AIIMS 2018 Chemistry First Order Reactions MCQ Question

For a first-order reaction, the rate of reaction is proportional to the concentration of the reactant, and the half-life (t₁/₂) is constant and independent of concentration. Given that the rate decreases from 0.4 M s⁻¹ to 0.04 M s⁻¹ over 20 minutes, we can use the integrated rate law for first-order reactions, which indicates that the half-life can be calculated using the formula t₁/₂ = 0.693/k, where k is the rate constant. The observed decrease in rate suggests a consistent half-life of approximately 8 minutes, making option A correct. Options B and C are incorrect as they do not reflect the calculated half-life based on the provided rate changes.