Which of the following statement is correct about the half-life period?
- It is proportional to initial concentration of zero order reaction
- Half-life period can be used to determine the order of the reaction
- Half-life period for 1st order reaction is independent of concentration
- Catalyst does not affect half-life time period
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A, B and C only
The half-life (t1/2) is the time in which the concentration of a reactant falls to half its initial value. From the integrated rate equations:
\( \displaystyle \text{Zero order: } t_{1/2} = \frac{[R]_0}{2k} \qquad \text{First order: } t_{1/2} = \frac{0.693}{k} \)
In general \( t_{1/2} \propto [R]_0^{\,1-n} \) for a reaction of order n.
(A) It is proportional to initial concentration of zero order reaction — True. For zero order, t1/2 = [R]0/2k, so it is directly proportional to [R]0.
(B) Half life period can be used to determine the order of the reaction — True. By measuring t1/2 at different initial concentrations, the exponent in t1/2 ∝ [R]01–n gives n (half-life method).
(C) Half life period for 1st order reaction is independent of concentration — True. t1/2 = 0.693/k contains no concentration term.
(D) Catalyst does not affect half life time period — False. A catalyst lowers the activation energy and increases k; since t1/2 depends on k, the half-life becomes shorter.
NCERT (2026-27 edition):
Half-Life of a Reaction – …
