Answer the questions based on the passage given below:
Adsorption is a surface phenomenon and it differs from absorption which occurs throughout the body of the substance that absorbs. In physisorption the attractive forces are mainly van der Waals forces while in chemisorption ionic / covalent bonds are formed between particles of adsorbent and adsorbate. The catalytic activity of finely divided iron in Haber's process of ammonia manufacture can be explained by adsorption theory. Adsorption being an exothermic process, the heat of adsorption is utilized in enhancing the rate of the reaction. Adsorption has many applications being used in gas masks, control of humidity, chromatograph separation, curing diseases etc.
The modern adsorption theory lists the following steps for heterogeneous catalysis
A. Adsorption of reactant molecules on the surface of a catalyst.
B. Occurrence of chemical reaction on the surface of a catalyst
C. Desorption of products from a catalytic surface
D. Diffusion of reactants to the surface of the catalyst.
Choose the correct sequence of steps from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → D, A, B, C
The adsorption theory of heterogeneous catalysis says the catalyst works only at its surface, so the reactants must first reach that surface and the products must leave it:
D comes first — reactant molecules have to reach the surface before anything else can happen.
A second — only molecules at the surface can be adsorbed.
B third — the reaction needs the reactants already held on the surface.
C last — the products can leave only after they are formed; desorption frees the surface for more reaction.
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):
Adsorption Theory of Heterogeneous Catalysis – …
