A-LevelChemistryReaction kinetics9701/43 · Oct/Nov 2025 · Q311 Marks

(a) Solid manganese(IV) oxide, MnO2, catalyses the decomposition of hydrogen peroxide. 2H2O2(aq) → 2H2O(l) + O2(g) State the type of catalysis for this reaction. Explain your answer. [1] (b) Hydrogen peroxide reacts with iodide ions in acidic conditions as shown. H2O2 + 2I¯ + 2H+ → 2H2O + I2 The initial rate of this reaction is investigated with different concentrations of H2O2, I and H+. The results obtained are shown in Table 3.1. Table 3.1 experiment [H2O2]/mol dm⁻³ [I]/mol dm⁻³ [H+]/moldm⁻³ initial rate/mol dm⁻³ s⁻¹ 1 0.0450 0.0300 0.0125 2.42 x 10⁻³ 2 0.0225 0.0600 0.0125 2.42 x 10⁻³ 3 0.0225 0.120 0.0125 4.84 x 10⁻³ 4 0.0450 0.120 0.0500 9.68 x 10⁻³ (i) Use the information in Table 3.1 to deduce the rate equation for this reaction. Explain your reasoning. [4] (ii) Use your rate equation from (b)(i) and the data from Experiment 1 to calculate the rate constant, k, for this reaction. Include the units of k. k = units [2] (c) The rate of the thermal decomposition of azomethane, CH3N=NCH3, is investigated. CH3N=NCH3 → N2 + C2H6 Fig. 3.1 shows the results obtained. The reaction is first order with respect to CH3N=NCH3. [Figure 3.1] (i) Use Fig. 3.1 to calculate two half-lives, t₁/₂, to show that the reaction is first order. [2] (ii) Use your answer to (c)(i) to calculate the rate constant, k, for the decomposition of azomethane. k = s⁻¹ [1] (d) Describe the effect of increasing temperature on the rate constant and on the rate of a reaction. [1]

(a)Solid manganese(IV) oxide, MnO2, catalyses the decomposition of hydrogen peroxide. 2H2O2(aq) → 2H2O(l) + O2(g) State the type of catalysis for this reaction. Explain your answer.[1]
(b)(i)Use the information in Table 3.1 to deduce the rate equation for this reaction. Explain your reasoning.[4]
(b)(ii)Use your rate equation from (b)(i) and the data from Experiment 1 to calculate the rate constant, k, for this reaction. Include the units of k. k = units[2]
(c)(i)Use Fig. 3.1 to calculate two half-lives, t₁/₂, to show that the reaction is first order.[2]
(c)(ii)Use your answer to (c)(i) to calculate the rate constant, k, for the decomposition of azomethane. k = s⁻¹[1]
(d)Describe the effect of increasing temperature on the rate constant and on the rate of a reaction.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

[1] (a) heterogeneous AND MnO2 is in a different state / phase to the reactants / H2O2
[4] (b)(i) M1: (expt 2 and 3) [I-] × 2 initial rate x 2 so 1st order wrt [I-] M2: (expt 1 and 2) [I-] × 2, [H2O2] × 0.5 initial rate × 1 so 1st order wrt [H2O2] OR (expt 1 and 4) [I-] × 4 initial rate × 4 AND [H+] × 4 so zero order wrt [H+] OR (expt 1 and 3) [I-] × 4, [H2O2] × 0.5 initial rate x 2 so 1st order wrt [H2O2] M3: (expt 1 and 4) [I-] x 4, [H+] × 4 initial rate × 4 so zero order wrt [H+] OR (expt 3 and 4) [H2O2] × 2 initial rate × 2AND [H+] x 4 so zero order wrt [H+] OR (expt 3 and 4) [H2O2] × 2, [H+] × 4 initial rate × 2 so 1st order wrt [H2O2] M4: rate = k [H2O2] [I-] OR rate = k [H2O2] [I-] [H+]º
[2] (b)(ii) M1: k = 2.42 × 10–3 ÷ (0.045 × 0.030) = 1.79 min 2sf M2: units = dm³ mol-1 s-1
[2] (c)(i) M1: calculation of one t1/2 = 60 s ±5s M2: two t1/2 calculated that are constant
[1] (c)(ii) k = In2 ÷ 60 = 0.0116 OR k = 0.693 ÷ 60 = 0.0116 min 2sf
[1] (d) increases k and increases the rate of reaction

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