The rate determining step for a complex reaction is ____

The correct answer is Option (1) → the slowest step

Most reactions do not happen in a single collision. They take place through a series of simple elementary steps, together called the mechanism. The complete reaction cannot go faster than its slowest step, just as traffic on a road moves only as fast as the narrowest point allows. So the slowest step is the rate-determining step, and the rate law of the whole reaction is written from it.

For example, the decomposition of \( \ce{H2O2} \) catalysed by iodide ions occurs in two steps:

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

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

The rate depends only on the slow first step: Rate = k\( \ce{[H2O2][I^{-}]} \).

Why the other options are incorrect

• Option (2) the fastest step — Incorrect. A fast step finishes quickly and waits for the slow step; it cannot limit the rate of the whole reaction.

• Option (3) the first step — Incorrect. The slow step can come anywhere in the mechanism. It is the slowest step, not the first one, that controls the rate; in the \( \ce{H2O2} \) example the slow step only happens to be the first.

• Option (4) the step involving the major reactant — Incorrect. The rate is not decided by which species is present in the largest amount, but by the slowest step.

NCERT (2026-27 edition):
Molecularity of a Reaction – “… chances to win the relay race competition by a team depend upon the slowest person in the team. Similarly, the overall rate of the reaction is controlled by the slowest step in a reaction called the rate determining step. … The first step, being slow, is the rate determining step. Thus, the rate of formation of intermediate will determine the rate of this reaction.”

Note: The same/similar question has been asked in different years by NTA as per the details given below: