Chemical Kinetics

What you'll learn

A from-scratch guide to reaction rates for CBSE Class 12: what rate means, how to write a rate law, the difference between order and molecularity, the rate constant and its units, and the first-order integrated equation with half-life — complete with worked numericals.

This chapter has

Chemical Kinetics — solved practice questions

8 Class 12 Chemistry questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   The rate of a chemical reaction is expressed as the change in:

   • AConcentration of a reactant or product per unit time
   • BTemperature per unit time
   • CPressure per unit volume
   • DMass per unit mole
   Show answer & solution

   Correct answer: (A) Concentration of a reactant or product per unit time

   The rate of reaction is defined as the change in concentration of a reactant or product per unit time (e.g. mol L^-1 s^-1).

2. Q2easy

   For a first-order reaction, the unit of the rate constant k is:

   • Amol L^-1 s^-1
   • BL mol^-1 s^-1
   • CL^2 mol^-2 s^-1
   • Ds^-1
   Show answer & solution

   Correct answer: (D) s^-1

   For a first-order reaction, rate = k[A], so k has units of (concentration/time)/concentration = time^-1 = s^-1.

3. Q3medium

   The order of a reaction is defined as:

   • AThe number of reactant molecules taking part in the elementary step
   • BThe sum of the powers of the concentration terms in the experimentally determined rate law
   • CAlways equal to the molecularity
   • DThe number of products formed
   Show answer & solution

   Correct answer: (B) The sum of the powers of the concentration terms in the experimentally determined rate law

   Order is the sum of the exponents of the concentration terms in the experimentally determined rate law; it can be zero, fractional or integral, unlike molecularity.

4. Q4medium

   For a zero-order reaction, a plot of concentration of reactant [A] against time gives:

   • AAn exponential curve
   • BA straight line passing through the origin with positive slope
   • CA curve with increasing slope
   • DA straight line with negative slope equal to k
   Show answer & solution

   Correct answer: (D) A straight line with negative slope equal to k

   For a zero-order reaction, [A] = [A]0 - kt, so a plot of [A] versus t is a straight line with slope -k (negative slope).

5. Q5medium

   The half-life of a first-order reaction is related to its rate constant k by:

   • At(1/2) = 0.693/k
   • Bt(1/2) = k/0.693
   • Ct(1/2) = [A]0/2k
   • Dt(1/2) = 2k/[A]0
   Show answer & solution

   Correct answer: (A) t(1/2) = 0.693/k

   For a first-order reaction, t(1/2) = 0.693/k. Notably, this half-life is independent of the initial concentration.

6. Q6medium

   According to the Arrhenius equation k = A e^(-Ea/RT), increasing the temperature increases the rate constant mainly because:

   • AThe activation energy decreases
   • BThe fraction of molecules with energy greater than or equal to Ea increases
   • CThe frequency factor A becomes zero
   • DThe reaction becomes exothermic
   Show answer & solution

   Correct answer: (B) The fraction of molecules with energy greater than or equal to Ea increases

   Raising the temperature increases the fraction of molecules possessing kinetic energy equal to or greater than the activation energy Ea, so more effective collisions occur and k increases.

7. Q7medium

   A first-order reaction has a rate constant k = 6.93 x 10^-3 s^-1. Its half-life is approximately:

   • A50 s
   • B200 s
   • C100 s
   • D693 s
   Show answer & solution

   Correct answer: (C) 100 s

   t(1/2) = 0.693/k = 0.693/(6.93 x 10^-3) = 100 s.

8. Q8easy

   For a reaction, the rate law is found to be rate = k[A][B]^2. The overall order of the reaction is:

   • A1
   • B2
   • C3
   • D0
   Show answer & solution

   Correct answer: (C) 3

   The overall order is the sum of the powers: 1 (for A) + 2 (for B) = 3, so it is a third-order reaction.

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