Which of the following are the characteristics of chemisorption?
(A) Highly specific in nature
(B) Low activation energy
(C) High heat of adsorption
(D) Reversible in nature
Choose the correct answer from the options given below
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A and C only
In chemisorption the adsorbate is held on the surface by chemical bonds, as in a surface compound. This explains its properties: it occurs only when the adsorbate can bond with that surface (specific), it releases a large amount of heat (80–240 kJ mol−1), it needs activation energy like any chemical reaction, and the bonds are not easily broken (irreversible). Physisorption, held by weak van der Waals forces, shows the opposite properties.
(A) Highly specific in nature — Correct. It occurs only if a bond can form between adsorbent and adsorbate (\( \ce{O2} \) on metals, \( \ce{H2} \) on transition metals).
(B) Low activation energy — Incorrect. Chemisorption needs a high activation energy, which is why it is slow at low temperatures.
(C) High heat of adsorption — Correct. Bond formation releases 80–240 kJ mol−1, against 20–40 kJ mol−1 for physisorption.
(D) Reversible in nature — Incorrect. Chemisorption is irreversible; physisorption is the reversible one.
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):
Types of Adsorption – …
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.”