Passage
Chemical Kinetics
Some reactions such as ionic reactions occur very fast, for example, precipitation of silver chloride occurs instantaneously by mixing of aqueous solutions of silver nitrate and sodium chloride. On the other hand, some reactions are very slow, for example, rusting of iron in the presence of air and moisture. The sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction. Another property of a reaction called molecularity helps in understanding its mechanism. The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction. The half-life (t1/2) of a reaction is the time in which the concentration of a reactant is reduced to one-half of its initial concentration.
In a chemical reaction, which is the rate-determining step?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 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 (1) The simplest step — Incorrect. All elementary steps are simple; being simple does not make a step control the rate.
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 (4) The main reaction 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:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 19-05 S1 11-05 S1 | 14-05 S1 | – | – | – |