1. In case of chemisorption, adsorption first increases and then decreases, with increase in temperature. Explain.
2. What are the other methods of activating the solid adsorbent?
3. Why does physisorption decreases with increase in temperature?
4. Why are powdered substances more effective adsorbent than their crystalline form?
5. Many precious metal catalysts are often deposited as very thin films on a substance of high surface area per unit mass such as aluminate or silica. Why is this an effective way of utilizing the catalyst material?
6. Why is it necessary to remove carbon monoxide when ammonia is obtained from haber process?
7. What is the role of desorption in the process of catalysis?
8. Physical adsorption is multimolecular while chemisorption is unimolecular. Explain.
9. What is Gold number?
10. Why is it essential to wash the precipitate with water before estimating it quantitatively?
SOLUTIONS:-
Ans1:- The initial increase in chemisorption is due to the fact that heat supplied is used as activation energy. The later decrease in adsorption is due to the fact that the adsorption in equilibrium is exothermic in nature.
Ans2:- Other methods for activating solid adsorbent is :-
i) by making its surface more rough
ii) by subdividing the given adsorbent into fine particles so that the surface area is increased and hence increase the extent of adsorption.
Ans3:- With increase in temperature, the strength of the Vander Waal's forces decreases. Hence with increase of temperature, the physisorption which because of Vander Waal's forces also decreases.
Ans4:- Powdered substance have more surface area as compare to their crystalline forms and, therefore, powdered substances are more effective adsorbent than their crystalline forms.
Ans5:- Use of chemically stable supports make it possible to obtain very large surface area per unit mass of the precious metal catalyst, because the metal can be deposited in a very thin, even, monomolecular, layer on the surface of the support.
Ans6:- Carbon monoxide acts as a poison and lower the efficiency of the catalyst. So, it must be removed when ammonia is obtained by Haber process.
Ans7:- When desorption occurs from the surface of catalyst, the surface of catalyst is ready to adsorb fresh reactants and acts as catalyst.
Ans8:- Chemisorption involves the formation of a compound on it's surface which does not react further with the adsorbate and hence unimolecular in nature. On the other hand, physical adsorption is due to Vander Waal's forces which are always there in one layer and fresh adsorbate.
Ans9:- Gold number is defined as the minimum number of milligrams of the protective colloid which must be added to 10 mL of a standard gold Sol to prevent its coagulation when 1 ml of 10% sodium chloride solution is added to it.
Ans10:- The precipitate may be contaminated with the adsorbed particles of some other substances which might have been used or formed during the process. Therefore, it is essential to wash the precipitate to remove these adsorbed particles.