AQA A-Level Chemistry AS Paper 2, June 2025: Question 19

1 mark · Medium difficulty · Multiple Choice

Identify which letter represents the mean energy of molecules at the lower temperature on a Maxwell–Boltzmann distribution curve.

Practise this question

Question

A graph showing two Maxwell–Boltzmann distribution curves plotting number of molecules against energy for a gas at two temperatures. The taller curve represents the lower temperature, with dashed vertical line A marking its peak and dashed line B positioned slightly to the right of the peak. The broader curve represents the higher temperature, with dashed vertical line C at its peak and dashed line D to the right of its peak. Four multiple-choice options corresponding to letters A, B, C, and D are listed below.
Question text

19 The Maxwell–Boltzmann distribution of molecular energies for a sample of gas at

two different temperatures is shown.

Which letter represents the mean energy of the molecules at the lower temperature?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme table showing question number 19 with the correct answer B and 1 mark under AO1.

19 B 1 (AO1)

How to answer it

Maxwell–Boltzmann Distribution: Identifying Molecular Energies

📋 What this question tests

This question assesses your understanding of Maxwell–Boltzmann distribution curves at different temperatures, specifically distinguishing between the most probable energy (Emp) and the mean energy (Emean) of particles, and understanding why the distribution's asymmetry affects their positions.

Question 19 • Multiple Choice [1 Mark]

Identifying Mean Energy at Lower Temperature

AQA Chemistry A-Level • Physical Chemistry • Kinetics

✅ Correct Answer

B

  • Lower Temperature Curve: The curve with the higher, narrower peak shifted towards the left represents the gas sample at the lower temperature.
  • Line A: Represents the most probable energy (Emp) at the lower temperature (aligns with the peak of the curve).
  • Line B: Represents the mean energy (Emean) at the lower temperature (positioned just to the right of the peak).
Mark Scheme: 1 mark for selecting B (AO1).

💡 Key Knowledge

  • Temperature effect: As temperature decreases:
    • The peak becomes taller and shifts to the left.
    • The proportion of molecules with lower energies increases.
  • Emp vs Emean:
    • Most probable energy (Emp): The energy possessed by the greatest number of molecules; exactly at the maximum peak height.
    • Mean energy (Emean): The average energy of all molecules; it always lies to the right of the peak because the curve is asymmetric with a long high-energy tail extending to the right.

🧠 Exam Technique & Decoding the Graph

Break the four labeled vertical dashed lines down into their exact roles:

  • A: Emp (most probable) for the lower temperature.
  • B: Emean (mean energy) for the lower temperature → Target Answer.
  • C: Emp (most probable) for the higher temperature.
  • D: Emean (mean energy) for the higher temperature.

Tip: Always eliminate two options first by identifying which curve represents the specified temperature (Lower temp = peak shifted left → lines A and B only).

❌ Common Errors & Examiner Traps

  • Selecting A: The most frequent mistake. Students assume the mean is at the peak of the curve, confusing mean with mode / most probable.
  • Selecting C or D: Failing to identify which curve belongs to the lower temperature. The broader, flatter curve shifted right is the higher temperature.
  • Assuming a normal distribution: Maxwell–Boltzmann curves are not symmetrical; the long tail on the right "pulls" the mean energy to a higher value than the peak.

Topics

Physical Chemistry · 3.1.5 Kinetics

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.