Chemical Kinetics
Kinetic studies not only help us to determine the speed or rate of a chemical reaction but also describe the conditions under which the reaction rates can be altered. Some reactions such as ionic reactions are very fast while some others like rusting of iron are very slow. Also, there are reactions like inversion of cane sugar and hydrolysis of starch, which proceed at a moderate speed. The rate of a reaction can be defined as the change in concentration of a reactant or product per unit time. The unit used to express the rate of a reaction is concentration time−1. The rate of a reaction can be determined in terms of the average rate of reaction or can be determined at a particular instant of time called the instantaneous rate of reaction. The rate of a chemical reaction at a given temperature depends on the concentration of one or more reactants. The representation of the rate of reaction in terms of concentration of the reactants is known as rate law. Rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically, but must be determined experimentally.
Which of the following statement is incorrect?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Order is applicable to elementary reactions only whereas molecularity is applicable to complex reactions only.
Order comes from the experimental rate law and applies to both elementary and complex reactions; molecularity is a theoretical count of colliding species and has meaning only for an elementary reaction.
Option (1) Order of a reaction can be fractional. — A correct statement. For example, the decomposition of ethanal has Rate = k[CH3CHO]3/2, so its order is 3/2.
Option (2) Molecularity of a reaction can never be zero where as order can be zero. — A correct statement. At least one species must take part in a step, so molecularity cannot be zero; but the rate of a reaction can be independent of concentration, e.g. the decomposition of \( \ce{NH3} \) on a hot platinum surface at high pressure is zero order.
Option (3) Order of a reaction is an experimentally determined quantity. — A correct statement. The order cannot be predicted from the balanced equation; it comes from measured rates, as the passage also says.
Option (4) Order is applicable to elementary reactions only whereas molecularity is applicable to complex reactions only. — The incorrect statement. It swaps the two ideas: it is molecularity that applies to elementary reactions only, while order applies to complex reactions too.
NCERT (2026-27 edition):
Molecularity of a Reaction – …
