For a complex reaction, the order of reaction is equal to

The correct answer is Option (4) → The molecularity of slowest step of reaction

A complex reaction takes place in a series of elementary steps. The rate of the whole reaction cannot be faster than the slowest step, which is called the rate determining step. For an elementary step, order and molecularity are the same, so the order of the complex reaction equals the molecularity of its slowest step.

For example, the decomposition of \( \ce{H2O2} \) catalysed by iodide ion in alkaline medium goes in two steps:

    \( \displaystyle \ce{H2O2 + I^- -> H2O + IO^-}\quad\text{(slow)} \)

    \( \displaystyle \ce{H2O2 + IO^- -> H2O + I^- + O2}\quad\text{(fast)} \)

The slow step is bimolecular, so the reaction is first order in \( \ce{H2O2} \) and first order in \( \ce{I-} \): Rate = k[\( \ce{H2O2} \)][\( \ce{I-} \)].

Why the other options are incorrect

• Option (1) Sum of stoichiometric coefficients in balanced chemical reaction — Incorrect. Order is found by experiment; it equals the sum of the coefficients only for an elementary reaction, never as a rule for a complex one.

• Option (2) The molecularity of overall reaction — Incorrect. Molecularity has a meaning only for an elementary step; a complex reaction as a whole has no molecularity.

• Option (3) Order of fastest step of the reaction — Incorrect. A fast step does not limit the rate; the slowest step does.

NCERT (2026-27 edition):
Molecularity of a Reaction – “… Similarly, the overall rate of the reaction is controlled by the slowest step in a reaction called the rate determining step. … (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.”