NEET 2018 · ChemistryIntegrated ratePrevious Year Question

Which one of the following statements correctly describes the difference between a first-order and a second-order reaction?

Answer: (B) The half-life of a first-order reaction does not depend on $[A]_{0}$ ; the half-life of a second-order reaction does depend on $[A]_{0}$ . \textbf{Answer:} (B) The half-life of a first-order reaction does not depend on $[A]_{0}$ ; that of a second-order reaction does \textbf{Solution:} For a first-order reaction: $t_{1/2} = \frac{0.693}{k}$ which is independent of the initial concentration $[A]_{0}$ .

A.A first-order reaction can be catalysed; a second-order reaction cannot be catalysed
B.The half-life of a first-order reaction does not depend on $[A]_{0}$ ; the half-life of a second-order reaction does depend on $[A]_{0}$ ✓
C.The rate of a first-order reaction does not depend on reactant concentrations; the rate of a second-order reaction does depend on reactant concentrations
D.The rate of a first-order reaction does depend on reactant concentrations; the rate of a second-order reaction does not depend on reactant concentrations

Correct Answer

(B) The half-life of a first-order reaction does not depend on $[A]_{0}$ ; the half-life of a second-order reaction does depend on $[A]_{0}$

Solution & Explanation

\textbf{Answer:} (B) The half-life of a first-order reaction does not depend on $[A]_{0}$ ; that of a second-order reaction does \textbf{Solution:} For a first-order reaction: $t_{1/2} = \frac{0.693}{k}$ which is independent of the initial concentration $[A]_{0}$ . For a second-order reaction: $t_{1/2} = \frac{1}{k [ A ] _{0}}$ which depends on $[A]_{0}$ . Option (C) is false because the rate of a first-order reaction does depend on concentration, $rate = k [A]$ .

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