Identify the order of reaction if its rate constant is k = 2 × 10−2 s−1.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → First order
The unit of rate is always mol L−1 s−1. Since Rate = k[A]n, the unit of the rate constant depends on the order n:
\( \displaystyle \text{Unit of }k=\frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{(\mathrm{mol\,L^{-1}})^{n}}=(\mathrm{mol\,L^{-1}})^{1-n}\,\mathrm{s^{-1}} \)
The given rate constant, 2 × 10−2 s−1, has the unit s−1 only, so the reaction is first order.
Why the other options are incorrect
Option (1) Zero order — Incorrect. Its rate constant has the same unit as the rate, mol L−1 s−1.
Option (3) Second order — Incorrect. Its rate constant has the unit mol−1 L s−1.
Option (4) Half order — Incorrect. With n = 1/2 the unit is mol1/2 L−1/2 s−1.
NCERT (2026-27 edition):
Order of a Reaction – …
