Chemisorption has the following traits:
(A) High enthalpy of adsorption
(B) Reversible
(C) Favoured by high temperature
(D) Requires low activation enthalpy
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, C only
In chemisorption the adsorbate is held by chemical bonds. Like a chemical reaction it releases much heat, needs activation energy and is not easily reversed. Because of the activation energy, chemisorption first increases with temperature (as a reaction does), unlike physisorption, which falls as temperature rises.
(A) High enthalpy of adsorption — Correct. About 80–240 kJ mol−1, the order of a chemical bond.
(B) Reversible — Incorrect. Chemisorption is irreversible; a surface compound is formed.
(C) Favoured by high temperature — Correct. A higher temperature supplies the activation energy, so chemisorption rises with temperature (up to a point).
(D) Requires low activation enthalpy — Incorrect. It needs a high activation enthalpy.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Characteristics of Chemisorption – “… (ii) Irreversibility: As chemisorption involves compound formation, it is usually irreversible in nature. Chemisorption is also an exothermic process but the process is very slow at low temperatures on account of high energy of activation. Like most chemical changes, adsorption often increases with rise of temperature. Physisorption of a gas adsorbed at low temperature may change into chemisorption at a high temperature. Usually high pressure is also favourable for chemisorption. … (iv) Enthalpy of adsorption: Enthalpy of chemisorption is high (80-240 kJ mol–1) as it involves chemical bond formation.”