NCERT Solutions For Class 12 | Chemistry | Chapter 5 | Surface Chemistry

NCERT Solutions for Class 12 Chemistry Chapter 5 Surface Chemistry is the essential study material needed to perfect Surface Chemistry topics.

NCERT Class 12 Chemistry solutions provided here have correct answers to NCERT textbook questions. Solutions curated in a comprehensive manner will help students to understand the subtopics in this chapter in a better way.

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1. Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.

Answer –

ADSORPTION	ABSORPTION
Adsorption is the surface phenomenon where the substance gets accumulated on the surface rather than in the mass or bulk of the liquid or solid.	Absorption is the process in which the molecules get evenly distributed inside the bulk of solid or liquid.
The concentration of adsorbate increases on the surface of the adsorbent.	The concentration of molecules absorbed increases throughout the bulk of the material.
Example- Water vapours are adsorbed by anhydrous calcium chloride.	Example- Water vapours are absorbed by silica gel.

2. What is the difference between physisorption and chemisorption?

Answer –

PHYSISORPTION	CHEMISORPTION
The adsorption in which adsorbate is attached to the surface of the adsorbent with the help of weak van der Waal’s force.	The adsorption in which adsorbate is attached to the surface of the adsorbent with the help of chemical bonds.
As van der Waal’s force is universal, it is not specific in nature.	It is highly specific in nature and occurs only if there is a possibility of chemical bonding.
It is reversible in nature.	It is irreversible in nature.
Low temperature favours physisorption.	High temperature favours chemisorption.
Multi-layer adsorption under high pressure.	Unimolecular and mono-layer adsorption.
Adsorption enthalpy is low, i.e. 20-40kJ mol-1	Adsorption enthalpy is high, i.e. 80-240kJ mol-1

3. Give reason why a finely divided substance is more effective as an adsorbent.

Answer – Being a surface phenomenon, adsorption increases with surface area, i.e. directly proportional. A finely divided surface has a large surface area which makes it a good adsorbent. Both physisorption and chemisorption are directly proportional to the surface area.

4. What are the factors which influence the adsorption of a gas on a solid?

Answer – The various factors which influence the adsorption of a gas on a solid are as follows-

• NATURE OF THE GAS – Easily liquefiable gases with higher critical temperatures are readily adsorbed because van der Wall’s force is stronger at near-critical temperatures.
• SURFACE AREA OF THE SOLID – The adsorption is directly proportional to the surface area of the solid. Therefore, the adsorption rate increases with the surface area.
• EFFECT OF THE PRESSURE – According to Le-Chatelier’s principle, as the pressure increases and the volume decrease, the adsorption rate increases.
• EFFECT OF THE TEMPERATURE – As adsorption is an exothermic process, the adsorption rate decreases with the increases in temperature according to Le-Chatelier’s principle.

5. What is an adsorption isotherm? Describe Freundlich adsorption isotherm.

Answer –

Adsorption isotherm is a curve which is used to express the variation in the amount of gas adsorbed by the adsorbent with pressure at a constant temperature.

The plot between the extent of adsorption (x/m) against the pressure of the gas (P) at constant temperature (T) is the adsorption isotherm as shown below:

Credit – NCERT

Freundlich adsorption isotherm: An empirical relationship between the quantity of gas adsorbed by the unit mass of solid adsorbent and pressure at a specific temperature. It was given by Freundlich in 1909. It can be expressed by the following equation:

Where, x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, k and n are constants which depend on nature of the adsorbent and the gas at a particular temperature.

Credit – NCERT

From the graph:-

• If the graph is a straight line, then the Freundlich isotherm is valid. The slope of the graph gives the value of 1/n and the intercept gives the value of log k.
• When 1/n= 0, x/m = constant, the adsorption is independent of pressure.
• When 1/n= 1, x/m= k p, i.e. x/m∝ p, the adsorption varies directly with pressure.

6. What do you understand by activation of adsorbent? How is it achieved?

Answer – Activating an adsorbent is the process of increasing the adsorbing power of the adsorbent.
The following are the ways to achieve the activation of the adsorbent:-

• By increasing the surface area of the adsorbent.
• By giving special treatments like wood charcoal’s activated by heating it between 650 K and 1330 K in vacuum or air.

7. What role does adsorption play in heterogeneous catalysis?

Answer – The heterogenous catalysis is the catalysis in which the catalyst and the reactants are present in different phases or states. The adsorption theory of heterogenous catalysis has the following mechanism:

• The reactants diffuse to the surface of the catalyst.
• The adsorption of the reactants on the surface of the catalyst.
• The occurrence of the chemical reaction on the surface of the catalyst by the formation of the intermediate compound.
• The desorption of the product from the catalyst’s surface.

This theory explains why the catalyst remains unchanged in mass and chemical compa=ostion at the end of the reaction and why it is effective in small quantities.

8. Why is adsorption always exothermic?

Answer – Adsorption is an exothermic process because-

• Adsorption is always accompanied by a decrease in surface energy i.e. there is always a decrease in residual force on the surface which releases as heat. Therefore, H is always negative.
• When a gas is adsorbed, the freedom of movement of its molecules becomes restricted. This amounts to a decrease in the entropy of the gas after adsorption, i.e., ∆S is negative.
• As ΔG=ΔH–TΔS, for a process to be spontaneous ΔG should be negative. Hence, ΔH and ΔS are negative, therefore ΔG is negative.

9. How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?

Answer – The following table shows the classification of the eight types of colloidal solutions on the basis of physical states of the dispersed phase and dispersion medium:

	Dispersed phase	Dispersion medium	Type of colloid	Example
1.	Solid	Solid	Solid Sol	Gemstone, Coloured glasses
2.	Solid	Liquid	Sol	Paint, cell fluids
3.	Solid	Gas	Aerosol	Smoke, dust
4.	Liquid	Solid	Gel	Cheese, jellies
5.	Liquid	Liquid	Emulsion	Milk, hair cream, butter
6.	Liquid	Gas	Aerosol	Fog, mist, cloud, insecticide sprays
7.	Gas	Solid	Solid foam	Pumice stone, foam rubber
8.	Gas	Liquid	Foam	Froth, whipped cream, soap lather

10. Discuss the effect of pressure and temperature on the adsorption of gases on solids.

Answer –

• EFFECT OF THE PRESSURE – According to Le-Chatelier’s principle, as the pressure increases and the volume decrease, the adsorption rate increases.
• EFFECT OF THE TEMPERATURE – As adsorption is an exothermic process, the adsorption rate decreases with the increases in temperature according to Le-Chatelier’s principle.

11. What are lyophilic and lyophobic sols? Give one example of each type. Why are hydrophobic sols easily coagulated?

Answer –

I. LYOPHILIC SOLS –

Lyophilic sols are liquid-loving. These are colloidal sols that are made directly by mixing substances such as gum, gelatin, starch, etc. with a suitable liquid (dispersion medium). They are reversible sols. They are stable and cannot be coagulated easily.

II. LYOPHOBIC SOLS –

Lyophobic sols are liquid-hating. When substances such as metals and their sulphides are mixed with the dispersion medium, they do not form colloidal sols. They have to be made from special treatment and methods. They are irreversible sols. They are not stable and are easily precipitated or coagulated. For stabilising, they need agents for preservation.

The salvation of a colloidal particle and the presence of gas determines the stability of lyophilic sols. Whereas the presence of charge determines the stability of the lyophobic sols. If the charge is removed from the particles of lyophobic sols, the particles come together and aggregate which leads to precipitation.

12. What is the difference between multimolecular and macromolecular colloids? Give one example of each. How are associated colloids different from these two types of colloids?

Answer –

MULTIMOLECULAR COLLOIDS	MACROMOLECULAR COLLOIDS
On dissolution, the colloidal particles are an aggregate of atoms or small molecules having a size in the colloidal range (1-1000nm).	They have a high molecular mass. Macromolecules in suitable dispersion medium form sols in which the size of macromolecules is in the colloidal range.
Gold sols, sulphur sols	Starch, cellulose

• At low concentrations, some substances behave as normal strong electrolytes. But at higher concentrations, they form aggregates and behave like colloidal solutions. The aggregated particles are micelles and such colloids are called associated colloids.

13. What are enzymes? Write in brief the mechanism of enzyme catalysis.

Answer – Enzymes are complex, nitrogenous organic compounds which are produced by living plants and animals. They are proteins with high molecular mass which form a colloidal solution with water. As numerous reactions inside the living body are catalysed by enzymes, they are also known as biochemical catalysts.

MECHANISM OF ENZYME CATALYSIS:-

• Number of cavities present on the surface of the colloidal particles of enzymes are characterized by shape and groups like –NH2, −COOH, -SH, etc. These are the active sites of the enzymes.
• Just like a key fits into a lock, the reactant molecules attach to these cavities. This forms an activated complex.
• This activated complex gets decomposed to produce products.

14. How are colloids classified on the basis of

• (i) physical states of components
• (ii) nature of dispersed phase and
• (iii) interaction between dispersed phase and dispersion medium?

Answer –

(i) On the basis of the physical states of components, the colloids are divided into eight types as follows:

	Dispersed phase	Dispersion medium	Type of colloid	Example
1.	Solid	Solid	Solid Sol	Gemstone, Coloured glasses
2.	Solid	Liquid	Sol	Paint, cell fluids
3.	Solid	Gas	Aerosol	Smoke, dust
4.	Liquid	Solid	Gel	Cheese, jellies
5.	Liquid	Liquid	Emulsion	Milk, hair cream, butter
6.	Liquid	Gas	Aerosol	Fog, mist, cloud, insecticide sprays
7.	Gas	Solid	Solid foam	Pumice stone, foam rubber
8.	Gas	Liquid	Foam	Froth, whipped cream, soap lather

(ii) On the basis of the nature of the dispersed phase, they can be divided as-

Dispersion medium	Sols name
Water	Aquasol or hydrosol
Alcohol	Alcosol
Benzene	Benzosol
Gases	Aerosol

(iii) On the basis of the interaction between the dispersed phase and dispersion medium, the colloidal sols can be of two types:

• Lyophobic sols- solvent attracting
• Lyophilic sols- solvent repelling

15. Explain what is observed

• (i) when a beam of light is passed through a colloidal sol.
• (ii) an electrolyte, NaCl is added to hydrated ferric oxide sol.
• (iii) electric current is passed through a colloidal sol?

Answer –

• (i) When a beam of light is passed through a colloidal solution, scattering of light is observed. This phenomenon is known as the Tyndall effect. This scattering illuminates the path of the beam in the colloidal solution.
• (ii) NaCl dissociates to give Na+ and Cl– ions. With the presence of negatively charged Cl- ions, ferric oxide sols which are positively charged are coagulated.
• (iii) The colloidal particles are charged with either a positive or negative charge. When the electric current is passed, the colloidal particles move towards the oppositely charged electrode and get coagulated as they lose their charge.

16. What are emulsions? What are their different types? Give example of each type.

Answer – Emulsions are liquid-liquid colloidal systems, i.e. both the dispersed phase and dispersion medium are liquids.

The two main types of emulsions are

• O/W TYPE- It is oil dispersed in water. Example- milk
• W/O TYPE- It is water dispersed in oil. Example- Butter.

17. How do emulsifires stabilise emulsion? Name two emulsifiers.

Answer – The emulsifiers or emulsifying agents are the components used to stabilise the emulsion which is unstable. They form an interfacial film between suspended particles and the medium which decreases the tension between them and stables the emulsions.

Two examples of emulsifiers are proteins, gums, etc for O/W type and heavy metal salts of fatty acids for W/O type.

18. Action of soap is due to emulsification and micelle formation. Comment.

Answer – The cleansing activity of soap is determined by emulsification and micelle production. The basic component of soap is a combination of long-chain fatty acid salts in sodium and potassium (R-COO-Na+). The alkyl end of the molecule is non-polar, and sodium is connected to the polar end of the molecule. This means that a soap molecule has both polar and non-polar components, or hydrophilic and hydrophobic components, respectively.

When soap is added to dirty water, a micelle is created. The soap molecules surround the dirt particles so that the hydrophobic parts of the dirt molecules connect to them and the hydrophilic parts point away from them. As a result, the polar group dissolves in the dirt particle while the non-polar group does not. Now that these micelles are negatively charged and do not coalesce, a stable emulsion is created.

19. Give four examples of heterogeneous catalysis.

Answer –

(I) Sulfur trioxide is formed by the oxidation of sulfur dioxide. In this reaction, Pt acts as a catalyst.

(ii) Combination between dinitrogen and dihydrogen to form ammonia in the presence of finely divided iron in Haber’s process.

(iii)Oxidation of ammonia into nitric oxide in the presence of platinum gauze in Ostwald’s process

(iv) Hydrogenation of vegetable oils in the presence of finely divided nickel as catalyst.

20. What do you mean by activity and selectivity of catalysts?

Answer –

(a) Catalyst Activity:

The ability to speed up the rate of a particular reaction is called its activity. The activity of the catalyst is determined by the main factor, i.e. chemisorption. When the reactants are adsorbed on the surface of the catalyst, the process must be neither too strong to make it immobilised nor too weak. It should just be strong enough to make the catalyst active.

(b) Catalyst selectivity:

Catalyst selectivity is defined as the ability of a catalyst to direct a reaction to produce a specific product. For example, by using different catalysts, we can obtain different products for the reaction between H2 and CO.

Credit – NCERT

21. Describe some features of catalysis by zeolites.

Answer – Zeolites are microporous alumino-silicates. Zeolites are also shape-selective catalysts because they have a honeycomb structure. Some silicon atoms are replaced by aluminium atoms in the expanded 3D silicate lattice, giving them an Al-O-Si framework. The size of the pores and cavities of the zeolite makes the reactions very sensitive. Zeolites are commonly used in the petrochemical industry as ZSM-5 and convert alcohol into gasoline by dehydrating them.

22. What is shape selective catalysis?

Answer – A catalytic reaction that depends on the molecules of the product, the size of the reactants and the pore structure of the catalyst are called shape-selective catalysis. For example, zeolite catalysis is form-selective catalysis. Therefore, molecules with a pore size larger than 260-740 µm cannot enter the zeolite and undergo the reaction.

23. Explain the following terms:

• (i)Electrophoresis
• (ii) Coagulation
• (iii) Dialysis
• (iv)Tyndall effect

Answer –

(i) Electrophoresis –

An applied electric field affects randomly moving colloidal particles, this is called electrophoresis. The negatively charged particles move towards the anode while the positively charged particles move towards the cathode. When the particles approach oppositely charged electrodes, they become neutral and coagulate.

(ii) Coagulation –

The process of converting colloids into precipitates is called coagulation of colloidal particles.

(iii) Dialysis –

When a solute is removed across the membrane from a colloidal solution by diffusion, it is called dialysis. Small molecules and ions can pass through animal membranes unlike colloidal particles, which is the basic principle of this process.

(iv)Tyndall effect –

The Tyndall effect is observed when a beam of light is allowed to pass through a colloidal solution, it becomes visible as a column of light. This phenomenon occurs when colloidal-sized particles scatter light in all directions.

24. Give four uses of emulsions.

Answer – Uses of emulsions are as follows;

• (i) Emulsion formation determines the cleaning action of the soap.
• (ii) The emulsification process is responsible for the digestion of fats in the intestines.
• (iii) Emulsions are formed when antiseptics and disinfectants are added to water.
• (iv) The emulsion process used to manufacture the drug and medicines.

25. What are micelles? Give an example of a micellers system.

Answer – When soap and detergent are dissolved in water, micelles are formed. The molecules of these substances contain part hydrophobic and part hydrophilic. When present in water, these substances arrange themselves in a spherical structure in such a way that their hydrophilic parts face outwards, while their hydrophobic parts face the centre. This is called micelle formation.

26. Explain the terms with suitable examples:

• (i) Alcosol
• (ii) Aerosol
• (iii) Hydrosol

Answer –

(I) ALCOSOL –

An alcosol is a type of colloidal solution that has alcohol as the dispersion medium and a solid as the dispersed phase.
example: colloidal sol of cellulose nitrate in ethyl alcohol

(II) AEROSOL –

A colloidal solution with a solid as the dispersed phase and gas as the dispersion medium is called an aerosol.
example: fog

(III) HYDROSOL –

A colloidal solution with a solid as the dispersion phase and water as the dispersion medium is called a hydrosol.
example: starch sol or gold sol

27. Comment on the statement that “colloid is not a substance but a state of substance”.

Answer – In a benzene medium, common salt, a typical crystalloid in an aqueous medium, acts as a colloid. Therefore, we can conclude that a colloidal material is not a distinct class of substances. The solute particle behaves as a colloid when its size is between 1 nm and 1000 nm.

As a result, we can conclude that a colloid is not a substance but rather a condition of a substance that depends on the particle size. The transitional condition between a real solution and a suspension is called a colloidal state.

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