When we consider a balanced chemical equation of an elementary reaction which statement is applicable on it?
(A) Molecularity can never be zero
(B) Order is less than molecularity
(C) Order is equal to molecularity
(D) Order is greater than the molecularity
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (C) only
An elementary reaction takes place in a single step, as its balanced equation is written. Its molecularity is the number of reacting species that collide at the same time in that step, so it is always a whole number, 1, 2 or rarely 3. The order is the sum of the powers of the concentrations in the experimental rate law. For an elementary reaction the rate law follows directly from the equation, so order and molecularity are the same. For example, the decomposition of HI is a bimolecular elementary reaction:
\( \displaystyle \ce{2HI -> H2 + I2} \qquad \text{Rate} = k[\ce{HI}]^{2} \)
Its molecularity is 2 and its order is 2.
(A) Molecularity can never be zero — Correct. At least one molecule must take part in a reaction step, so molecularity cannot be zero (or a fraction); order, which is found by experiment, can be zero.
(B) Order is less than molecularity — Incorrect. For an elementary reaction the two are equal. Order can differ from molecularity only for complex reactions, or when one reactant is in large excess (pseudo first order reactions).
(C) Order is equal to molecularity — Correct. The rate law of an elementary step is written from its balanced equation, so the order equals the molecularity.
(D) Order is greater than the molecularity — Incorrect. For the same reason, order cannot exceed molecularity in an elementary reaction.
NCERT (2026-27 edition):
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. (iii) For complex reaction, order is given by the slowest step and molecularity of the slowest step is same as the order of the overall reaction.”