Consider the following statements:
(A) The order of a reaction can be known from experimental results and not from the stoichiometry of the reaction.
(B) Molecularity applies to each elementary step of a complex reaction, or to a reaction that takes place in a single step.
(C) For the reaction mA + nB → products, the overall order of the reaction is m + n.
Which of the above statements are correct?
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A and B only
The order of a reaction is the sum of the powers of the concentration terms in the experimentally found rate law. The molecularity is the number of reacting species that collide at the same time in an elementary reaction.
(A) The order of a reaction can be known from experimental results and not from the stoichiometry of the reaction. — True. Order is an experimental quantity; the rate law cannot be written from the balanced equation.
(B) Molecularity applies to each elementary step of a complex reaction, or to a reaction that takes place in a single step. — True. Molecularity is defined only for elementary reactions. A complex reaction as a whole has no molecularity, but each of its elementary steps, such as the slowest step, has one.
(C) For the reaction mA + nB → products, the overall order of the reaction is m + n. — False. The exponents in the rate law need not equal the stoichiometric coefficients m and n; the order is m + n only if experiment shows Rate = k[A]m[B]n.
NCERT (2026-27 edition):
3.2.4 Molecularity of a Reaction – “(i) Order of a reaction is an experimental quantity.”
“(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.”
3.2.2 Rate Expression and Rate Constant – “… In these reactions, the exponents of the concentration terms are not the same as their stoichiometric coefficients. Thus, we can say that: Rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically but must be determined experimentally.”