The incorrect statement(s) pertaining to the adsorption of a gas on a solid surface are:
(A) Adsorption is always exothermic
(B) Physisorption may transform into chemisorption at high temperature
(C) Physisorption increases with increase in temperature
(D) Chemisorption is more exothermic than physisorption however it is very slow due to higher energy of activation
(E) Physisorption is reversible in nature
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (C) Only
Adsorption of a gas on a solid lowers the freedom of the gas molecules (entropy decreases), so it can be spontaneous only if heat is released. Each statement:
(A) Adsorption is always exothermic — True. Both physisorption (20–40 kJ mol−1) and chemisorption release heat.
(B) Physisorption may transform into chemisorption at high temperature — True. Hydrogen held on nickel by van der Waals forces at low temperature dissociates and becomes chemisorbed as the temperature rises.
(C) Physisorption increases with increase in temperature — False. Being exothermic, physisorption occurs readily at low temperature and decreases with rise in temperature (Le Chatelier’s principle).
(D) Chemisorption is more exothermic than physisorption, but slow because of its higher activation energy — True. Its enthalpy (80–240 kJ mol−1) is high, and it needs activation, so it is slow at low temperature.
(E) Physisorption is reversible in nature — True. The gas held by weak forces can be removed by lowering the pressure.
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):
Reversible Nature – “Physical adsorption of a gas by a solid is generally reversible. … Since the adsorption process is exothermic, the physical adsorption occurs readily at low temperature and decreases with increasing temperature (Le-Chatelier’s principle).”
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.”