Which of the following is correct in regard to the rate determining s…

The correct answer is Option (2) → It is the slowest step in a reaction.

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 (1) It is the fastest step in a reaction. — Incorrect. A fast step finishes quickly and waits for the slow step; it cannot limit the rate of the whole reaction.

• Option (3) It can be the fastest or slowest step in reaction. — Incorrect. The rate determining step is always the slowest step, never the fastest.

• Option (4) It is always the last step in the reaction. — Incorrect. The slowest step can come at any stage of the mechanism; in the \( \ce{H2O2} \) example above it is the first 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: