The order of a reaction with rate-constant, k = 4x10-5 mol L-1 s-1 is

The correct answer is Option (3) → Zero order

The rate of a reaction is the change in concentration per unit time, so the unit of rate is always mol L⁻¹ s⁻¹.

For a reaction of order n:

    \( \displaystyle \text{Rate}=k[\text{A}]^{n},\quad k=\frac{\text{Rate}}{[\text{A}]^{n}} \)

    \( \displaystyle \text{Unit of }k=\frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{(\mathrm{mol\,L^{-1}})^{n}}=(\mathrm{mol\,L^{-1}})^{1-n}\,\mathrm{s^{-1}} \)

Zero order (n = 0):

    \( \displaystyle \text{Unit of }k=(\mathrm{mol\,L^{-1}})^{1}\,\mathrm{s^{-1}}=\mathrm{mol\,L^{-1}\,s^{-1}} \)

Third order (n = 3):

    \( \displaystyle \text{Unit of }k=(\mathrm{mol\,L^{-1}})^{-2}\,\mathrm{s^{-1}}=\mathrm{mol^{-2}\,L^{2}\,s^{-1}} \)

The given rate constant, 4 × 10⁻⁵ mol L⁻¹ s⁻¹, has the same unit as the rate itself. So the reaction is zero order.

Shortcut: When k has the same unit as the rate (mol L⁻¹ s⁻¹), the rate does not depend on concentration, so the order is zero.

Why the other options are incorrect

• Option (1) First order — Incorrect. The unit of k would be s⁻¹.

• Option (2) Second order — Incorrect. The unit of k would be mol⁻¹ L s⁻¹.

• Option (4) Third order — Incorrect. The unit of k would be mol⁻² L² s⁻¹.

NCERT (2026-27 edition):
Order of a Reaction – …

Note: The same/similar question has been asked in different years by NTA as per the details given below: