Question

The correct statement(s) pertaining to the adsorp tion of a gas on a solid surface is (are):

• A. Adsorption is always exothermic
• B. Physisorption may transform into chemisorption at high temperature
• C. Physisorption increases with increasing temperature but chemisorption decreases with increasing temperature
• D. Chemisorption is more exothermic than physisorption, however it is very slow due to higher energy of activation

Answer 1

Answer: (A), (B), (D)

Chemisorption is more exothermic than physisorption, however it is very slow due to higher energy of activation

(a) In the process of adsorption, a bond is formed between adsorbate and adsorbent, hence always exothermic.
(b) Physisorption require very low activation energy while chemisorption require high activation energy. Therefore a physisorption may transform into chemisorption but only at high temperature.
(c) It is wrong statement as at higher temperature, physically adsorbed substance starts desorbing.
(d) In physical adsorption, van der Waals' force hold the adsorbate and adsorbent together which is a weak electrostatic attraction. In chemisorption, strong chemical bond binds the adsorbate to the adsorbent. Therefore, chemisorption is more exothermic than physical adsorption.