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The plot between the extent of adsorption (
x/m
) against the pressure of gas (P) at constant temperature (T) is called the adsorption isotherm.
Freundlich adsorption isotherm:
Freundlich adsorption isotherm gives an empirical relationship between the quantity of gas adsorbed by the unit mass of solid adsorbent and pressure at a specific temperature.
From the given plot it is clear that at pressure P
S
,(
x/m
) reaches the maximum valve. Ps is called the saturation pressure. Three cases arise from the graph now.
Case I- At low pressure:
The plot is straight and sloping, indicating that the pressure in directly proportional to
x/m
i.e.,
x/m
a p.
x/m = k p( k is constant)
Case II- At high pressure:
When pressure exceeds the saturated pressure,
x/m
becomes independent of P values.
(ii) Formation of ammonia by the combination of dinitrogen and dihydrogen in the presence of finely divided iron.
This process is called the Haber’s process.
(iii) Oswald’s process: Oxidation of ammonia to nitric oxide in the presence of platinum.
(iv) Hydrogenation of vegetable oils in the presence of Ni.
Question
20. What do you mean by activity and selectivity of catalysts?
Solution :
(a) Activity of a catalyst:
The activity of a catalyst is its ability to increase the rate of a particular reaction. Chemisorption is the main factor in deciding the activity of a catalyst. The adsorption of reactants on the catalyst surface should be neither too strong nor too weak. It should just be strong enough to make the catalyst active.
(b) Selectivity of the catalyst:
The ability of the catalyst to direct a reaction to yield a particular product is referred to as the selectivity of the catalyst. For example, by using different catalysts, we can get different products for the reaction between H
2
and CO.
Question
21. Describe some features of catalysis by zeolites.
Solution :
Zeolites are alumino-silicates that are micro-porous in nature. Zeolites have a honeycomb-like structure, which makes them shape-selective catalysts. They have an extended 3D-network of silicates in which some silicon atoms are replaced by aluminium atoms, giving them an Al−O−Si framework. The reactions taking place in zeolites are very sensitive to the pores and cavity size of the zeolites. Zeolites are commonly used in the petrochemical industry.
Question
22. What is shape selective catalysis?
Solution :
A catalytic reaction which depends upon the pore structure of the catalyst and on the size of the reactant and the product molecules is called shape-selective catalysis. For example, catalysis by zeolites is a shape-selective catalysis. The pore size present in the zeolites ranges from 260-740 pm. Thus, molecules having a pore size more than this cannot enter the zeolite and undergo the reaction.
Question
23. Explain the following terms:
(i) Electrophoresis (ii) Coagulation
(iii) Dialysis (iv) Tyndall effect.
Solution :
(i) Electrophoresis:
The movement of colloidal particles under the influence of an applied electric field is known as electrophoresis. Positively charged particles move to the cathode, while negatively charged particles move towards the anode. As the particles reach oppositely charged electrodes, they become neutral and get coagulated.
(ii) Coagulation:
The process of settling down of colloidal particles i.e., conversion of a colloid into a precipitate is called coagulation.
(iii) Dialysis
The process of removing a dissolved substance from a colloidal solution by the means of diffusion through a membrane is known as dialysis. This process is based on the principle that ions and small molecules can pass through animal membranes unlike colloidal particles.
(iv) Tyndall effect:
When a beam of light is allowed to pass through a colloidal solution, it becomes visible like a column of light. This is known as the Tyndall effect. This phenomenon takes place as particles of colloidal dimensions scatter light in all directions.
Question
24. Give four uses of emulsions.
Solution :
Four uses of emulsions:
Question
26. Explain the terms with suitable examples:
(i) Alcosol (ii) Aerosol (iii) Hydrosol
Solution :
(i) Alcosol:
A colloidal solution having alcohol as the dispersion medium and a solid substance as the dispersed phase is called an alcosol.
For example: colloidal sol of cellulose nitrate in ethyl alcohol is an alcosol.
(ii) Aerosol:
A colloidal solution having a gas as the dispersion medium and a solid as the dispersed phase is called an aerosol.
For example:
fog
(iii) Hydrosol
A colloidal solution having water as the dispersion medium and a solid as the dispersed phase is called a hydrosol.
For example:
starch sol or gold sol
Question
27. Comment on the statement that “colloid is not a substance but a state of substance”.
Solution :
Common salt (a typical crystalloid in an aqueous medium) behaves as a colloid in a benzene medium. Hence, we can say that a colloidal substance does not represent a separate class of substances. When the size of the solute particle lies between 1 nm and 1000 nm, it behaves as a colloid.
Hence, we can say that colloid is not a substance but a state of the substance which is dependent on the size of the particle. A colloidal state is intermediate between a true solution and a suspension.
Question
28. Write any two characteristics of Chemisorption.
Solution :
1. Chemisorption is highly specific in nature. It occurs only if there is a possibility of chemical bonding between the adsorbent and the adsorbate.
2. Like physisorption, chemisorption also increases with an increase in the surface area of the adsorbent.
Question
29. 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 temperature. This means that physisorption occurs more readily at a lower temperature.
Question
30. 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.
Question
31. 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.
Question
32. Why is the ester hydrolysis slow in the beginning and becomes faster after sometime?
Solution :
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.
Question
33. 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
Question
34. 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 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.’
Question
35. 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.
