A-LevelChemistryReaction kinetics9701/41 · Oct/Nov 2020 · Q18 Marks

Nitrogen monoxide, NO, reacts with oxygen to form nitrogen dioxide, NO2. 2NO(g) + O2(g) ⇋ 2NO2(g) The rate equation for the forward reaction is shown. rate = k[NO]2[O2]

(a)Complete the following table. the order of reaction with respect to [NO] the order of reaction with respect to [O2] the overall order of reaction[1]
(b)(i)Two separate experiments are carried out at 30 °C to determine the rate of the forward reaction. experiment [NO]/mol dm-³ [O2]/mol dm-³ rate/mol dm-³ s-¹ 1 0.00300 0.00200 1.51 x 10-² 2 0.00500 6.05 x 10-µ Use the data for experiment 1 to calculate the value of the rate constant, k. State the units of k.[2]
(b)(ii)Calculate the value of [NO] in experiment 2.[1]
(c)Define the term rate-determining step.[1]
(d)(i)Peroxodisulfate ions, S2O82-, react with iodide ions, I-. S2O82- + 2I- → 2SO42- + I2 The rate equation for the reaction in the absence of any catalyst is shown. rate = k[S2O82-][I-] Suggest equations for a two-step mechanism for this reaction, stating which of the two steps is the rate-determining step.[2]
(d)(ii)A large excess of peroxodisulfate ions is mixed with iodide ions. Immediately after mixing, [I-] = 0.00780 mol dm-³. Under the conditions used, the half-life of [I-] is 48 seconds. Calculate the iodide ion concentration 192 seconds after the peroxodisulfate and iodide ions are mixed.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[1] ALL CORRECT
[2] (a) the order of reaction with respect to [NO]: 2; the order of reaction with respect to [O2]: 1; the overall order of reaction: 3
[1] M1: (rate/([NO]^2[O2])) = (1.51 × 10–4) / (0.00300^2 × 0.00200)
[1] M2: k = 8389 (min 2sf)
[1] M3: units = mol-2 dm6 s-1
[1] M1: 8400 = (6.05 × 10–5) / (x² × 0.005)
[1] M2: x = √ (6.05 × 10–5) / (8400 × 0.005)
[1] M3: x = 0.00120 / 1.20 × 10–3 (min 2sf ecf from Q1bi)
[1] slow(est) step
[1] correct RDS identified as step 1 with only one S2O82– and one I–
[1] overall mechanism adds up to chemical equation and no cancellable species on LHS / RHS in each of the equations
[2] (d)(i) e.g. step 1 S2O82– + I– → SO42– + SO4I– (RDS = step 1); step 2 SO4I– + I→ SO42– + I2
[1] no. of t1/2 = 192/48 = 4
[1] [I–] = 0.0078/16 = 4.9 × 10–4 (min 2sf)

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