CBSE 2025, Set 2

Question

**Assertion (A):** In a first order reaction, if the concentration of the reactant is doubled, its half-life is also doubled.

**Reason (R):** The half-life of a first order reaction does not depend upon the initial concentration of the reactant. A). Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). B). Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). C). Assertion (A) is true, but Reason (R) is false. D). Assertion (A) is false, but Reason (R) is true.

Edvaya · Accepted Answer

The correct answer is Assertion (A) is false, but Reason (R) is true.. **A is FALSE:** For a first order reaction: $$t_{1/2} = \frac{0.693}{k}$$ The half-life is **independent of the initial concentration**. Doubling the concentration does **not** change the half-life. So the assertion is wrong. **R is TRUE:** The half-life of a first order reaction depends only on the rate constant $k$, not on the initial concentration. This is the distinguishing feature of first-order kinetics. **Answer: (D) — A is false, R is true.**

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Chemistry > Physical Chemistry > Chemical Kinetics

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