The rate of a reaction is found to be equal to its rate constant at any concentration of the reactant. The order of the reaction is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Zero order
The order of a reaction is the power to which the concentration of the reactant is raised in the rate law:
\( \displaystyle \text{Rate}=k[\mathrm{R}]^n \)
If the rate equals k at any concentration, [R]n must be 1 for every value of [R]. This is true only for n = 0, since any quantity raised to the power zero is unity:
\( \displaystyle \text{Rate}=k[\mathrm{R}]^0=k \)
The rate does not depend on the concentration of the reactant, so the reaction is zero order.
Why the other options are incorrect
Option (2) First order — Incorrect. Rate = k[R], so the rate changes with [R]; it equals k only when [R] = 1 mol L−1.
Option (3) Second order — Incorrect. Rate = k[R]2, which rises four times when [R] is doubled; it is not equal to k at every concentration.
Option (4) Third order — Incorrect. Rate = k[R]3, which also depends on [R].
NCERT (2026-27 edition):
3.3.1 Zero Order Reactions – “Zero order reaction means that the rate of the reaction is proportional to zero power of the concentration of reactants.”