Chemical Kinetics — Class 12 MCQs with Answers

Chemical Kinetics — important questions & MCQs with answers (Class 12 Chemistry)

14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 82 are timed and scored when you sign in.

1. Q1Easy

   Rate of reaction is:

   A.Change in concentration per unit time✓ Correct

   B.Total mass

   C.Volume change

   D.Temperature change

   Answer: A. Change in concentration per unit time

   Explanation: Rate is −d[R]/dt for a reactant or +d[P]/dt for a product: concentration change per unit time. Total mass and volume don't change during reaction, and temperature is not what rate measures.

2. Q2Easy

   Units of rate constant for first-order:

   A.unitless

   B.M·s⁻¹

   C.M⁻¹·s⁻¹

   D.s⁻¹✓ Correct

   Answer: D. s⁻¹

   Explanation: Since rate = k[A]¹, units of k = (mol·L⁻¹·s⁻¹)/(mol·L⁻¹) = s⁻¹ — concentration cancels out, leaving inverse time, unlike second-order k which keeps L·mol⁻¹·s⁻¹.

3. Q3Easy

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

   A.pressure of gases per unit area

   B.mass of reactant per unit volume

   C.temperature per unit time

   D.concentration of reactant or product per unit time✓ Correct

   Answer: D. concentration of reactant or product per unit time

   Explanation: Rate of reaction is the change in concentration of any reactant or product divided by the time interval — not pressure per area, mass per volume, or temperature change.

4. Q4Easy

   Order of a reaction is:

   A.number of reactant molecules in the balanced equation

   B.sum of stoichiometric coefficients of reactants

   C.sum of powers of concentration terms in the rate law✓ Correct

   D.always equal to molecularity

   Answer: C. sum of powers of concentration terms in the rate law

   Explanation: Order of reaction is the sum of the powers of concentration terms in the experimentally determined rate law — not the coefficients in the balanced equation, and it need not equal molecularity.

5. Q5Easy

   The SI unit of rate of a reaction in solution is:

   A.mol·s⁻¹

   B.L·mol⁻¹·s⁻¹

   C.mol·L⁻¹·s⁻¹✓ Correct

   D.s⁻¹

   Answer: C. mol·L⁻¹·s⁻¹

   Explanation: Rate = Δconcentration/Δtime, so its SI unit is mol·L⁻¹·s⁻¹. L·mol⁻¹·s⁻¹ and s⁻¹ are units of the rate CONSTANT k for particular orders, not of rate itself.

6. Q6Easy

   Units of the rate constant k for a zero-order reaction are:

   A.mol·L⁻¹·s⁻¹✓ Correct

   B.s⁻¹

   C.L·mol⁻¹·s⁻¹

   D.L²·mol⁻²·s⁻¹

   Answer: A. mol·L⁻¹·s⁻¹

   Explanation: For zero order, rate = k[A]⁰ = k, so k carries exactly the same units as rate itself: mol·L⁻¹·s⁻¹ — there is no concentration term left to change the units.

7. Q7Medium

   Arrhenius equation: k =

   A.A/Ea

   B.A·e^(Ea/RT)

   C.A·e^(−Ea/RT)✓ Correct

   D.Ea/RT

   Answer: C. A·e^(−Ea/RT)

   Explanation: k = A·e⁻ᴱᵃ/ᴿᵀ has a negative exponent because higher Ea suppresses k — a positive exponent would make k grow with activation energy, backwards from collision theory.

8. Q8Medium

   Half-life of first-order reaction:

   A.k²

   B.1/k[A]₀

   C.k·[A]₀

   D.0.693/k (independent of [A]₀)✓ Correct

   Answer: D. 0.693/k (independent of [A]₀)

   Explanation: Integrating −d[A]/dt = k[A] gives t½ = 0.693/k, independent of [A]₀ — the fingerprint separating first order from zero-order (t½ = [A]₀/2k) and second-order (t½ = 1/k[A]₀).

9. Q9Medium

   In a reaction, [A] falls from 0.50 mol·L⁻¹ to 0.40 mol·L⁻¹ in 10 minutes. The average rate of disappearance of A is:

   A.0.5 × 10⁻² mol·L⁻¹·min⁻¹

   B.1.0 × 10⁻¹ mol·L⁻¹·min⁻¹

   C.1.0 × 10⁻² mol·L⁻¹·min⁻¹✓ Correct

   D.2.0 × 10⁻² mol·L⁻¹·min⁻¹

   Answer: C. 1.0 × 10⁻² mol·L⁻¹·min⁻¹

   Explanation: Average rate = −Δ[A]/Δt = (0.50 − 0.40 mol·L⁻¹)/10 min = 0.010 mol·L⁻¹·min⁻¹ — dividing the concentration drop by the time elapsed, not by the initial or final concentration alone.

10. Q10Medium

    A first-order reaction is 50% complete in 30 min. The rate constant k (min⁻¹) is approximately:

    A.0.0693

    B.0.0231✓ Correct

    C.0.0100

    D.0.500

    Answer: B. 0.0231

    Explanation: 50% completion is one half-life, so k = 0.693/t½ = 0.693/30 min ≈ 0.0231 min⁻¹ — using 0.693 instead of 1, or forgetting to divide by t½, are the usual slips here.

11. Q11Medium

    For 2A + B → 3C, if rate of disappearance of A is 0.04 mol·L⁻¹·s⁻¹, the rate of formation of C is:

    A.0.04 mol·L⁻¹·s⁻¹

    B.0.06 mol·L⁻¹·s⁻¹✓ Correct

    C.0.02 mol·L⁻¹·s⁻¹

    D.0.12 mol·L⁻¹·s⁻¹

    Answer: B. 0.06 mol·L⁻¹·s⁻¹

    Explanation: For 2A + B → 3C, rate = −(1/2)d[A]/dt = (1/3)d[C]/dt, so d[C]/dt = (3/2)×0.04 = 0.06 mol·L⁻¹·s⁻¹ — C forms 1.5× as fast as A disappears, from the 3:2 mole ratio.

12. Q12Hard

    Catalyst:

    A.Increases ΔG

    B.Lowers activation energy without changing equilibrium position✓ Correct

    C.Adds mass to products

    D.Shifts equilibrium

    Answer: B. Lowers activation energy without changing equilibrium position

    Explanation: A catalyst opens a lower-Ea pathway for both forward and reverse reactions equally, so it speeds up both directions and leaves the equilibrium position and ΔG unchanged.

13. Q13Hard

    For zero-order reaction, rate is:

    A.Independent of concentration✓ Correct

    B.Square of concentration

    C.Inverse

    D.Same as first order

    Answer: A. Independent of concentration

    Explanation: Zero order means rate = k[A]⁰ = k: rate stays constant as [A] falls, right up to the moment reactant runs out — unlike first order, where rate keeps dropping with concentration.

14. Q14Hard

    A first-order reaction has Ea = 104.6 kJ·mol⁻¹ and t½ = 6.93 h at 300 K. The rate constant at 350 K is approximately (R = 8.314 J·K⁻¹·mol⁻¹):

    A.≈ 5.0 × 10⁻¹ s⁻¹

    B.≈ 2.78 × 10⁻⁵ s⁻¹

    C.≈ 1.1 × 10⁻² s⁻¹✓ Correct

    D.≈ 1.0 × 10⁻⁸ s⁻¹

    Answer: C. ≈ 1.1 × 10⁻² s⁻¹

    Explanation: First get k at 300 K: 0.693/(6.93×3600) = 2.78×10⁻⁵ s⁻¹. Then ln(k₂/k₁) = (104600/8.314)(1/300 − 1/350) ≈ 5.99, a factor of about 400, so k₂ ≈ 1.1×10⁻² s⁻¹. Converting hours to seconds first is essential.

Chemical Kinetics — FAQs

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