Surface Chemistry

Chemistry

NCERT

1   Write any two Characteristics of Chemisorption.

Solution :

(i) Chemisorption is highly specific in nature. If chemical bonds exist between adsorbate and adsorbent then the adsorption is chemical adsorption.$\\$ (ii) On increase in temperature initially the rate of reaction increases and on further increase in temperature desorption occurs.

2   Why does physisorption decrease with the increase of temperature?

Solution :

Physisorption is exothermic in nature. Therefore, in accordance with Le-Chateliere’s principle, it decreases with an increase in temperate. The means that physisorption occurs more readily at a lower temperature.

3   Why are powdered substances more effective adsorbents than their crystalline forms?

Solution :

Powdered substances are more effective adsorbents than their crystalline forms because when a substance is powdered, its surface area increases and physisorption is directly proportional to the surface area of the adsorbent.

4   Why is it necessary to remove $CO$ when ammonia is obtained by Haber’s process?

Solution :

It is important to remove $CO$ in the synthesis of ammonia as $CO$ adversely affects the activity of the iron catalyst, used in Haber’s process.

5   Why is the ester hydrolysis slow in the beginning and becomes faster after sometime?

Solution :

Ester hydrolysis can be represented as:$\\$ Ester + Water $\to$ Acid + Alcohol$\\$ The acid produced in the reaction acts as a catalyst and makes the reaction faster. Substances that act as catalysts in the same reaction in which they are obtained as products are know as auto catalysts.

6   What is the role of desorption in the process of catalysis?

Solution :

The role of desorption in the process of catalysis is to make the surface of the solid catalyst free for the fresh adsorption of the reactants on the surface.

7   What modification can you suggest in the Hardy-Schulze law?

Solution :

Hardy-Schulze law states that the greater the valence of the flocculating ion added, the greater is its power to cause precipitation’$\\$ This law takes into consideration only the charge carried by an ion, not its size. The smaller the size of an ion, the more will be its polarizing power. Thus, Hardy-Schulze law can be modified in terms of the polarizing power of the flocculating ion. Thus the modified Hardy-Schulze law can be stated as the greater the polarizing power of the flocculating ion added, the greater is its power to cause precipitation’

8   Why is it essential to wash the precipitate with water before estimating it quantitatively?

Solution :

When a substance gets precipitated, some ions that combine to form the precipitate get adsorbed on the surface of the precipitate. Therefore it becomes important to wash the precipitate before estimating it quantitatively in order to remove these adsorbed ions or other such impurities.

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

Solution :

Adsorption is a surface phenomenon of accumulation of molecules of a substance at the surface rather than in the bulk of a solid or liquid. The substance that gets adsorbed is called the ‘adsorbate’ and the substance on whose surface the adsorption takes place is called the adsorbent. Here, the concentration of the adsorbate on the surface of the adsorbent increases. In adsorption, the substance gets concentrated at the surface only. It does not penetrate through the surface to the bulk of the solid or liquid. For example, when we dip a chalk stick into and ink solution, only its surface becomes coloured. If we break the chalk stick, it will be found to be white from inside, On the other hand, the process of absorption is a bulk phenomenon. In absorption, the substance gets uniformly distributed throughout the bulk of the solid or liquid.

10   What is the difference between physisorption and chemisorption?

Solution :

$\text{Physisorption:-}$ $\\$ 1.In this type of adsorption, the adsorbate is attached to the surface of the adsorbent with weak van der Waal’s forces of attraction.$\\$ 2.No new compounds is formed in the process.$\\$ 3.Rate of adsorption depends on ease of liquefaction of gases. Easy liquefiable gases have great extent of adsorption$\\$ 4.It is generally found to be reversible in nature.$\\$ 5.Enthalpy of adsorption is low as weak van der Wall’s forces of attraction are involved. The values lie in the range of $20-40 kJ$ mol $^{ -1}$ $\\$ 6. It is favoured by low temperature conditions.$\\$ 7.It is an example of multi–layer adsorption.$\\$ $\text{Chemisorption:-}$ $\\$ 1.In this type adsorption, strong chemical bonds are formed between the adsorbate and the surface of the adsorbent.$\\$ 2.New compounds are formed at the surface of the adsorbent.$\\$ 3.Rate of adsorption does not depend on liquefaction of gases.$\\$ 4.It is usually irreversible in nature.$\\$ 5.Enthalpy of adsorption is high as chemical bonds are formed. The values lie in the range of $40-400 kJ $ mol$^ {-1}$ $\\$ 6.It is favoured by high temperature conditions.$\\$ 7.It is an example of mono-layer adsorption.