A-LevelChemistryReaction kinetics9701/42 · Oct/Nov 2017 · Q113 Marks

The compound chlorine dioxide, ClO2, can be prepared by the reaction shown. NaClO2 + ½Cl2 → ClO2 + NaCl

(a)Using oxidation numbers, explain why this reaction is a redox reaction.[2]
(b)The central atom in the molecule of ClO2 is chlorine. Draw the 'dot-and-cross' diagram for ClO2. Show outer electrons only.[2]
(c)(i)What is meant by the term order of reaction with respect to a particular reagent?[1]
(c)(ii)Use the results of experiment 1 to calculate the rate constant, k, for this reaction. Include the units of k.[2]
(c)(iii)Use the data in the table to calculate • the initial rate in experiment 2, initial rate = ..................................................... mol dm⁻³ s⁻¹ • [ClO2] in experiment 3. [ClO2] = ...................................................... mol dm⁻³[2]
(d)(i)What is meant by the term rate-determining step?[1]
(d)(ii)The equation for the reaction between ClO2 and F2 is shown. 2ClO2 + F2 → 2ClO2F rate = k[ClO2][F2] The mechanism for this reaction has two steps. Suggest equations for the two steps of this mechanism, stating which of the two steps is the rate-determining step.[2]
(e)By considering the rate equation, explain why the rate increases with increasing temperature.[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 (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] Cl +3 to +4 (and oxidised)
[1] Cl0 to -1 (and reduced)
[1] 19 electrons total [1]
[1] correct diagram [1]
[1] the exponent / power to which a concentration is raised in the rate equation
[1] (0.0022 = k (0.01) × (0.06))
[1] k = 3.7 (3.67)
[1] mol⁻¹ dm³ s⁻¹
[1] initial rate = 5.50 × 10⁻³
[1] [ClO2] = 0.048
[1] slowest step (in a multi-step reaction)
[1] 1 mole of F2 and 1 mole ClO2 reacting in the rate-determining step
[1] 1st step is rate-determining step and a balanced mechanism consistent with overall equation e.g. ClO2 + F2 → ClO2F2 ClO2 + ClO2F2 → 2ClO2F or ClO2 + F → ClO2F + F ClO2 + F → ClO2F
[1] k increases (as rate increases)

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