Arrange the following rate constant units in increasing order of their order of reaction :
(A) sec−1
(B) mol L−1 sec−1
(C) mol−1 L sec−1
(D) mol−2 L2 sec−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)
Since Rate = k[A]n and the unit of rate is mol L−1 s−1, the unit of k depends on the order n:
\( \displaystyle \text{Unit of }k=(\mathrm{mol\,L^{-1}})^{1-n}\,\mathrm{s^{-1}} \)
(B) mol L−1 s−1: 1 − n = 1, so n = 0 (zero order): first
(A) s−1: 1 − n = 0, so n = 1 (first order): second
(C) mol−1 L s−1: 1 − n = −1, so n = 2 (second order): third
(D) mol−2 L2 s−1: 1 − n = −2, so n = 3 (third order): last
Shortcut: Each extra order adds one factor of mol−1 L. Only Option (3) starts with the zero order unit (B) and ends with (D).
NCERT (2026-27 edition):
Order of a Reaction – …
