Arrange the orders of the reactions, whose unit of rate constants are given below, in increasing arrangement.
(A) s−1
(B) mol L−1 s−1
(C) mol−1 L s−1
(D) mol−2 L2 s−1
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B), (A), (C), (D)
The unit of rate is always mol L−1 s−1. For a reaction of order n, the rate law gives the unit of the rate constant:
\( \displaystyle \mathrm{Rate}=k[A]^n \quad\Rightarrow\quad k=\frac{\mathrm{Rate}}{[A]^n} \)
\( \displaystyle \text{Unit of }k=\frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{(\mathrm{mol\,L^{-1}})^n}=(\mathrm{mol\,L^{-1}})^{1-n}\,\mathrm{s^{-1}} \)
Putting in each order:
\( \displaystyle n=0:\quad \mathrm{mol\,L^{-1}\,s^{-1}} \)
\( \displaystyle n=1:\quad \mathrm{s^{-1}} \)
\( \displaystyle n=2:\quad \mathrm{mol^{-1}\,L\,s^{-1}} \)
\( \displaystyle n=3:\quad \mathrm{mol^{-2}\,L^2\,s^{-1}} \)
Matching the given units:
(B) mol L−1 s−1: zero order
(A) s−1: first order
(C) mol−1 L s−1: second order
(D) mol−2 L2 s−1: third order
NCERT (2026-27 edition):
Units of Rate Constant
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 22-05 S2 20-05 S2 | – | – | – | – |