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NEET Chemistry · Chemical Kinetics

Zero and first order reactions and half-lives

Taught in NCERT Class 12 Chemistry, Chapter 3: Chemical Kinetics.

3 verified questions: Easy 0 · Medium 2 · NEET level 1

Sample questions

Sample 1Medium

For the decomposition of a reactant R, a graph of ln[R] (y-axis) against time in minutes (x-axis) is a straight line falling from ln[R]₀ with a slope of −0.0693 min⁻¹. What are the order of the reaction and its half-life?

  1. A

    Second order; 10 min

  2. B

    First order; 10 min

  3. C

    Zero order; 10 min

  4. D

    First order; 14.4 min

Show the answer
The answer is B.

A straight line for ln[R] against t shows a first order reaction with slope −k, so k = 0.0693 min⁻¹. Then t½ = 0.693/k = 0.693/0.0693 = 10 min.

  • A.

    A straight ln[R] against t plot is the signature of first order kinetics, not second order.

  • C.

    A zero order reaction gives a straight line when [R], not ln[R], is plotted against time.

  • D.

    14.4 min is 1/k; the half-life of a first order reaction is 0.693/k.

NCERT: Class 12 Chemistry, Chapter 3

Sample 2NEET level

A first order reaction has a rate constant of 0.0693 min⁻¹. The time required for 75% of the reactant to be consumed is (take ln 2 = 0.693)

  1. A

    10 min

  2. B

    15 min

  3. C

    20 min

  4. D

    40 min

Show the answer
The answer is C.

For a first order reaction t1/2 = 0.693 / k = 0.693 / 0.0693 min⁻¹ = 10 min. 75% completion leaves one quarter of the reactant, which takes two half-lives, so t = 2 × 10 min = 20 min.

  • A.

    10 min is one half-life, which gives only 50% completion.

  • B.

    15 min is 1.5 half-lives; 75% completion needs two half-lives.

  • D.

    40 min is four half-lives, which gives 93.75% completion.

NCERT: Class 12 Chemistry, Unit 3 (Chemical Kinetics), Integrated Rate Equations: Half-Life of a Reaction

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