Chemisorption is favoured by

The correct answer is Option (3) → high pressure

Chemisorption is adsorption through chemical bond formation between the adsorbate and the surface. Its extent depends on the conditions as follows:

• Pressure: a higher pressure brings more gas molecules to the surface, so high pressure favours chemisorption.

• Temperature: chemisorption has a high energy of activation, so it is very slow at low temperatures and often increases with rise of temperature (like most chemical reactions).

• Surface area: a larger surface area of the adsorbent gives more sites, so chemisorption increases with surface area.

• Enthalpy: it involves bond formation, so its enthalpy is high, 80–240 kJ mol⁻¹.

Why the other options are incorrect

• Option (1) low enthalpy of adsorption — Incorrect. Chemisorption has a high enthalpy of adsorption; a low enthalpy (20–40 kJ mol⁻¹) is typical of physisorption.

• Option (2) decreased surface area of the adsorbent — Incorrect. Fewer surface sites mean less adsorption; chemisorption increases with an increase of surface area.

• Option (4) low temperature — Incorrect. Because of its high activation energy, chemisorption is very slow at low temperatures; it is physisorption that is favoured by low temperature.

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):
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.”