Which of the following is not characteristic of chemisorption ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Low enthalpy of adsorption
In chemisorption the adsorbed gas is held on the surface by chemical bonds, not by weak van der Waals forces as in physisorption. Its properties follow from this: it needs a chance of bond formation (specific), it forms surface compounds (irreversible), it stops at one layer, and it releases a large amount of heat.
Bond formation releases much more energy than van der Waals attraction, so the enthalpy of chemisorption is high, 80–240 kJ mol−1. A low enthalpy of adsorption (about 20–40 kJ mol−1) belongs to physisorption. So “Low enthalpy of adsorption” is not a characteristic of chemisorption.
Option (1) Highly specific — A characteristic of chemisorption. It occurs only where the adsorbent and adsorbate can form a bond, e.g. \( \ce{O2} \) on metals (oxides), \( \ce{H2} \) on transition metals (hydrides).
Option (2) Irreversible — A characteristic of chemisorption. A surface compound is formed, so the process is usually irreversible.
Option (3) Results in unimolecular layer — A characteristic of chemisorption. Once every surface site is bonded, no further bonding is possible, so it gives a single layer, whereas physisorption can give multimolecular layers.
Option (4) Low enthalpy of adsorption — Not a characteristic of chemisorption. A low enthalpy of adsorption belongs to physisorption.
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 – “(i) High specificity: Chemisorption is highly specific and it will only occur if there is some possibility of chemical bonding between adsorbent and adsorbate. For example, oxygen is adsorbed on metals by virtue of oxide formation and hydrogen is adsorbed by transition metals due to hydride formation.
(ii) Irreversibility: As chemisorption involves compound formation, it is usually irreversible in nature. …
(iv) Enthalpy of adsorption: Enthalpy of chemisorption is high (80-240 kJ mol−1) as it involves chemical bond formation.”