Order and molecularity in the given reaction is
\( \ce{CHCl3 + Cl2 -> CCl4 + HCl} \)
\( \text{Rate}=k[\ce{CHCl3}][\ce{Cl2}]^{\frac{1}{2}} \)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( \displaystyle \frac{3}{2} \) and molecularity not applicable (complex reaction)
Note: None of the given options was correct in the NTA question paper. To provide additional question practice to the students, the question has been slightly modified so that the question has one clear answer.
Order and molecularity are different quantities:
The order is the sum of the powers of the concentrations in the experimentally found rate law; it can be zero or a fraction.
The molecularity is the number of reacting species that must collide simultaneously in an elementary reaction; it is always a whole number (1, 2 or 3). For a complex reaction, molecularity has no meaning.
\( \displaystyle \text{Rate} = k[\ce{CHCl3}]^1[\ce{Cl2}]^{1/2} \;\Rightarrow\; \text{order} = 1 + \frac{1}{2} = \frac{3}{2} \)
The fractional order shows that this reaction is not elementary: it is a complex reaction that goes through more than one step. So it has no molecularity. Two molecules (\( \ce{CHCl3} \) and \( \ce{Cl2} \)) appear on the left of the balanced equation, but that count is not the molecularity of a complex reaction.
Option (1) \( \displaystyle \frac{3}{2} \) and 0 — Incorrect. The order is right, but molecularity can never be zero.
Option (2) \( \displaystyle \frac{3}{2} \) and molecularity not applicable (complex reaction) — Correct. The order is the sum of the powers in the rate law, 1 + 1/2 = 3/2. The fractional order shows that the reaction is complex, and molecularity applies only to elementary reactions, so for this reaction it has no meaning.
Option (3) \( \displaystyle \frac{3}{2} \) and \( \displaystyle \frac{3}{2} \) — Incorrect. Molecularity cannot be a fraction; only the order can.
Option (4) 2 and \( \displaystyle \frac{3}{2} \) — Incorrect. The order from the rate law is 3/2, not 2, and molecularity cannot be a fraction.
NCERT (2026-27 edition):
Molecularity of a Reaction – “Another property of a reaction called molecularity helps in understanding its mechanism. The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.”
Molecularity of a Reaction – “… Thus, from the discussion, till now, we conclude the following:
(i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non integer.
(ii) Order is applicable to elementary as well as complex reactions whereas molecularity is applicable only for elementary reactions. For complex reaction molecularity has no meaning.”