Consider the following statements for the order of a reaction and choose the correct statements.
(A). The order of a reaction is an experimental quantity.
(B). Order can be zero.
(C). Order can be fractional value.
(D). Order is applicable to elementary as well as complex reactions.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B), (C) and (D)
The order of a reaction is the sum of the powers of the concentration terms in the rate law. For Rate \( = k[\mathrm{A}]^{x}[\mathrm{B}]^{y} \), the order is \( x + y \). The powers are not taken from the balanced equation; they are found by experiment, and they can be any number that the experiment gives.
(A) The order of a reaction is an experimental quantity. — Correct. The rate law, and so the order, can only be found from measured rates; it cannot be written from the stoichiometry. For example, \( \ce{2NO + O2 -> 2NO2} \) happens to be third order, but \( \ce{CHCl3 + Cl2 -> CCl4 + HCl} \) has the order 3/2.
(B) Order can be zero. — Correct. In a zero order reaction the rate does not depend on concentration, as in the decomposition of \( \ce{NH3} \) on a hot platinum surface: Rate \( = k[\ce{NH3}]^{0} = k \).
(C) Order can be fractional value. — Correct. For \( \ce{CH3CHO -> CH4 + CO} \), Rate \( = k[\ce{CH3CHO}]^{3/2} \), so the order is 3/2.
(D) Order is applicable to elementary as well as complex reactions. — Correct. Every reaction has an experimental rate law, whether it occurs in one step or in many. It is molecularity that applies only to elementary reactions.
NCERT (2026-27 edition):
Molecularity of a Reaction – “… (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.”