Using the Maxwell-Boltzmann distribution, explain why a small increase in temperature produces a large increase in the rate of reaction.
[3 marks]
Total for this question: 3
Chemistry
Kinetics
AQA 3.1.5
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
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The topic on one screen
- Reactions happen when particles collide with energy ≥ the activation energy (Ea) and the correct orientation.
- The Maxwell-Boltzmann distribution shows the spread of molecular energies: it starts at the origin, has no maximum energy, and the area under it = the total number of molecules.
- Higher temperature shifts the peak right and lower; a much greater proportion of molecules now exceed Ea, so the rate rises steeply (exponentially).
- Higher concentration (or pressure for gases) means more particles per unit volume → more frequent collisions.
- A catalyst provides an alternative route with a LOWER Ea, so a greater proportion of molecules can react — the distribution itself is unchanged.
- On a Maxwell-Boltzmann graph, mark the catalysed Ea to the left of the uncatalysed Ea; the extra area to its right is the extra reacting molecules.
Where students actually lose marks
The final mark for the temperature effect needs detail: many more particles now have energy greater than (or equal to) Ea, and the rate increases exponentially with temperature. 'Particles move faster' is not enough.
June 2022 Paper 2 examiner report (Q01.5)
On the Maxwell-Boltzmann sketch, students got the axis labels the wrong way round and failed to show how a temperature rise changes the distribution. y-axis = number of molecules; x-axis = energy.
June 2023 Paper 2 examiner report (Q09.4)
Try it — exam-style
Hard
ORIGINAL · after June 2022 Paper 2 Q01.5
Easy
ORIGINAL · after June 2023 Paper 2 Q09.4
State what is plotted on each axis of a Maxwell-Boltzmann distribution.
[1 mark]
Total for this question: 1
Medium
ORIGINAL
Explain, in terms of activation energy, how a catalyst increases the rate of a reaction.
[2 marks]
Total for this question: 2
Easy
ORIGINAL
Use collision theory to explain why increasing the concentration of a reactant increases the rate of reaction.
[2 marks]
Total for this question: 2
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