A-LevelChemistryReaction kinetics9701/42 · Oct/Nov 2019 · Q29 Marks

Explain what is meant by the following terms. half-life of a reaction rate-determining step

(a)Explain what is meant by the following terms. half-life of a reaction rate-determining step[2]
(b)The reaction between hydroxide ions and bromomethane is first order with respect to [OH-] and first order with respect to [CH3Br]. CH3Br + OH- → CH3OH + Br¯ Suggest a practical method that would confirm that the reaction is first order with respect to [OH-]. • Your method should include details of measurements that would be taken in order to calculate the rate of the reaction. • You should include a method of presenting the results to show that the reaction is first order with respect to [OH-].[4]
(c)The hydrolysis of methyl ethanoate, CH3CO₂CH₃, by hydroxide ions, OH-, is first order with respect to [CH3CO2CH3] and also first order with respect to [OH-]. In a particular experiment, • [CH3CO2CH3] = 0.100 moldm-³ • [OH-] = 0.100 mol dm-3 • rate of reaction = 2.06 × 10-3 mol dm-3s-1. Write a rate equation for this reaction and calculate the value of the rate constant, k, under these conditions. State the units of k. rate = k = units =[3]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[2] M1: the time taken for the amount/concentration or a reactant to halve
[2] M2: the slowest step
[4] • Use an excess of CH3Br
[4] • (Several experiments with) different initial [OH]
[4] • control / equilibrate temperatures
[4] • measure time
[4] • find [OH-] by sample and titrate or use of pH probe or find [Br] by sample and reference to use of Ag⁺.
[4] • processing of results – plot graph of [OH-] vs rate or evaluate rate is proportional to [OH-] numerically
[4] Alternative approach: • Use an excess of CH3Br
[4] • One experiment with known initial [OH-]
[4] • control / equilibrate temperatures
[4] • measure time
[4] • find [OH-] by sample and titrate or use of pH probe or find [Br] by sample and reference to use of Ag+ and describes how to calculate [OH-] .
[4] • processing of results – plot graph of [OH-] vs time and look for constant half-life
[4] Award 1 mark for each correctly identified point.
[3] M1: rate = k[ester][OH-]
[3] M2: value of k = 0.206
[3] M3: units mol-1dm³s-1

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