AQA A-Level Chemistry Paper 2, 2023: Question 2

5 marks · Easy difficulty

Reaction Kinetics and Mechanism This question examines the kinetics of a reaction between A and B in the presence of an acid catalyst. It involves completing a table with rate data based on the rate equation: rate = k[B]²[H⁺]. Additionally, it requires analysing a proposed reaction mechanism to deduce the rate-determining step and justifying the choice based on the reaction's dependence on the concentrations of reactants.

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Question

Question text

02 A and B react together in the presence of an acid catalyst.

A(aq) + 2B(aq) → C(aq) + D(aq)

The rate equation for this reaction is

rate = k[B]2[H+]

Table 1 shows how the values of the relative initial rate vary with different

concentrations of each reagent at the same temperature.

Table 1

[A] [B] [H+] Relative initial

Experiment –3 –3 –3

/ mol dm / mol dm / mol dm rate

10.40 0.20 0.10 1.00

20.50 0.20 0.10

30.40 0.10 0.64

40.50 0.30 0.06

02.1 Complete Table 1 by calculating the missing values.

[3 marks]

02.2 A suggested mechanism for the reaction is shown.

Step 1 B + H+ → BH+

Step 2 BH+ + B → B H+

Step 3 B H+ + A → C + D

Deduce the rate-determining step for this reaction.

Give a reason for your answer.

[2 marks]

Rate-determining step

Reason

Mark scheme

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Question Answers Additional Comments/Guidelines Mark

M1 Relative rate = 1.00

02.1 M2 [B] = 0.16

(3 x AO2)

M3 Relative rate = 1.35

M1 Step 2

02.2

+ (2 x AO2)

M2 (By the end of step 2) 1 × H and 2 × B have been used Allow slowest step

How to answer it

Reaction Mechanisms and Rates

Question

(a) Complete the table by calculating the missing values based on the given rate equation:

rate = k[B]²[H⁺]

(b) Deduce the rate-determining step for the reaction mechanism and give a reason for your answer.

Answer: Part (a)

To calculate the missing values:

Use the rate equation:
rate = k[B]²[H⁺]
For experiment 2:
rate = (0.20)²(0.10) = 0.16
For experiment 4:
rate = (0.30)²(0.06) = 1.35

Completed Table:

Experiment	[A] / mol dm^-3	[B] / mol dm^-3	[H⁺] / mol dm^-3	Relative initial rate
1	0.40	0.20	0.10	1.00
2	0.50	0.20	0.10	0.16
3	0.40	0.10	0.10	0.64
4	0.50	0.30	0.06	1.35

Misconception: Students often forget to square [B] in the rate equation. Always check the order of reaction with respect to each reagent.

Answer: Part (b)

The rate-determining step is:

Step 2: BH⁺ + B → B₂H⁺

Reason:

The rate equation depends on [B]² and [H⁺], which matches the reactants in step 2.
By the end of step 2, 2 × B and 1 × H⁺ have been used, which fits the stoichiometry of the rate equation.

Tip: The rate-determining step involves the species in the rate equation. Look for steps that use the correct number of molecules indicated by the reaction orders.

Final Notes

Key points to remember:

Use the rate equation to identify the relationship between the concentrations of reactants and the rate.
The rate-determining step is the slowest step in the mechanism and dictates the overall rate.
Double-check calculations for accuracy, especially when powers and stoichiometry are involved.

Topics

Physical Chemistry · 3.1.9 Rate Equations

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.