NEETChemistryChemical Kinetics
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): For a first-order reaction, the time required for 75 % completion is 1.5 times the time required for 50 % completion. Reason (R): The half-life of a first-order reaction is independent of the initial concentration of the reactant. In the light of the above statements, choose the most appropriate an
Options
- ABoth (A) and (R) are correct and (R) is the correct explanation of (A)
- BBoth (A) and (R) are correct but (R) is not the correct explanation of (A)
- C(A) is correct but (R) is not correct
- D(A) is not correct but (R) is correct
Correct answer
D. (A) is not correct but (R) is correct
Step-by-step solution
For a first-order reaction, 50 % completion corresponds to exactly one half-life ( t_ 1/2 ). At 75 % completion, 25 % of the reactant remains, which is 1 4 of the initial concentration. Reducing the concentration to one-fourth requires exactly two half-lives (from 100 % to 50 % , then 50 % to 25 % ). Thus, t_ 75 % = 2 t_ 50 % . Therefore, Assertion (A) is incorrect. The half-life of a first-order reaction is given by t_ 1/2 = 0.693 k , which does not contain any concentration term and is thus independent of the ini