse Class 10 Physical Science Solutions

WBBSE Chapter 6 Current Electricity Simple Numerical Problems

Question 1. If 6 coulomb of charge flows through a conductor in 3 seconds, find the strength of the electric current.
Answer: Given: 

Charge (Q) = 6C, time (t) = 3s

Current (I) = \((I)=\frac{\mathrm{Q}}{t}=\frac{6 \mathrm{C}}{3 \mathrm{~s}}=2 \mathrm{~A}\)

Question 2. During how much time, a charge of 20 coulomb flows through a conductor to constitute a current of 4 amperes? 
Answer: Given:

Charge (Q) = 20C, current (I) = 4A

Since Current (I) =\((\mathrm{I})=\frac{\mathrm{Q}}{t} \quad \quad t=\frac{\mathrm{Q}}{\mathrm{I}} \quad \text { or, } \quad t=\frac{20 \mathrm{C}}{4 \mathrm{~A}}=5 \mathrm{~s}\)

Question 3. An electric current of 12 ampere flows through a conductor for 15 seconds. Find the amount of charge that flows.
Answer: Given:

Current (I) = 12A, time (t) = 15s Q

As \(\mathrm{I}=\frac{\mathrm{Q}}{t} \text {, so } \mathrm{Q}=1 \times t \text { or, } \mathrm{Q}=12 \mathrm{~A} \times 15 \mathrm{~s}=1800\)

WBBSE Chapter 6 Current Electricity Ohm’s Law Ohm’s Law Concept Of Resistance

Ohm’s law deals with the relationship between voltage (p.d.) and current in an ideal conductor.

Statement: At a constant temperature, the current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor.

Remember that, Ohm’s law is not a fundamental law like Newton’s laws of motion

It is a statement of how voltage and current flowing in a circuit are related to each other, only when the temperature remains constant.

let a current I flow through a conductor when the p.d. across its ends is V, then according to Ohm’s law, \(I \propto V.\) If the current is doubled or tripled voltage will be doubled or tripled.

Similarly, if, the voltage is halved, the current will also be halved. So the reverse relation is also true. Reversing this relation we can get,

⇒ \(V \propto 1\) Or, V = K.l (K= a Proportionality constant) or, \(\frac{V}{\mathrm{l}}=\text { constant }=k\)

If V remains the same then I can increase only if the value of k decreases; the reverse is also true; so that the constant k quantitatively refers to the factor that opposes current.

It is named the resistance of the conductor and it is represented as R.

Thus the Mathematical expression of Ohm’s Law is V=IR

Resistance from Ohm’s law: According to Ohm’s law: V = IR or, R\(=\frac{v}{\mathrm{l}}.\)

That is, at a constant temperature, the ratio of the p.d. across two ends of a conductor.

The unit of resistance is Ohm (Symbol Ω).

⇒ \(1 \text { ohm }=\frac{1 \text { volt }}{1 \text { ampere }}\)

The resistance of a conductor is said to be 1 ohm if 1-ampere current flows through it when a p.d. of 1 volt is applied across its two ends.

The bigger units of resistance are  Kilo-ohm (KΩ²) and Mega-ohm (MΩ)

1KΩ² = 10³ Ω² and 1MΩ² = 10⁶ Ω²

Graphical representation of Ohm’s law: If a graph is plotted taking p.d. V along the x-axis and current I along the y-axis, we will get  \(I \propto V \Rightarrow I=\frac{V}{R}\)

The relation is similar to the equation y = mx. So, the l-V graph is a straight line (OA) passing through the origin and its slope is \(=\frac{y-\text { value }}{x-\text { value }}=\frac{1}{R} \text { : }\)

Ohmic conductors: The conductors that obey Ohm’s law are called ohmic conductors, for which the l-V graph is a straight line passing through the origin and the slope of the graph is the same for all values of V and I, that is R is the same, at a given temperature.

Examples: metallic wires.

Non-ohmic conductors: The conductors that do not obey Ohm’s law are called non-ohmic conductors, for which the l-V graph is not a straight line, but a curve, and the ratio V/l is not the same for all values of V and I, that is, R is variable.

Examples: Semiconductors.

Do all conductors follow Ohm’s law? the p.d. vs current graph for two different conductors A and B. Can you say which conductor has greater resistance?

According to Ohm’s law:  \(V=I R \Rightarrow R=\frac{V}{i}\)

Here, (Slope of A) > (Slope of B) because slope =\(\frac{y-\text { value }}{x-\text { value }}\)

So, A has greater resistance.

WBBSE Chapter 6 Current Electricity Simple Numerical Problems

Question 1. Find the resistance of a wire if 10mA current flows through it and the p.d. across its ends is 2V.
Answer: Given:

Wbbse Class 10 Physical Science Solutions

Current (1) = 10mA = 10×10^-3A, P.d (V) =2V

∴ The Value  Of resistance, R = V/1 = \(=\frac{2 \mathrm{~V}}{10 \times 10^{-3} \mathrm{~A}}=200 \mathrm{ohm}\)

Question 2. Two wires have the same terminal p.d. If the ratio of current flowing through them is 1 : 3, calculate the ratio of their Resistances.
Answer:  By Ohm’s Law: \(I=\frac{V}{R}\)

For the first wire:  \(\mathrm{I}_1=\frac{V}{R_1}\) and for the time second wire: \(\mathrm{I}_2=\frac{V}{R_2}\)

Given:  \(\frac{\mathrm{I}_1}{\mathrm{I}_2}=\frac{1}{3} \quad \frac{\mathrm{I}_1}{\mathrm{I}_2}=\frac{\mathrm{V}}{\mathrm{R}_1} \times \frac{\mathrm{R}_2}{\mathrm{~V}}=\frac{\mathrm{R}_2}{\mathrm{R}_1}\)

⇒ \(\frac{1}{3}=\frac{R_2}{R_1} \Rightarrow \frac{R_1}{R_2}=\frac{3}{1}\)

⇒ \(\text { i.e. } R_1: R_2=3: 1\)

Question 3. 20V of p.d. across a conductor maintains a current of 0-2A. How much p.d. will be required to maintain a current of 250 mA in the same conductor?
Answer: Given: 

V=20V, I=0.2A, R=?

By ohm’s law: \(\mathrm{R}=\frac{\mathrm{V}}{\mathrm{I}}=\frac{20 \mathrm{~V}}{0 \cdot 2 \mathrm{~A}}=100 \Omega\)

if I= 250 mA =250×10^-3A, R=100Ω, then V=?

∴ The required p.d Across the conductor is v=1R=(250×10¯³ A)×(100Ω)=25V

WBBSE Chapter 6 Current Electricity EMF and Internal Resistance of a Cell

EMF of a cell is the potential difference between two terminals of the cell when no current flows i.e. the cell is in an open circuit.

At the time of buying a dry cell from the market, we get the emf value of the cell. But when the cell is connected to an external circuit, it gives a potential difference of less than the EMF value.

Why? Due to internal resistance of the cell (the obstruction offered to current due to electrolytes which act as non-electrical agency).

Suppose a cell of emf 8 is connected to an external resistance R and a current I is drawn from the cell.

Under such conditions, the electrolyte inside the cell offers a resistance to the flow of current, known as the internal resistance of the cell denoted as r.

The connecting wires are considered to have no resistance, so in the circuit, there are two resistances external resistance (R) and internal resistance of the cell (r).

∴ Total Resistance of the circuit  =R+r

∴ Current Drawn From the cell \(I=\frac{\text { e.m.f. of the cell }}{\text { Total resistance }}=\frac{\varepsilon}{R+r}\) Or, ε=V+Ir Or, V=ε-Ir Or, ε=I (R+r) IR+Ir

Here the term ‘IR’ is called terminal voltage (V) or potential difference in the external circuit.

So, the terminal voltage is the potential difference across two terminals of a cell when current is drawn from the cell.

The term ‘Ir’ is the voltage drop inside the cell, called the ‘internal potential drop’ or ‘lost volt’. Because this Ir portion of £ is lost due to the obstruction offered by r.

So we can write: emf of a cell = terminal voltage of the cell + lost volt.

In an open circuit (when no current flows in the circuit): I = 0; Ir = 0; ε = V. That is, the emf of a cell in an open circuit = terminal voltage of the cell.

If R = 0 then V = 0  ε= Ir.

That is, m.f. is just the work done inside a cell by the non-electrical agency, in moving a unit positive charge from the negative terminal to the positive terminal of the cell when there is no external circuit.

WBBSE Chapter 6 Current Electricity Resistivity And Conductivity

We have learned about the resistance of a conductor. Here we will discuss other characteristic properties of conductors.

The factors affecting the resistance of a conductor are—

1. nature of the material,
2. length,
3. area of cross-section and
4. temperature.

Dependence on length and area of cross-section I (Temperature and nature of material remain same): Suppose we have a conductor of length l and cross-sectional area A.

Its original resistance is R.  1. If l is made to keep the same A, then resistance will also increase.

Thus, resistance is direct R\(\propto\)I (when A remains unchanged).

This means the more the length, the more the resistance, 2. If A is made to increase while keeping the same /, then resistance will decrease. Thus resistance is inversely proportional to area.

Mathematically, \(R \propto \frac{1}{A}\) (when I remain unchanged) is proportional to length. Mathematically, R remains unchanged).

This means, the larger the area, the smaller the resistance. Combining these two or specific we get, \(R \propto \frac{1}{A} \Rightarrow R=\rho \cdot \frac{1}{A} \Rightarrow \rho=\frac{R A}{I}\) resistance of the material of the wire.

In the reaction R=ρI/A, If I=1 and A=1 Then R=ρ. Thus, resistivity is the resistance of a conductor of unit length and unit area of the cross-section at a constant temperature.

Both resistance and resistivity are resistances. Then, what is their difference? Resistance will change with the change of / and A.

But when / and A have unit values, then the resistance is resistivity. If we change the material, we find different p.

So, resistivity is a characteristic property of a material. Resistivity depends on the nature of the material and the temperature of the conductor but not upon the dimensions (/, A).

S. I Unit of Resistivity: From the relation R= ρ I/A, We have ρ=RA/I

∴ Unit of ρ =  \(\frac{\text { Unit of } R \times \text { Unit of } A}{\text { Unit of } I}=\frac{\text { ohm } \times \mathrm{m}^2}{\mathrm{~m}}\)

The Statement ‘Resistivity of copper = 1.62×10¯⁶  ohm. cm’ means that a Conductor made of copper having a length, equal to

The statement ‘resistivity of copper = 1.62 x 10^-6 ohm. cm’ means that a conductor made of copper having a length, equal to 1 cm and an area of 1 cm² has a resistance of 1.62 x 10^-6 ohm between its two opposite faces at a particular temperature.

If a wire is stretched to double its length, what will be the change in R and p ? Since the material of the wire remains unchanged, so p will not change. But as R\(\propto\) so R will be doubled.

What would happen to the resistance of a wire if its radius or diameter is 1. doubled and 2. halved?

If r = radius and d = diameter of a write then its area of cross-section is \(A=\pi r^2=\pi\left(\frac{d}{2}\right)^2=\frac{\pi d^2}{4}\).

Resistance of wire \(R=\frac{\rho /}{\pi r^2}=\frac{4 \rho /}{\pi d^2}\)

That is, \(\mathrm{R} \propto 1 / r^2 \text { and } \mathrm{R} \propto 1 / d^2\) (when ρ  and I are fixed)

So, if the radius or diameter of a wire is doubled, its Resistance lessens to 1/4th of its initial value.

If the radius, or diameter of a wire is doubled, its resistance increases to 4 times its initial value.

If a wire is stretched to 3 times its length. Will there be any change in resistivity and resistance? Here resistivity (p) will not change, since the material of the wire remains unchanged.

Here, resistivity (ρ) will not change, since the material of the wire remains unchanged. here,

⇒ \(R_1=\frac{\rho l}{A} \text { and } R_2=\frac{\rho 3 l}{A / 3}=9 \cdot \frac{\rho l}{A}=9 \cdot R_1 \Rightarrow \frac{R_2}{R_1}=9 \Rightarrow R_2=9 R_1 \text {. }\)

Hence, resistance will be 9 times its initial value.

The resistance of a metallic wire increases with an increase in temperature. This is so why a “fixed temperature” is mentioned in defining resistivity.

Electrical conductivity: Just like resistance in a conductor there is another factor known as conductance (G).

Reciprocal resistance is called \(\left(\frac{1}{R}\right)\)conductivitance. The \(\sigma=\frac{1}{\rho}=\frac{1}{\mathrm{RA}}\)reciprocal of resistivity is called the electrical conductivity.

It is represented as a property of the conductor whereas ρ is a property of the non-conductor/ insulator. A good Conductor should have a high value of σ and a low value of ρ.

Units of electrical conductivity: In S.l. the system, ohm^-1.m^-1 or mho.m^-1 or S.m^-1, and CGS system, ohm^-1.cm^-1 or mho.cm^-1.

Conductors and Insulators: Substances with low resistivity and high electrical conductivity are good conductors. They allow an electric current to flow easily.

Most metals (Ag, Cu, Al ….) are good conductors. On the other hand, insulators have very high resistivity and their electrical conductivity is very low (almost nil).

Examples— are wood, rubber, plastic, etc. Insulators are mainly used to protect us from electrical shock because of their poor electrical conductivity.

Resistivity is a more fundamental property as compared to resistance. Because the resistance of wire changes with the change of its length and area of cross-section; the resistivity remains unchanged for such changes.

Resistivity depends on oh temperature,

For metals, resistivity increases with the increase in temperature, and For semiconductors, it decreases with the increase in temperature.

For alloys, it remains nearly unaffected by the change in temperature. For example, the resistivity of constant (Cu + Ni), and manganin (Cu + Mn + Ni) remains almost constant with the rise in temperature.

Represents the resistivity of some substances. The values of resistivity help us to recognize conductors, semiconductors, and insulators separately.

Resistivity depends on the material of the substance, For metals, it is very low (-10^-8Ω.m), for semiconductors, it is low (-10^-5Ω.m) and for insulators, it is very high (-10^-13Ω.m).

For electrical connections and power transmission, the wires used should possess negligible resistance and very small resistivity.

Copper or aluminum possesses such qualities. This is the fact why the wires are generally made of copper or aluminum.

Generally, the standard resistors are made from alloys, such as manganin, constant, nichrome, etc. These are so chosen as their resistivity remains practically constant with the change in temperature.

The filament of a bulb is made by using tungsten. Because it has-

1. High melting point and
2. High resistivity.

The resistance of the tungsten filament of a bulb is more when it is glowing as compared to when it is not glowing.

This is so because in the glowing condition, the filament is at a high temperature, and for it, the resistance increases.

The resistivity of an alloy is more than that of its constituent metal. For example, the resistivity of the constant (Cu + Ni) is nearly 30 times more than that of Cu.

The heating element of the heater is made by using a nichrome (Ni + Cr + Fe) wire because of its high resistivity.

A fuse wire is made from an alloy of lead and tin in a ratio: of 1. Because it has a low melting point and high resistivity.

Superconductor: Generally, the resistivity of metals decreases with the decrease in temperature.

There is a category of substance whose resistivity becomes almost zero at a lower temperature less than a particular value, called critical temperature and this type of substance is called superconductor.

For example, mercury below 4-2K behaves as a superconductor. Superconductors have infinite conductivity.

Resistivity-temperature graph: the resistivity-temperature graph.

The utility of the graph is to identify the metallic conductors, semiconductors, and superconductors on the basis of the variations of their resistivity with different temperatures.

WBBSE Chapter 6 Current Electricity Series And Parallel Combination Of Resistances

Sometimes two or more resistances are connected together to make a combination of resistances for different purposes in electric circuits.

There are two ways of a combination of resistances—

1. series combination and
2. parallel combination.

Series combination:

Two resistances R₁ and R₂ are connected in series (end-to-end). In this combination, let us look at the p.d. across each resistance and the current flowing through them.

Since the resistances are connected end-to-end, the same current (suppose I, the main current) flows through each resistance.

If V is the supplied p.d. of the battery then V is divided into two parts V₁ and V₂ across R₁ and R₂ respectively V= V₁ + V₂.

According to Ohm’s law: V1 = \(l_1=\frac{V}{R_1}\) and V₂ = \(l_2=\frac{V}{R_2}.\)

If R_s be the equivalent resistance of this series circuit, then V = \(l_s=\frac{V}{R_s}\)

Since l = l₁ + l₂

∴ \(\frac{V}{R_p}=\frac{V}{R_1}+\frac{V}{R_2} \Rightarrow \frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}\)

If three resistances R₁,R₂ And R₂ are connected in series then \(\frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}\)

In a series combination, the equivalent resistance is more than the highest value of the resistance connected.

But in a parallel combination, the equivalent resistance is less than even the smallest resistance connected.

That is R_s >R₁ R₂, R₃, … and R_p <R₁ R₂,R₃,…….

In series combination, p.d. gets divided but the same current flows through each resistance. In

parallel combination, p.d. remains the same across the resistances but the current gets divided.

When the resistance of the combination is to be increased, the series combination is preferred, and when the resistance is to be reduced,

i.e., if more current is to pass through the circuit, the parallel combination is preferred.

WBBSE Chapter 6 Current Electricity Simple Numerical Problems

Question 1. Two resistances 3Ω and 6Ω are connected in (1) series and (2) parallel with a supply voltage of 12V. Draw the circuit diagrams. Then find a current and potential drops in all resistances.
Answer: (1) Series Combination 

Here Equivalent resistance,

R_e = R₁ + R₂= 3+6= 9Ω

Current Flows Through each resistance,

The potential drop in 3Ω,

The potential drop in 6Ω,

Wbbse Class 10 Physical Science Solutions

(2) Parallel Combination 

⇒ \(\frac{1}{R_e}=\frac{1}{R_1}+\frac{1}{R_2}=\frac{1}{3}+\frac{1}{6}=\frac{2+1}{6}=\frac{3}{6}=\frac{1}{2}\)

R_e=2Ω

Main Current, \(\mathrm{I}=\frac{\mathrm{V}}{\mathrm{R}_e}=\frac{12}{2}=6 \mathrm{~A}\)

⇒ \(l_1=\frac{V}{R_1}=\frac{12}{3}=4 \mathrm{~A} \text { and } I_2=\frac{V}{R_2}=\frac{12}{6}=2 \mathrm{~A}\)

Potential Drop across 3Ω and 6Ω resistance is the same = 12 V.

Wbbse Class 10 Physical Science Solutions

Question 2. What will be the equivalent resistance of the combination of resistors 1Ω, 10Ω, and 100Ω (1) in series combination and (2) in parallel combination?

(1) The equivalent resistance in a series combination

R_s = 1Ω+10Ω+100Ω=111Ω

(2) The equivalent resistance in parallel combination

⇒ \(\frac{1}{\mathrm{R}_p}=\frac{1}{1}+\frac{1}{10}+\frac{1}{100}=\frac{100+10+1}{100}=\frac{111}{100}=0 \cdot 9 \Omega\)

Activity: We make two circuits Two identical bulbs are connected (1) in series (end-to-end) and (2) in parallel combination (between two common points).

We are to compare how the brightness of each bulb varies in the two combinations. Here the bulbs act as resistors since the resistance of the connecting wire is neglected.

In a series combination, the same current flows through each bulb, while the equivalent resistance becomes more than either of the two resistances.

So, the electric current becomes less, and as a result of which, bulbs glow dimly, and the brightness of each bulb diminishes.

In parallel combination, each of the two bulbs is connected across the two ends of a cell. The equivalent resistance of this combination becomes lower than either of the three resistances.

So, the electric current becomes more. The current in each bulb remains unaffected by the presence of another bulb.

So, the bulbs glow as bright as they were connected individually to the supply voltage.

Due to this fact, in your house bulbs/fans/TV/computer, etc. all are connected in parallel combination with the supply voltage.

WBBSE Chapter 6 Current Electricity Heating Effect Of Electric Current Joule’s Law of Heating Effect Of Current Concept Of Electrical Energy

When an electric current flows through a conductor in any direction, it gets heated up and this fact is known as the heating effect of current.

Why this heating effect is happening?  Whenever a conductor is connected to a voltage source, the electrons present in the conductor start moving towards the higher potential (+ ve terminal of the battery).

During this movement, electrons collide with atoms, ions, etc. present in the conductor.

As a result of which, electrons have to overcome the resistance of the conductor, and a loss of energy of electrons takes place.

This energy gets converted into heat energy for which the heating of the conductor takes place.

The heating effect of current plays an important role in home appliances like electric kettles, electric stoves, electric irons, electric cookers, etc.

Whenever you keep your mobile connected to a charger for a long time or when you watch TV for a long time while touching your mobile/TV you feel hot; even a fan or every electronic gadget gets hot.

Mathematics: Suppose a p.d. V is applied across a conductor. Here, the cell is not producing charge it only gives energy to electric charge.

Thus, the cell does some electrical work.

⇒ \(\mathrm{V}-0=\frac{\mathrm{W}_{\text {ext.agency }}}{\mathrm{Q}}=\frac{\mathrm{W}_{\text {done by cell }}}{\mathrm{Q}} \Rightarrow \mathrm{W}_{\text {done by cell }}=\mathrm{E}_{\text {supplied by cell }}=\mathrm{VQ} \ldots \ldots . \text { (i) }\)

Suppose the cell allows I current to flow through a resistance R  of the Conductor for time t. Then the amount of charge flown in time t will be \(1=\frac{Q}{t} \quad Q=I t\)

Using the value of ‘Q’ From (2) Into (1):  W=V×I×t……….(3)

According to Ohm’s Law: V= IR. So that W= IR×I×t⇒ W= I²Rt…………(4)

Again using Ohm’s Law: \(I=\frac{V}{R}.\) Then \(\mathrm{W}=\frac{\mathrm{V}^2}{\mathrm{R}^2} \times \mathrm{R} \times t \Rightarrow \mathrm{W}=\frac{\mathrm{V}^2}{\mathrm{R}} t\)…………………(5)

This work appears as the heat energy in the conductor.

From the work-heat equivalence of heat\(W=J H \Rightarrow H=\frac{W}{J}\) (as stated in class IX), relations (3), (4).and (5) can be expressed in terms of heat produced.

We know that J = 1 in the SI system and J = 4-2 joule/cal in the CGS system. So the relations for heat produced in a conductor are

H= \(\mathrm{VIt}(\mathrm{SI}) \text { and } \mathrm{H}=\frac{\mathrm{VIt}}{4 \cdot 2} \mathrm{cal}(\mathrm{CGS})\)

H = \(\mathrm{I}^2 \mathrm{R} t(\mathrm{SI}) \text { and } \cdot \mathrm{H}=\frac{\mathrm{I}^2 \mathrm{R} t}{4 \cdot 2} \mathrm{cal}(\mathrm{CGS})\)

H= \(\frac{\mathrm{V}^2}{\mathrm{R}} t(\mathrm{SI}) \text { and } \mathrm{H}=\frac{1}{4 \cdot 2} \cdot \frac{\mathrm{V}^2}{\mathrm{R}} t \mathrm{cal} \text { (CGS) }\)

These relations are known as Joule’s law of the heating effect of the current

According to this equation:

⇒ \(H \propto R\)(R, t constant),

⇒ \(H \propto R\)(I, t constant) and

⇒ \(H \propto t\)(I, R constant)

Joule’s laws of heating effect of current: There are three laws.

First Law: The amount of heat H produced in a conductor is directly proportional to the square of the current (I) flowing through it when resistance and time of flow of current remain the same, i.e. \(\mathbf{H} \propto \mathbf{I}^2\) (When R and t remain constant)

Second Law: The amount of heat H produced in the conductor is directly proportional to the resistance R of the conductor when the current and time of flow of current remain the same, i.e.,  \(H \propto R\)(When I and t remain constant),

Third Law: The amount of heat H produced in the conductor is directly proportional to the time of flow t of current when current and resistance remain the same, i.e., \(H \propto R\) (When I and R remain constant)

WBBSE Chapter 6 Current Electricity Domestic Uses Of Heating Effect Of Current

The common household electrical appliances which make use of the heating effect of current are electric heaters, irons, bulbs, geysers, toasters, ovens, immersion heaters, etc.

Electric heater: The heating coil in an electric heater is made of nichrome (an alloy of 60% Nickel, 25% iron, and 15% chromium) wire (it is also called a heating element).

1. Because nichrome
2. Has a high resistivity,
3. A high melting point, and

Does not get oxidized up to a temperature of about 1000°C. The nichrome wire is wound in the form of a helical coil and placed in some insulating material (it protects the user from electric shock).

When an electric current passes through the coil, it becomes very hot.

Electric iron: Electric iron is used for pressing clothes. The heating element in iron is a flat nichrome coil wound on a mica sheet.

The coil is made flat so that the heat spreads over a large surface area. When an electric current passes through the heating element, it produces heat energy.

Electric bulb An incandescent electric bulb consists of a very fine coil of tungsten. Tungsten has a Very high resistivity,

Wbbse Class 10 Physical Science Notes

The very high melting point of about 3300°C. The bulb is filled with argon gas at very low pressure and completely evacuated.

When an electric current is passed through this, the filament gets heated up and it emits heat energy and light energy. This is due to the heating effect of the current.

Fuse or cut-out: The fuse is a safety device in an electric circuit. It protects electric circuits and electrical appliances by stopping the flow of electric current.

Wbbse Class 10 Physical Science Notes

The material of the fuse wire is made from an alloy of lead and tin in a ratio 3: of 1. It has

1. A low melting point and
2. A high value of resistance.

When the circuit becomes overloaded due to short-circuiting or fluctuation of current /the fuse melts and breaks the circuit and thus the electrical appliances are saved.

When the live wire and the neutral wire come in direct contact, it is called a short circuit.

Wbbse Class 10 Physical Science Notes

The fuse is always connected to the live wire in an electric circuit, at the point where current enters the circuit.

If the fuse is connected to the neutral wire, it will melt when there is overloading. Under such conditions, if an electrical appliance is touched, even in the OFF position, the person will get a shock as the appliance is connected to the live wire.

Wbbse Class 10 Physical Science Notes

To protect electric appliances like T. V. sets, Refrigerators, geysers, Iron, mixers, etc. cartridge fuses are used. These are fixed within the appliance.

Such fuses have a maximum tolerance current.

To protect a circuit, the fuse should be of a lower value of current than the maximum current that the circuit can withstand. Fuse ratings (e.g. 5A, 10A, 15A, ) ensure it.

For example, a 5A fuse can withstand a maximum of 5A current. If a current of more than 5A flows, the fuse melts.

These days, the use of MCB or Miniature Circuit Breaker Mechanical Circuit Breaker is very popular rather than traditional fuses.

It is used to protect each individual circuit; If the circuit is overloaded, the MCB falls down to switch off the circuit without causing damage.

Wbbse Class 10 Physical Science Notes

WBBSE Chapter 6 Current Electricity Electrical Power

In Physics, power is defined as the rate of doing work. So, Electrical power is defined as the rate of doing electrical work or the amount of electrical work done in 1 second. Mathematically

Wbbse Class 10 Physical Science Notes

P\(\frac{\text { Electric work done or energy supplied by the cell }}{\text { time }}=\frac{W}{t} \text { or, } \frac{E}{t}\)

But, \(\text { But, } W=V I t \quad P=\frac{V I t}{t}=V I \quad P=\frac{I^2 R t}{t}=I^2 R\)

P=\(\frac{V^2}{R^2} \times R=\frac{V^2}{R} \text { for fixed } V \text {. }\)

Any relation can be used to calculate electrical power.

Remember: Electrical power is supplied by a cell or any other voltage source and power is always dissipated in a resistor, across which the source is connected.

The S.I. unit of electrical power is the watt (or W). Bigger units of power are kilowatts (KW), and megawatt MW where 1 kW = 10³ W and 1 MW = 10⁶W.

Electrical power is said to be 1 watt when 1 joule of electrical work is done in 1 second, Or, the cell supplies 1 joule of energy in 1 second to the electric charge.

Units of electrical energy: An electrical bill is prepared on the basis of the amount of electrical energy (E) consumed in our homes/schools/industries.

The basic formula is E = P x t i.e. electrical energy consumed = electrical power x time. The commercial unit of electrical energy is watt-hour (W-h) and kilowatt-hour (kW-h).

1 watt-hour = 1 watt x 1 hour = 1 watt x 3600s = 3600 joule.

1 kilowatt-hour = 1 kilowatt x l hour = 100 Js^-1 x 3600s = 3-6 x 10⁶ = 3.6 MJ.

B. O. T. (or Board of Trade) unit: The bigger unit of electric energy is a kilowatt-hour. It is also known as B.O.T.

(Board of Trade Unit), which is the electric energy spent by an electric appliance of power 1 kilowatt used for 1 hour.

⇒ \(1 \text { B.O.T. unit }=1 \mathrm{kWh}=\frac{\text { watt } \times \mathrm{h}}{1000}=\frac{\text { volt } \times \text { ampere } \times \mathrm{h}}{1000}\)

WBBSE Chapter 6 Current Electricity Simple Numerical Problems

Question 1. A battery of 12V supplies a 2A current. Calculate its power.
Answer: Given: V = 12V, I = 2A So, power supplied by the battery P = V x l = 12 x 2 = 24 watt = 24 J/s.

Question 2. A torch bulb of 4-5V draws a current of 0-3A. If the bulb is switched on for 10 minutes, find out the energy released by the bulb.
Answer: Given: V – 4.5V, I = 0.3A, t = 10 min = 600s

The energy released by the bulb, E = Vlt = 4-5 x 0-3 x 600 = 810J

Question 3. An electric bulb of resistance of 500Q draws a current of 0.4A. Calculate its power.
Answer: Given: R = 5000, I = 0.4A, P = ?

Electric power P = l²R = (0-4)²x 500 = 80 watt

Wbbse Class 10 Physical Science Solutions

Question 4. A family uses one 100W bulb, one 100W fan, and one 1000W heater for 8h daily. Calculate the daily household electric bill, if one unit costs Rs. 3-00.?
Answer: The electrical energy consumed per day is

⇒ \(=\frac{\text { watt } \times h}{1000}=\frac{(1 \times 100+1 \times 100+1 \times 1000) \times 8}{1000}=9.6\)

Daily electric bill costs = Rs. 3.00 x 9.6 = Rs. 28.80.

Question 5. Two electric bulbs of 100W and three electric fans of 60W are used daily for 5 hours. What will be the cost for it in one month (30 days)? The cost of each unit is Rs. 3.50.
Answer:  The electrical energy consumed per day is

⇒  \(=\frac{\text { watt } \times h}{1000}=\frac{2 \times 100 \times 5+3 \times 60 \times 5}{1000}=1.9\)

The total electrical energy consumed in 30 days =1.9×30=57

∴ The cost of it is = Rs. 3.50 × 57 = Rs.199.50

If The Bulb is Connected To a p.d. less than 220V (say 110V), the build will consume less power and will glow dimly.

Then power Consumed will be, \(P=\frac{V^2}{R}=\frac{(110)^2}{484}=25 \mathrm{~W}\)

Rating And Its Significance: 

• In general, electrical appliances such as electric bulbs, irons, heaters, geysers, washing machines, etc. are rated by their power and voltage.
• This is known as power-voltage rating or simply power rating. This power rating is done to calculate
• The resistance of the appliance and
• The safe limit of current that can pass through it.

For example, an electric bulb rated as 100W, 220V means that when the bulb is connected to a 220V main line, it glows fully and consumes 100W power or 100J of electrical energy in 1 second.

While glowing the resistance of the filament of the bulb is, \(R=\frac{V^2}{P}=\frac{(220)^2}{100}=484 \Omega\)

The safe limit of current that flows in the Bulb is \(I=\frac{P}{V}=\frac{100}{220}=0.45 \text { (approx) }\)

• Three different types of light bulbs are available in the market. These are Incandescent bulbs, CFL (Compact fluorescent lamps), and LED (Light Emitting Diode).
• CFL and LED are more efficient than incandescent bulbs from the point of the energy economy. Incandescent bulbs contain tungsten filaments.
• When an electric current is passed the filament gets heated up and emits light. More than 95% of the energy gets converted into heat energy.
• Only less than 5% of the energy is converted into light energy. That’s why an incandescent bulb becomes hot when switched on.

A CFL contains argon and a small amount of mercury vapor when a current is passed, this generates UV light and excites the fluorescent coating inside the bulb which produces visible light.

• On the other hand, LEDs are semiconductors.
• When current (i.e. electrons) is passed, they emit light. Out of these three, LED is better than others.
• CFL is not eco-friendly like LED, because CFL contains toxic mercury vapors.

Significance of the energy rating mark given in household electrical appliances: The Bureau of Energy Efficiency (BEE) under Govt, of India, suggests marking the energy star level of household electrical appliances, such that, 5 stars (*) appliances save maximum energy while 1-star (*) appliances save the least energy.

From the point of the energy economy, a buyer should use 5-star appliances to save on electricity bills.

Activity:

(1)Value development against misuse of electrical energy: We are all aware of today’s energy crisis, lack of fossil fuels, load shedding scenario, along needless misuse of electricity at homes, schools, government offices, etc.

The best thing to do for us is to minimize wastage. It requires common people’s awareness. Government-level initiatives are also required.

These days more efficient CFL and LED bulbs are available. Although these are expensive, they are 4-6 times more efficient than filament-based bulbs, They are useful against misuse of electrical energy.

An incandescent bulb of power 100 W has the same brightness as a CFL bulb of power 24 W and an LED bulb of 16 W.

(2) Suppose two bulbs rated as SOW and 100W are connected in series and in parallel with the same voltage line. In the two cases which bulb glows more brightly than the other?

1. In a series of connections between the bulbs, the same current flows through each bulb. That is,

I= Constant. As\(P=V^2 / R, \text { so } P \propto \frac{1}{R}\) for the same V i.e. resistance of 60 W bulb is more than the 100W bulb.

As \(P=1^2 R \text {, so } P \propto R \text {. }\) Hence, the 60W bulb glows more brightly.

2. In parallel connection of the bulbs, the current in each of them does not remain the same (although the p.d. V remains constant).

Remember: Resistance of a Particular build remains the same as \(R=\rho \frac{1}{A}. \text { As } P=\frac{V^2}{R}\) \(P \propto \frac{1}{R} .\) Thus, for the same v, the 100W bulb glows more brightly.

WBBSE Chapter 6 Current Electricity Electromagnetism Action Of Electric Current On Magnet

Magnetism has been known since ancient times and electricity has been discovered in the 1700s. Until 1820, electricity and magnetism were studied as two separate branches.

Electricity was considered a phenomenon related to electric current, while magnetism was considered to be related to magnets.

In 1820, famous physicist Hans Christian Oersted first experimentally proved that a current-carrying wire behaves like a magnet, or a current-carrying wire produces a magnetic field around itself as long as the current is passed through it.

This is known as the magnetic effect of electric current. Oerested proved that electricity and magnetism are related to one another.

Today we have a new branch of study known as electromagnetism.

Activity: Oersted’s experiment: Materials required: A magnetic compass (tiny magnet), Cu wire, battery with crocodile clips, and a few carrom coins.

Procedure: Fix carrom coins on a table with the help of glue tape over which stick the Cu wire. Place the magnetic compass below the Cu wire.

At this position, the magnetic compass needle points in a geographical north-south direction.

Then connect the H-Ve and -Ve terminals of the battery with two ends of Cu wire and allow current to flow.

Observations: The compass needle at once gets deflected to one side deflection towards the East). A magnet can only influence another magnet.

So we can say that a current-carrying wire exerts a force on a magnet.

If the amount of current is increased, then the magnetic compass shows a larger deflection.
So, the magnetic field produced in the wire is directly proportional to the electric current.

Current is passed through the Cu wire and the compass is taken slightly away from the Cu wire. The deflection of the magnetic needle decreases the Greater the distance weaker the magnetic field.

Place the compass above the wire. The magnetic needle deflects in the opposite direction (deflection towards the west).

As if the direction of the electric current is reversed, the direction of deflection of the magnetic needle gets reversed.

So we can say that the direction of the magnetic field can be reversed by reversing the direction of the current flow.

Rules for determining the direction of deflection of magnetic needle: The direction of deflection of the magnetic needle due to current can be determined by any one of the following two rules:

1. Ampere’s swimming rule: If a swimmer stretching his arm is swimming along a current carrying wire in the direction of the current, facing the magnetic needle, then the direction in which the left hand of the swimmer points gives the direction of deflection of the north pole of the magnetic needle

Explanation on the basis of observation of Oerested’s experiment: Suppose the wire is placed over the compass through which current flowing from S→N due to which [following SNOW Rule (S: South, N: North, O: Over and W: West – Where anyone change of S/N/ O/W, deflection of N-pole will be changed to either E or W and for two changes deflection will be a pole of the magnetic compass will be deflected towards West.

2. Wire is over the compass and current flowing from N→ S for which [following NSOE Rule] N-pole of the magnetic compass will be deflected towards East,

3. Wire is placed below the compass and current flows from S to N for which N-pole will be deflected towards East [following SNBE Ruie.]

4. (following NSBW Rule) N-pole will be deflected towards the West. (As a whole this rule is very complex).

Right-hand grasp rule: If we hold a current-carrying conductor in the right hand such that the thumb points in the direction of flow of current, then the direction in which other fingers curl/wrap gives the direction of the magnetic field.

Explanation: According to the right-hand grasp rule, keeping the thumb towards the direction of current, the direction along which other fingers curl for gripping the wire, produces a circular path.

For which the current is flowing in an upward direction and for which the current is flowing in a downward direction.

This circular path gives the direction of the magnetic field produced due to current this is the direction along which a north pole moves. [This rule is easier).

Magnetic field around a long straight wire: A straight wire passes vertically through a hole made at the center of the cardboard. Some iron filings are sprinkled on the cardboard.

A current is passed through the wire in an upward direction and the cardboard is gently tapped. The iron filings get arranged along some concentric circles around the wire.

The arrows show the direction of the magnetic field. It is observed that a straight current-carrying wire produces a magnetic field that looks like concentric circles around the wire.

Here, the magnetic field strength is very weak.

Magnetic field around a circular wire: A straight wire is bent into a circular loop. The loop passes through two points on cardboard.

Some iron filings are sprinkled on the cardboard. A current is passed through the loop and the cardboard is gently tapped.

Shows that the iron filings get arranged along two sets of concentric circles for each end of the wire.

The arrows show the direction of the magnetic field. The magnetic field of a circular loop is stronger than a straight wire.

How? Imagine the loop is divided into small segments which are nearly straight wires. Each straight segment produces a magnetic field in the shape of concentric circles.

At the center of the loop, the magnetic field lines are almost straight and they add up which the magnetic field becomes stronger.

If instead of one loop, there are N loops then the magnetic field will be N times stronger.

The similarity between a field due to a magnetic pole and a field due to a circular current-carrying loop:

In a circular current-carrying loop, the magnetic field lines at the center, are along the axis of the loop and normal to the plane of the loop.

If the loop is replaced by a thin magnet, similar types of magnetic field lines will be obtained. That is, the behavior of a current-carrying loop is similar to that of a magnetic pole.

Polarities in a current-carrying loop: If you look at the face of the loop and assume that the current flows in an anticlockwise direction then the face of the loop acts as a north pole and for the flow of current in a clockwise direction, the loop acts as a south pole.

WBBSE Chapter 6 Current Electricity Action Of Magnet On Current

We learned that a current-carrying wire exerts a force on a magnet. Is the reverse also true? Can a magnet exert a force on a current-carrying wire?

In 1821 Michael Faraday experimentally observed that whenever a current-carrying wire is placed in a magnetic field the current-carrying wire experiences a mechanical force acting on it which is known as a magnetic force.

If there is no current in the wire, there is no force. If the wire is free to move, it will produce motion wire.

This is known as the action of a magnet on current. The direction of force or motion of the conductor can be found by Fleming’s left-hand rule.

Fleming’s left-hand rule: Stretch the first three fingers of the left hand mutually perpendicular to each other such that if the forefinger indicates the direction of the magnetic field and the middle finger indicates the direction of current, then the thumb will indicate the direction of force or motion of the conductor.

Principle of working of Barlow’s wheel: Whenever an electric conductor is brought in a magnetic field, the conductor gets motion due to the presence of the magnetic field. One such example is Barlow’s wheel.

It is a suitable instrument that shows how motion is produced in a current-carrying conductor placed in a magnetic field. In it, electrical energy is converted into mechanical energy.

The direction of rotation of the wheel can be determined by applying Fleming’s left-hand rule.
Barlow’s wheel consists of a star-shaped copper wheel, capable of rotating freely in a vertical plane about a horizontal axis.

A pool of mercury is kept in a small groove on the wooden base of the apparatus such that the point of each spoke of the wheel just dips into the pool of mercury while rotating.

The pool of mercury is kept in between the NS poles of a strong magnet When the axis of the wheel and the mercury are connected to a battery, the circuit is completed and the wheel starts rotating due to the action of the magnet on the current.

While rotating, when a spoke of the wheel just leaves mercury, the circuit breaks out, but due to inertia of motion, the next spoke comes in contact with mercury, thereby rotation of the wheel continues.

The speed of rotation of Barlow’s wheel depends upon the—

1. The strength of the magnetic field, and
2. The strength of the current.

The rotation of Barlow’s wheel increases with increasing the current. The wheel rotates in the opposite direction when the current is reversed.

The rotation of Barlow’s wheel is possible in D. C. only, but not in A.C.

Reason: In A. C., the direction of rotation of the wheel changes its direction during each half-cycle. Ultimately, no rotation of the wheel is possible in A.C.

WBBSE Chapter 6 Current Electricity Electric motor

What makes an electric fan move? Electric motor. In many instruments like tulle pumps, toys, washing machines, mixers, grinders, etc.

electric motors are used. An electric motor works due to the action of a magnet on current.

An electric motor is a device that directly converts electrical energy (d.c.) into mechanical energy (specifically rotational kinetic energy).

Working principle of an electric motor: When an electric current is passed through a rectangular coil placed in a magnetic field, two equal and opposite forces act on two arms of the coil, as a result of which the coil begins to rotate continuously and thus mechanical energy (rotational K.E. is obtained.)

The direction of rotation of the armature coil could be determined with the help of Fleming’s left-hand rule. Due to this fact, the left-hand rule is also called the ‘rule of motor’.

A rectangular coil made of insulated Cu wire, called armature coil, ABCD is placed in between two ponies of a strong magnet NS.

The ends A and D of the coil are connected to a commutator made of Cu which has two split parts R₁ and R₂.

This combination is mounted on a shaft so that it is capable of rotating around the shaft. A pair of carbon brushes and  B₂ press gently against R₁ and R₂ respectively.

A d. c. source is connected across the carbon brasses B₁ and B₂. Here note that R₁,R₂ is connected with the armature coil and B₁, B₂ with the external circuit.

Working: When current is passed through the armature coil in the direction DCBA, then according to Fleming’s left-hand rule, the force on the arm AB acts in the inward direction and that force acts on the arm CD in the outward direction.

Force on sides BC and AD are zero as these are directed along the magnetic field. The equal and opposite forces acting on arms AB and CD form a couple.

This couple creates rotational motion in the coil in the anticlockwise direction. While rotating, it, the coil begins to rotate and the arm AB comes out and the arm CD goes in.

When the coil reaches a vertical position, the carbon brushes lose contact with the commutator and momentarily the current gets cut off.

However, due to the inertia of motion, the coil continues rotating in the same direction.

After half rotation, R₁ comes in contact with B₂ and R₂ with B₁ At this position the force on the arm CD acts in the outward direction, and on the arm AB it acts in an inward direction.

Due to this, the coil continues rotating in the same direction. The strength of an electric motor can be Increased by

1. Increasing the number of turns in the armature coil,
2. Increasing the strength of current through the armature,
3. Increasing the area of the coil and

Increasing magnetic field strength by inserting a soft iron core inside the armature coil. Because soft iron has the property to get magnetized easily.

Activity: To make a model of a motor using a battery, magnet, and wires: Wrap a 3-4ft long insulated copper wire around a hollow pipe in 25-30 turns.

Ejecting the pipe, bring out the tightly packed coil. This coil acts as an armature. Strip out the insulation wire from two ends of the coil wire.

Two safety pins are inserted in a thermocol tightly with a gap from one another. Safety pins combined act as a commutator.

Two ends of the coil are brought in to support over two safety pins such that there is free space for rotation of the coil.

Keep a disc magnet into the thermocouple, just below the coil Then, + ve and -ve terminals of an electric cell through a switch are connected with two ends of the safety pins.

When the switch is made ON, current flows through the coil and it starts rotating and as soon as the switch is OFF, the rotation stops. This is a hand-made motor. Can it work with an alternating current? No.

WBBSE Chapter 6 Current Electricity Electromagnetic Induction Concept Of Induced EMF And Induced Current

We knew what happens when a current-carrying wire is placed in a magnetic field. Oersted’s experiment proved that electric current produces a magnetic field.

Can the reverse be also true? Let us see what will happen, as per the activity

Simple Demonstration: Take a coil with which a current-sensitive galvanometer (G) is connected in series having no source and a simple bar magnet (for which the direction of magnetic field lines or magnetic flux is from the N-pole to the S-pole).

Now, move either pole of the magnet (N or S) towards or away from the coil. A deflection in G-needle is seen (either towards the left or right).

But, if the magnet’s motion is stopped, G- the needle does not deflect. Keeping the magnet fixed, when the coil is moved towards or away from the magnet, deflection in G-needle is also seen.

Make the faster motion of either the magnet or the coil, and faster deflection in the G-needle is seen.

Replacing the magnet with a current-carrying coil, the same type of result is obtained.
Does it happen because of motion?

Actually, when there is a relative motion between a coil and a magnet, the magnetic field lines linking to the cross-section of the coil (i.e. magnetic flux) change (increases/decreases), and because of this change, an emf (potential difference) is induced across two ends of the coil.

When the motion is stopped, no change in magnetic flux takes place, i.e., the existence of emf lasts as long as there is a change in magnetic flux.

The emf produces an electric current called induced current, for which G-needle deflects, and it lasts as long as a change in magnetic flux takes place.

The phenomenon is known as electromagnetic induction. As a whole, mechanical energy (or work) is directly converted into electrical energy.

What a surprising fact Without using a voltage source (d.c.), electric current can be obtained In 1822, Michael Faraday, an English scientist, first time invented it.

Electromagnetic induction is a milestone discovery of Faraday’s laws of electromagnetic induction:

Faraday’s First Law: Whenever there is a change in the magnetic flux linked with a conductor, an e.m.f. is induced in the conductor, as long as there is a change in the magnetic flux.

If the conductor forms a closed coil, an induced current flows through it.

Faraday’s Second Law: The magnitude of induced e.m.f. is directly proportional to the rate of change of magnetic flux linked with the conductor or coil.

Lenz’s law as a consequence of conservation of energy: when the N-pole of the magnet is brought towards the coil, an induced current would flow through the coil such that the magnetic flux would decrease.

This is possible if the induced current would try to oppose the increase of magnetic flux i.e. an N-pole is induced at the face of the coil which opposes the motion of the N-pole of the magnet approaching the coil.

The mechanical energy spent in doing work against such opposing force is transformed into electrical energy, due to which a current flows in an anti-clockwise direction in the coil.

Similarly, when the S-pole of the magnet is brought towards the coil, the induced current flows in a clockwise direction.

Here, in the absence of external force, the K.E. of the moving magnet decreases while the P.E. of the coil increases at the same rate.

Hence, we can conclude that Lenz’s law is based on the law of conservation of energy.

Remember: Lenz’s law is one of the fundamental laws of the universe.

Faraday’s First Law: States the cause of induced EMF or current in a closed coil.

Faraday’s Second Law: States how much emf or current is induced.

Lenz’s Law: States the direction of induced emf or current.

WBBSE Chapter 6 Current Electricity Direct Current (D.C.) AND Alternating Current (A.C.)

Advantages of a.c. over d.c.: In practical use, generally, 220V a.c. is preferred, but not the d.c. The reason is that the a.c.

voltage can be increased or decreased by the use of a step-up or step-down transformer. It reduces the loss of electrical energy in the transmission lines.

On the other hand, the D.C. voltage can not be increased or decreased and this causes a huge loss of electrical energy in the line wires while passing d.c.

But in machines where big-size motors are used (to run trains,. tram), d.c. is more useful.

WBBSE Chapter 6 Current Electricity Electric Generator

A.C. generator: A.C. generator is a device that converts mechanical energy into electrical energy, working ‘according to the principle of electromagnetic induction.

There are two kinds of generators

1. A. C. generator and
2. D. C. generator.

How does it work? : In an A.C. generator, a rectangular coil is rotated in a magnetic field, because of which the magnetic flux changes with time, and an e.m.f.

Is induced between the ends of the coil. The basic construction of the main parts of an A.C. generator is

an armature coil ABCD made of insulated Cu wire wound over a soft iron core,

A field magnet,

Two co-axial slip rings and S₁ S₂, Two- carbon brushes B₁ B₂. This arrangement is mounted on an axle.

Working: Initially suppose when the armature coil starts rotating, the angle between the direction of magnetic flux lines and the axis of the coil is 0°.

There is no emf induced in the coil, because of no flux change. If the coil is rotated clockwise from 0° to 90°, maximum emf is induced in the coil.

When the coil is further rotated to 180°, the induced emf again becomes zero.

When the coil turns from 180° to 270°, then magnetic flux lines change in the opposite direction, and maximum emf is produced in the opposite direction.

Again when the coil turns to 360°, the rate of flux change again becomes zero, then emf also becomes zero.

During rotation of the coil, the arms AB and CD interchange their positions periodically. Thus the e.m.f. or current changes its polarity as well as magnitude periodically. It is called the alternating e.m.f.

The frequency of a. c. produced in a generator is determined by the number of rotations of the armature coil in one second.

In our country, the frequency of A.C. is 50 Hz. That is, the current changes its direction 100 times in 1 second.

The maximum value of induced e.m.f. in the armature coil of an A. C. dynamo can be increased by

1. Increasing the area of the cross-section of the coil,
2. Increasing the number of turns in the coil and
3. Increasing the speed of rotation of the coil,
4. Increasing the magnetic field strength.

D.C. generator: Like an a.c. generator, a d. c. generator has a field magnet and an armature coil.

The co-axial slip rings are replaced by two half-cylindrical slip rings which act as a commutator The commutator rotates with the rotation of the armature coil.

In each half-turn, the parts of the commutator pass from one brush to another. Positions of S_{1; and} S₂ reverse but the brushes B₂ stay in the same position so that the output is always in the same direction.

In d.c. the generator also the induced emf is a.c. in nature but this emf is converted into d.c. by using the commutator.

In d.c. generator the current flows in the external circuit only in one direction.

The basic principle of thermal and hydroelectric power generation:

Thermal power generation: In a thermal power station, the heat produced by the combustion of fossil fuel (coal) helps to boil water at very high pressure.

When the steam is allowed to rotate the blades of a turbine, the shaft of the turbine rotates at a very high speed. It helps to rotate the armature coil of the a.c. generator and produces thermo-electricity. Here mechanical energy is converted into electrical energy.

(2) Hydroelectric power generation: Hydroelectricity or hydroelectric power is produced by utilizing the mechanical energy of a high-speed stream of water.

In hilly areas, the water confined in a dam is released to flow through a tunnel at a very high speed, which then falls upon the blades of the turbine, and the turbine rotates.

In the next stage, the a.c. a generator connected to the rotating turbine produces hydroelectricity.

WBBSE Chapter 6 Current Electricity Domestic Electrical Circuit Components Used In Domestic Electrical Circuit

Switch: An electric switch is an on-off device for current in an electric circuit when desired. Commonly used electric switches are lever type (push button type switch).

A switch is always connected to the live wire. In the household circuits, a 5A switch is used for light/fan, and a 10 A / 15 A switch for fridge/washing machine/ geyser.

The main switch is used near the meter board. It has a live wire and a neutral wire. This switch keeps household circuits protected.

Three-pin plug: In a three-pin plug, the top thick pin is for earthing connection, the left-hand pin is for life and the right-hand pin is for neutral connection.

The earth pin is thicker than the other two pins in a three-pin plug: The earth pin is made thicker so that it can be connected first. This ensures the safety of the user.

Moreover, being thicker, the earth pin cannot be inserted into the hole of the socket for a live or neutral connection.

Holes in a socket: The upper bigger hole in the socket is for earth connection, the right-hand hole is for connection to the live wire, and the left-hand hole is for connection to the neutral wire.

Live wire, neutral wire: In domestic electrical circuits, there are three wires—Live or phase (220 V), neutral (0V), and earth (0V). The live wire carries the incoming electricity at 220V, so it is very dangerous.

The neutral wire is tied at zero potential. It is taken as a reference potential. The neutral completes the circuit, as it returns electricity to the generator after passing through the appliance.

The earth wire is connected to the earth and is used for precaution of the appliance.

Earthing of electrical appliance: By earthing an electrical appliance such as electric iron, heater, geyser, etc.

it is connected to a low potential. Ultimately by connecting it directly with a local earthing wire.

Local earthing: For local earthing, a thick copper wire kept inside a hollow insulating pipe is buried 2-3m deep in the earth.

It is connected to a thick copper plate of dimensions about 50 cm × 50 cm surrounded by a mixture of charcoal and salt.

If at any stage the iron/heater gets directly connected with the live wire, then the earthing system automatically maintains zero potential and saves from dangerous accidents.

The body/casing of an electrical appliance is always earthed.

If the switch is connected to the neutral wiring electrical appliance remains connected to the live wire and acquires supply voltage, even if the switch is off.

If a person touches the switch, the body comes in contact with the live wire of the appliance. Under such circumstances, the person may get a fatal shock.

To avoid accidents, the switch should always be connected to the live wire.

Color-coding of wires: According to the new international convention, the color coding of wires.

WBBSE Chapter 6 Current Electricity Household Circuits

In household wiring, all the appliances (e.g. bulbs, fans, TV, geyser, etc.) are connected in parallel at the mains, each with a separate switch.

A bulb, and a fan (with regulator) are connected parallel to the distribution box. For each branch line, a separate fuse and switch are connected to the live wire at the distribution box.

Neutral wire and earth wire are common to all branches or appliances.

The advantage of the parallel combination is all the appliances get the main voltage and if any one of the appliances goes out of work the electric supply to other appliances does not get affected and they remain functioning properly at the same voltage,

In parallel combination, R_eq. decreases. So heat loss also decreases.

An electric shock may be caused by an electrical appliance due to poor insulation of the wires.

It is very dangerous to test electrical instruments using domestic power lines.

Activity:  A simple model of an electrical circuit using a battery as a source.