Chemical Kinetics: Class 12 Important Questions with Answers

Practice set: test yourself

Try every question before looking at the answers below. 64 or more out of 80 means you are ready for the board.

C. Very short answer (1 mark each)

1. Define rate of reaction.
2. Define rate constant.
3. Give an example of a zero order reaction.
4. Write the unit of k for a second order reaction.
5. What is an activated complex?
6. Why is molecularity not defined for complex reactions?
7. Write the Arrhenius equation.
8. What is the effect of a catalyst on K?
9. What is an effective collision?
10. Give the relation between t½ and k (first order).

D. Short answer I (2 marks each)

1. Distinguish order and molecularity (two points).
2. A zero order reaction has k = 0.02 mol L⁻¹ s⁻¹ and [R]₀ = 0.5 M. Find t½.
3. Find t½ of a first order reaction with k = 0.0693 min⁻¹.
4. Rate = k[A]²[B]: give the order and the unit of k.
5. For 2N₂O₅ → 4NO₂ + O₂, relate the rates of the three species.
6. Draw the energy profile showing Ea with and without a catalyst.
7. Why does a 10 K rise nearly double the rate?
8. What is pseudo first order? Give an example.

Rough work

E. Short answer II (3 marks each)

1. A first order reaction is 20% complete in 10 min. Find k and t½.
2. k doubles from 300 K to 310 K. Calculate Ea.
3. Initial rates: [A] 0.1, 0.2, 0.1; [B] 0.1, 0.1, 0.2; rate 2 × 10⁻³, 8 × 10⁻³, 4 × 10⁻³ mol L⁻¹ s⁻¹. Find the rate law and k.
4. Derive the integrated rate equation of a zero order reaction.
5. A first order reaction has t½ = 30 min. How long for 75% completion?
6. Explain the effect of temperature using the Maxwell–Boltzmann curve.

F. Long answer (5 marks each)

1. (a) Derive k = (2.303/t) log([R]₀/[R]) for a first order reaction. (b) Show that t½ is independent of [R]₀. (c) A first order reaction has k = 1.0 × 10⁻³ s⁻¹. Find t½ and the time for 90% completion.
2. (a) State the Arrhenius equation and explain each term. (b) Ea = 75 kJ mol⁻¹: by what factor does k rise from 300 K to 310 K? (c) Give two effects of a catalyst.

G. HOTS (higher-order thinking)

1. Why can't a reaction of order 5/2 be elementary?
2. Why does the rate of a reaction fall with time even though k is constant?
3. Why is the rate of NH₃ decomposition on hot Pt independent of [NH₃] at high pressure?
4. Why do food items keep longer in a freezer?

20 MCQs

1. Unit of k for a zero order reaction
   (a) s⁻¹ (b) L mol⁻¹ s⁻¹ (c) mol L⁻¹ s⁻¹ (d) L² mol⁻² s⁻¹
2. For a first order reaction t½ is
   (a) 0.693/k (b) [R]₀/2k (c) 2.303/k (d) 1/k[R]₀
3. Order of Rate = k[A]^½[B]²
   (a) 2 (b) 3 (c) 1.5 (d) 2.5
4. Molecularity can be
   (a) 0 (b) 2 (c) 1.5 (d) −1
5. A catalyst changes
   (a) ΔH (b) Ea (c) K (d) ΔG
6. 99.9% of a first order reaction takes
   (a) 2 t½ (b) 3 t½ (c) 100 t½ (d) 10 t½
7. Slope of log k vs 1/T
   (a) −Ea/2.303R (b) −Ea/R (c) Ea/R (d) log A
8. Rate × 9 when [A] × 3: order
   (a) 1 (b) 3 (c) 2 (d) 0
9. Acid hydrolysis of ethyl acetate is
   (a) zero order (b) third order (c) second order (d) pseudo first order
10. For zero order the straight-line plot is
    (a) ln[R] vs t (b) [R] vs t (c) 1/[R] vs t (d) t½ vs t
11. k = 3 × 10⁻⁴ s⁻¹ means order
    (a) 0 (b) 2 (c) 1 (d) 3
12. Activation energy is the energy to form
    (a) the activated complex (b) products (c) reactants (d) a catalyst
13. Fraction of molecules with E ≥ Ea
    (a) e^(Ea/RT) (b) e^(−Ea/RT) (c) Ea/RT (d) −Ea/RT
14. Rate of 2HI → H₂ + I₂
    (a) −d[HI]/dt (b) −2 d[HI]/dt (c) −½ d[HI]/dt (d) d[HI]/dt
15. The rate constant depends on
    (a) concentration (b) volume (c) time (d) temperature
16. t₉₉/t₉₀ for first order
    (a) 2 (b) 1 (c) 10 (d) 0.5
17. Decomposition of NH₃ on hot Pt is
    (a) zero order (b) first order (c) second order (d) third order
18. Rate-determining step is the
    (a) fastest (b) last (c) first (d) slowest
19. ¼ of a first order reactant left after
    (a) 1 t½ (b) 2 t½ (c) 3 t½ (d) 4 t½
20. Arrhenius factor A is the
    (a) activation energy (b) rate (c) frequency factor (d) order

1 (c) · 2 (a) · 3 (d) · 4 (b) · 5 (b) · 6 (d) · 7 (a) · 8 (c) · 9 (d) · 10 (b) · 11 (c) · 12 (a) · 13 (b) · 14 (c) · 15 (d) · 16 (a) · 17 (a) · 18 (d) · 19 (b) · 20 (c)

Assertion–Reason (6)

(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.

1. A: t½ of a first order reaction does not depend on [R]₀. R: t½ = 0.693/k.
2. A: Molecularity of a reaction cannot be zero. R: At least one species must take part in an elementary step.
3. A: A catalyst increases the rate. R: A catalyst increases ΔG of the reaction.
4. A: Rate of reaction increases with temperature. R: More molecules have E ≥ Ea at higher temperature.
5. A: Order can be found from the balanced equation. R: Order is an experimental quantity.
6. A: Units of k and rate are the same for zero order. R: For zero order, Rate = k.

Key: 1 (a) · 2 (a) · 3 (c) · 4 (a) · 5 (d) · 6 (a)

Answers to the practice set