AQA A-Level Chemistry Paper 3, 2017: Question 16

1 mark · Easy difficulty · Multiple Choice

Identify which letter represents the most probable energy of gas molecules at the higher temperature on a Maxwell–Boltzmann distribution graph.

Practise this question

Question

A Maxwell-Boltzmann distribution graph plotting the number of molecules against energy for a gas at two different temperatures. One curve has a higher peak shifted to the left, with its peak projected to point A on the vertical axis and point C on the horizontal axis. The second curve has a lower, broader peak shifted to the right, with its peak projected to point B on the vertical axis and point D on the horizontal axis. The multiple-choice question asks which letter represents the most probable energy of the molecules at the higher temperature: A, B, C, or D.
Question text

16 The diagram shows the Maxwell−Boltzmann distribution of molecular energies in a gas

at two different temperatures.

Which letter represents the most probable energy of the molecules at the higher

temperature?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme table showing question number 16 with the correct answer D.

16 D

How to answer it

Maxwell–Boltzmann Distribution: Effect of Temperature

📋 What this question tests

This question assesses your understanding of the Maxwell–Boltzmann distribution of molecular energies at different temperatures. Specifically, it tests your ability to:

  • Identify what the axes and peaks represent on a Maxwell–Boltzmann curve.
  • Distinguish between curves representing lower and higher temperatures.
  • Identify the most probable energy (Emp) on the x-axis for a system at an elevated temperature.
Question 16 • Multiple Choice [1 Mark]

Identifying Most Probable Energy at Higher Temperature

Analysis of the Curves and Axes

✅ Correct Answer

D

Award 1 mark for selecting option D.

Why D is correct:

  • The curve with the broader, lower peak shifted to the right represents the higher temperature.
  • The peak of this curve corresponds to the mode or most probable energy.
  • Energy is plotted along the horizontal axis (x-axis). Projecting down from the peak of the higher temperature curve lands directly on D.

💡 Key Knowledge

  • Axes Meaning:
    • x-axis: Kinetic energy of molecules (E)
    • y-axis: Number (or fraction) of molecules with energy E
  • Peak Meaning: The highest point of the curve represents the most probable energy ( Emp ), NOT the mean or total energy.
  • When Temperature Increases:
    • The peak shifts to the right (higher energy).
    • The peak becomes lower (flattened curve).
    • Total area under the curve stays constant (same total number of molecules).

🧠 Exam Technique: Two-Step Elimination

  1. Identify the correct curve: As temperature rises, average kinetic energy increases, so the curve broadens and its maximum shifts to the right and downwards. The second curve (lower peak) is therefore the higher temperature curve.
  2. Identify what the question asks for: It asks for energy, not number of molecules.
    • Values on the y-axis (A, B) represent the number of molecules.
    • Values on the x-axis (C, D) represent energy.
    • Since the higher temperature peak lines up with D on the x-axis, D is the only possible answer.

❌ Common Errors & Misconceptions

  • Confusing the axes (Choosing B): B represents the number of molecules that have the most probable energy at the higher temperature, not the energy itself.
  • Selecting the lower temperature peak (Choosing C): C is the most probable energy at the lower temperature (where the peak is taller and further left).
  • Selecting the peak of the lower temperature on the y-axis (Choosing A): A is the maximum number of molecules at the lower temperature.
  • Conflating most probable energy with mean energy: Remember that Emp is at the peak, but mean energy is always slightly to the right of the peak because the curve is asymmetric with a long high-energy tail.

Topics

Physical Chemistry · 3.1.5 Kinetics

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.