The half life period of a reaction does not depend upon the initial concentration of the reactant. The order of the reaction is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → First
For a first order reaction the half-life is t1/2 = 0.693/k, with no [R]0 in it. For zero order, t1/2 ∝ [R]0, and for second order, t1/2 ∝ 1/[R]0.
Why the other options are incorrect
Option (1) Zero — Incorrect. For a zero order reaction, t1/2 is directly proportional to [R]0.
Option (3) Second — Incorrect. For a second order reaction, t1/2 is inversely proportional to [R]0.
Option (4) Third — Incorrect. For a third order reaction, t1/2 also depends on [R]0 (inversely on its square).
NCERT (2026-27 edition):
Half-Life of a Reaction – …
