A-LevelChemistryReaction kinetics9701/42 · May/June 2025 · Q410 Marks

In aqueous solution, iodide ions react with acidified hydrogen peroxide, as shown in reaction 1. reaction 1 2I® + H2O2 + 2H+ → I2 + 2H2O The rate equation for reaction 1 is shown. rate = k [I][H2O2] Explain what is meant by order of reaction.

(a)(i)In aqueous solution, iodide ions react with acidified hydrogen peroxide, as shown in reaction 1. reaction 1 2I® + H2O2 + 2H+ → I2 + 2H2O The rate equation for reaction 1 is shown. rate = k [I][H2O2] Explain what is meant by order of reaction.[1]
(a)(ii)Complete Table 4.1. Table 4.1[2]
(a)(iii)Sketch a line on Fig. 4.1 to show the relationship between [I¯] and time. [Figure 4.1][1]
(b)(i)Nitrogen dioxide, NO2, reacts with ozone, O3, as shown in reaction 2. reaction 2 2NO2 + O3 → O2 + N2O5 The rate equation for reaction 2 is shown. rate = k [NO2][03] Two experiments are carried out to measure the rate of reaction 2. In the first experiment, the initial rate is measured starting with known concentrations of NO2 and O3. In the second experiment, the concentrations of NO2 and O3 are both increased by a factor of four. Predict how the initial rate for reaction 2 would change.[1]
(b)(ii)The reaction mechanism for reaction 2 has two steps. Define rate-determining step.[1]
(b)(iii)Suggest equations for the two steps of the reaction mechanism for reaction 2. step 1 ............................................ step 2 ............................................[2]
(c)(i)Dinitrogen pentoxide, N2O5, can decompose to NO2 and O2. The rate equation for this decomposition is shown. rate = k [N2O5] Fig. 4.2 shows the graph of rate against [N2O5] for this decomposition. [Figure 4.2] Use Fig. 4.2 to calculate a value for the rate constant, k, for this decomposition. k = ................................. [1][1]
(c)(ii)Use your answer to (c)(i) to calculate the half-life, t½, in seconds, for this decomposition. t½ = ................................. s [1][1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

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

[1] the power/exponent to which a concentration AND of a reactant is raised in the rate equation
[1] the order of reaction with respect to [H+] 0
[1] the order of reaction with respect to [I-] 1
[1] the order of reaction with respect to [H2O2] 1
[1] overall order of the reaction 2
Any two [1] all three [2]
[1] negative gradient curve
[1] rate = 4 x 4 = 16 times
[1] slowest step / slow step in the mechanism / multi-step reaction
[1] M1: step 1 O3 + NO2 → NO3 + O2
[1] M2: step 2 NO3 + NO2 → N2O5 AND sum of step 1 and 2 is consistent with equation for reaction 2
[1] Use of graph k = 12 × 10–4/0.04 = 0.03
[1] k = 0.693/t1/2
[1] t1/2 = 0.693/0.030 = 23.1 (s) min 2sf