Chemistry Class 12 NCERT Solutions: Chapter 5 Surface-Chemistry Part 3

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Q: 7. What role does adsorption play in heterogeneous catalysis?

Answer:

Heterogeneous catalysis:

A catalytic process in which the catalyst and the reactants are present in different phases is known as a heterogeneous catalysis. This heterogeneous catalytic action can be explained in terms of the adsorption theory. The mechanism of catalysis involves the following steps:

(i) Adsorption of reactant molecules on the catalyst surface

(ii) Occurrence of a chemical reaction through the formation of an intermediate

(iii) De-sorption of products from the catalyst surface

(iv) Diffusion of products away from the catalyst surface

In this process, the reactants are usually present in the gaseous state and the catalyst is present in the solid state. Gaseous molecules are then adsorbed on the surface of the catalyst. As the concentration of reactants on the surface of the catalyst increases, the rate of reaction also increases. In such reactions, the products have very less affinity for the catalyst and are quickly desorbed, thereby making the surface free for other reactants.

Q: 8. Why is adsorption always exothermic?

Answer

Adsorption is always exothermic. This statement can be explained in two ways.

(i) Adsorption leads to a decrease in the residual forces on the surface of the adsorbent. This causes a decrease in the surface energy of the adsorbent. Therefore, adsorption is always exothermic.

(ii) of adsorption is always negative. When a gas is adsorbed on a solid surface, its movement is restricted leading to a decrease in the entropy of the gas i.e., is

Since is negative, has to be negative to make negative, Hence, adsorption is always exothermic.

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

Answer

One criterion for classifying colloids is the physical state of the dispersed phase and dispersion medium. Depending upon the type of the dispersed phase and dispersion medium (solid, liquid, or gas), there can be eight types of colloidal systems.

Q_9_Table of There Can Be Eight Types of Colloidal Systems
Q_9_Table of there can be Eight Types of Colloidal Systems

Dispersed Phase

Dispersion Medium

Type of Colloid

Example

1.

Solid

Solid

Solid Sol

Gemstone

2.

Solid

Liquid

Sol

Paint

3.

Solid

Gas

Aerosol

Smoke

4.

Liquid

Solid

Gel

Cheese

5.

Liquid

Liquid

Emulsion

Milk

6.

Liquid

Gas

Aerosol

Fog

7.

Gas

Solid

Solid foam

Pumice stone

8.

Gas

Liquid

Foam

Froth

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

Answer:

Effect of Pressure

Adsorption is a reversible process and is accompanied by a decrease in pressure. Therefore, adsorption increases with an increase in pressure.

Effect of temperature

Adsorption is an exothermic process. Thus, in accordance with Le-Chatelier’s principle, the magnitude of adsorption decreases with an increase in temperature.

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

Answer:

(i) Lyophilic Sols:

Colloidal sols that are formed by mixing substances such as gum, gelatin, starch, etc. with a suitable liquid (dispersion medium) are called lyophilic sols. These sols are reversible in nature i.e., if two constituents of the sol are separated by any means (such as evaporation), then the sol can be prepared again by simply mixing the dispersion medium with the dispersion phase and shaking the mixture.

(ii) Lyophobic Sols:

When substances such as metals and their sulphides etc. are mixed with the dispersion medium, they do not form colloidal sols. Their colloidal sols can be prepared only by special methods. Such sols are called lyophobic sols. These sols are irreversible in nature. For example: sols of metals.

Now, the stability of hydrophilic sols depends on two things& the presence of a charge and the salvation of colloidal particles. On the other hand, the stability of hydrophobic sols is only because of the presence of a charge. Therefore, the latter are much less stable than the former. If the charge of hydrophobic sols is removed (by addition of electrolytes), then the particles present in them come closer and form aggregates, leading to precipitation.

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

Answer:

(i) In multi-molecular colloids, the colloidal particles are an aggregate of atoms or small molecules with a diameter of less than 1 nm. The molecules in the aggregate are held together by van der Waal’s forces of attraction. Examples of such colloids include gold sol and sulphur sol.

(ii) In macro-molecular colloids, the colloidal particles are large molecules having colloidal dimensions. These particles have a high molecular mass. When these particles are dissolved in a liquid, sol is obtained. For example: starch, nylon, cellulose, etc.

(iii) Certain substances tend to behave like normal electrolytes at lower concentrations. However, at higher concentrations, these substances behave as colloidal solutions due to the formation of aggregated particles. Such colloids are called aggregated colloids.