Chemical Kinetics
Kinetic studies not only help us to determine the speed or rate of a chemical reaction but also describe the conditions under which the reaction rates can be altered. Some reactions such as ionic reactions are very fast while some others like rusting of iron are very slow. Also, there are reactions like inversion of cane sugar and hydrolysis of starch, which proceed at a moderate speed. The rate of a reaction can be defined as the change in concentration of a reactant or product per unit time. The unit used to express the rate of a reaction is concentration time−1. The rate of a reaction can be determined in terms of the average rate of reaction or can be determined at a particular instant of time called the instantaneous rate of reaction. The rate of a chemical reaction at a given temperature depends on the concentration of one or more reactants. The representation of the rate of reaction in terms of concentration of the reactants is known as rate law. Rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically, but must be determined experimentally.
In a chemical kinetics experiment, the rate constant is written in units of L mol−1 s−1. The order of the reaction is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Second
The rate of a reaction is the change in concentration per unit time, so its unit is always mol L−1 s−1. For a reaction of order n:
\( \displaystyle \mathrm{Rate}=k[\mathrm{A}]^n,\quad k=\frac{\mathrm{Rate}}{[\mathrm{A}]^n} \)
\( \displaystyle \text{Unit of }k=\frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{(\mathrm{mol\,L^{-1}})^n}=(\mathrm{mol\,L^{-1}})^{1-n}\,\mathrm{s^{-1}} \)
Putting in the common orders:
\( \displaystyle \text{Zero order }(n=0)\text{: }\mathrm{mol\,L^{-1}\,s^{-1}} \)
\( \displaystyle \text{First order }(n=1)\text{: }\mathrm{s^{-1}} \)
\( \displaystyle \text{Second order }(n=2)\text{: }\frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{\mathrm{mol^2\,L^{-2}}}=\mathrm{mol^{-1}\,L\,s^{-1}} \)
\( \displaystyle \text{Third order }(n=3)\text{: }\mathrm{mol^{-2}\,L^2\,s^{-1}} \)
The given rate constant has the unit L mol−1 s−1, which is the same as mol−1 L s−1. So the reaction is of second order.
NCERT (2026-27 edition):
Order of a Reaction
| Reaction | Order | Units of rate constant |
|---|---|---|
| Zero order reaction | 0 | \( \displaystyle \frac{\mathrm{mol\,L^{-1}}}{\mathrm{s}}\times\frac{1}{(\mathrm{mol\,L^{-1}})^{0}}=\mathrm{mol\,L^{-1}s^{-1}} \) |
| First order reaction | 1 | \( \displaystyle \frac{\mathrm{mol\,L^{-1}}}{\mathrm{s}}\times\frac{1}{(\mathrm{mol\,L^{-1}})^{1}}=\mathrm{s^{-1}} \) |
| Second order reaction | 2 | \( \displaystyle \frac{\mathrm{mol\,L^{-1}}}{\mathrm{s}}\times\frac{1}{(\mathrm{mol\,L^{-1}})^{2}}=\mathrm{mol^{-1}L\,s^{-1}} \) |
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 24-05 S2 22-05 S1 | 15-05 S1 | – | – | – |