Chemistry Class 12 NCERT Solutions: Chapter 5 Surface-Chemistry Part 1

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Q: 1. Write any two characteristics of Chemisorption.

Answer

1. Chemisorption is highly specific in nature. It occurs only if there is a possibility of chemical bonding between the adsorbent and the adsorbent.

2. Like Physisorption, chemisorption also increases with an increase in the surface area of the adsorbent.

Q: 2. Why does Physisorption decrease with the increase of temperature?

Answer

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

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

Answer

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.

Q: 4. Why is it necessary to remove when ammonia is obtained by Haber’s process?

Answer

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

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

Answer

Ester hydrolysis can be represented as:

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 known as autocatalysts.

Q: 6. What is the role of desorption in the process of catalysis?

Answer

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.

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

Answer:

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 polarising power. Thus, Hardy-Schulze law can be modified in terms of the polarising power of the flocculating ion. Thus, the modified Hardy-Schulze law can be stated as ‘the greater the polarising power of the flocculating ion added, the greater is its power to cause precipitation.’

Q: 8. Why is it essential to wash the precipitate with before estimating it quantitatively?

Answer:

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

Exercise-2

Q: 1. Distinguish between the meaning of the terms adsorption and absorption.

Give one example of each.

Answer:

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 an 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.

Q: 2. What is the difference between Physisorption and chemisorption?

Answer:

Q_2_Tabel of Physisorption and Chemisorption
Q_2_Tabel of Physisorption and Chemisorption

Physisorption

Chemisorption

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.

In this type of adsorption, strong

chemical bonds are formed between the adsorbate and the surface of the

adsorbent.

2.

No new compound is formed in the

process.

New compounds are formed at the

surface of the adsorbent.

3.

It is generally found to be reversible in nature.

It is usually irreversible in nature.

4.

Enthalpy of adsorption is low as weak

van der Waal’s forces of attraction are

involved. The values lie in the range of

.

Enthalpy of adsorption is high as

chemical bonds are formed. The values lie in the range of

5.

It is favoured by low temperature

conditions.

It is favoured by high temperature

conditions.

6.

It is an example of multi-layer

adsorption

It is an example of mono-layer

adsorption.

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

Answer:

Adsorption is a surface phenomenon. Therefore, adsorption is directly proportional to the surface area. A finely divided substance has a large surface area. Both physisorption and chemisorption increase with an increase in the surface area. Hence, a finely divided substance behaves as a good adsorbent.