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Class 12 Chemistry Chapter 3: Chemical Kinetics Assertion-reason questions with answers

2 assertion-reason questions on Chemical Kinetics, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): The time taken by a first order reaction to fall from 0.8 M to 0.4 M equals the time it takes to fall from 0.2 M to 0.1 M. Reason (R): For a first order reaction t½ = 0.693/k, which does not contain the initial concentration.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both intervals are one half-life; t½ = 0.693/k is independent of [R]₀, which is exactly why the two times are equal.

Question 2
Assertion (A): The decomposition of NH₃ on a hot platinum surface at high pressure goes at a rate that does not depend on the pressure of ammonia. Reason (R): Zero order reactions are uncommon and occur only under special conditions.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

A is true: the surface is saturated, so the amount of ammonia on it cannot change. R is also true, but it does not explain A; the saturated surface does.

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