Answer the question on the basis of the passage given below:
Adsorption is a surface phenomenon and it differs from absorption which occurs throughout the body of the substance which absorbs. In physisorption, the attractive forces are mainly van der Waals forces while in chemisorption actual bonding occurs between the particles of adsorbent and adsorbate. Gases which are easily liquefiable are adsorbed more easily on the surface of a solid as compared to the gases which are liquefied with difficulty. Adsorption increases with increase in pressure and decreases as the temperature is increased.
According to adsorption theory of catalysis, the speed of the reaction increases because
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Concentration of the reactant molecules at active centres of the catalyst becomes high due to adsorption.
In heterogeneous catalysis the reactants (gases or solutions) and the catalyst (a solid) are in different phases. The adsorption theory explains the action of such a catalyst through its surface: the reactant molecules are adsorbed on the free valencies (active centres) of the catalyst surface.
Once adsorbed, many reactant molecules are crowded together on a small surface. Their concentration at the surface becomes much higher than in the gas or solution, so they meet and react more often, and the reaction speeds up. The modern form of the theory adds that the adsorbed molecules form a loosely bound intermediate on the surface, react there, and the products then desorb and diffuse away, leaving the surface free for fresh reactant.
Why the other options are incorrect
Option (2) In the process of adsorption, the activation energy of the molecules becomes large — Incorrect. A larger activation energy would slow the reaction down. A catalyst always works through a path of lower activation energy.
Option (3) Adsorption absorbs heat which increases the speed of the reaction — Incorrect. Adsorption is an exothermic process; it releases heat (the heat of adsorption), it does not absorb it.
Option (4) Adsorption lower the activation energy of the reaction — Incorrect. The lower activation energy belongs to the reaction path through the surface intermediate. The adsorption theory explains the speed-up by the increased concentration of the reactants on the surface (and the heat of adsorption), not by adsorption itself lowering the activation energy.
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):
Heterogeneous Catalysis
