Topic 26: Reaction Kinetics

Question 1

Definitions I:
(a) Rate equation
(b) Order of reaction
(c) Rate constant

Answer 1

Definitions I:
(a) Equation relating rate to concentrations, e.g., rate = k[A]ᵐ.
(b) Power of concentration in rate equation, e.g., m in [A]ᵐ.
(c) Constant linking rate to concentrations, units depend on order.

Question 2

Definitions II:
(a) Half-life
(b) Rate-determining step
(c) Homogeneous catalyst

Answer 2

Definitions II:
(a) Time for half of reactant to react, e.g., 1st-order constant.
(b) Slowest step controlling overall reaction rate.
(c) Catalyst in same phase as reactants, e.g., Fe²⁺ in solution.

Question 3

Deduce the order for A if rate doubles when [A] doubles (rate = k[A]ᵐ). [2 Marks]

Answer 3

Rate = k[A]ᵐ, 2rate = k(2[A])ᵐ, 2 = 2ᵐ (1 mark)
m = 1, 1st order (1 mark)

Question 4

In a lab, [X] halves from 0.1 M to 0.05 M in 20 s (1st order). Calculate k. [2 Marks]

Answer 4

k = 0.693/t½, t½ = 20 s (1 mark)
k = 0.693/20 = 0.0347 s⁻¹ (1 mark)

Question 5

In a lab, [X] halves from 0.1 M to 0.05 M in 20 s (1st order). Calculate k. [2 Marks]

Answer 5

Step 1: 2NO → N₂O₂ (slow, rate = k[NO]²) (1 mark)
Step 2: N₂O₂ + O₂ → 2NO₂ (fast) (1 mark)
Matches rate and overall equation (1 mark)

Question 6

Calculate initial rate for rate = k[A][B]², k = 0.5 M⁻²s⁻¹, [A] = 0.2 M, [B] = 0.1 M. [2 Marks]

Answer 6

Rate = 0.5 × 0.2 × (0.1)² (1 mark)
Rate = 0.001 M/s (1 mark)

Question 7

In exhaust systems, how does Pt catalyse NO + CO → N₂ + CO₂? [2 Marks]

Answer 7

Pt adsorbs NO and CO, weakens bonds (1 mark)
Desorbs N₂ and CO₂, heterogeneous (1 mark)

Question 8

How does Fe²⁺ catalyse I⁻ + S₂O₈²⁻ → 2SO₄²⁻ + I₂ in solution? [2 Marks]

Answer 8

Fe²⁺ + S₂O₈²⁻ → Fe³⁺ + 2SO₄²⁻, then Fe³⁺ + 2I⁻ → Fe²⁺ + I₂ (1 mark)
Fe²⁺ reformed, homogeneous (1 mark)

Question 9

If t½ = 10 s for a 1st-order reaction, calculate k and [A] after 20 s from 0.1 M. [2 Marks]

Answer 9

k = 0.693/10 = 0.0693 s⁻¹ (1 mark)
[A] = 0.1 × e⁻⁰·⁰⁶⁹³×²⁰ = 0.025 M (1 mark)

Question 10

Deduce the overall order for rate = k[A]²[B]⁰ and predict t½ behavior. [2 Marks]

Answer 10

Overall order = 2 + 0 = 2 (1 mark)
t½ depends on [A], not constant (1 mark)

Question 11

Analyse this mechanism: NO₂ + NO₂ → N₂O₄ (slow), N₂O₄ + CO → NO + NO₂ + CO₂ (fast). Match rate = k[NO₂]². [3 Marks]

Answer 11

Slow step: NO₂ + NO₂ → N₂O₄, rate = k[NO₂]² (1 mark)
Fast step uses intermediate N₂O₄ (1 mark)
Matches rate and overall reaction (1 mark)

Question 12

Explain how temperature rise affects k for CH₃Cl + OH⁻ → CH₃OH + Cl⁻. [2 Marks]

Answer 12

Higher T increases k, more collisions > Ea (1 mark)
Rate increases exponentially (1 mark)