AQA A-Level Chemistry Paper 3, June 2022: Question 9

1 mark · Easy difficulty · Multiple Choice

Identify the correct statement about gas molecules at a higher temperature based on the Maxwell–Boltzmann distribution curve.

Practise this question

Question

Question 9 asks: 'Consider the change that occurs in the shape of the curve for the distribution of molecular energies in a gas when the temperature of the gas is increased. Which is a correct statement about the gas molecules at a higher temperature?' followed by four multiple choice options: A 'There are more molecules with any given energy.', B 'There are more molecules with the mean energy.', C 'There are more molecules with the most probable energy.', and D 'There is an increase in the most probable energy of the molecules.'
Question text

09 Consider the change that occurs in the shape of the curve for the distribution of

molecular energies in a gas when the temperature of the gas is increased.

Which is a correct statement about the gas molecules at a higher temperature?

[1 mark]

A There are more molecules with any given energy.

B There are more molecules with the mean energy.

C There are more molecules with the most probable energy.

D There is an increase in the most probable energy of the molecules.

Mark scheme

Show the mark scheme Mark scheme for question 9 showing the correct answer as option D (AO1) worth 1 mark, with the text 'There is an increase in the most probable energy of the molecules.'

9 D (AO1) 1 There is an increase in the most probable energy of the

molecules.

How to answer it

Maxwell-Boltzmann Energy Distribution & Temperature

📌 What this question tests

This question assesses A-Level Physical Chemistry (Kinetics) concepts, specifically:

  • Understanding how temperature affects molecular kinetic energy distributions (Maxwell-Boltzmann distribution).
  • Distinguishing between the energy value (x-axis) and the number/fraction of molecules (y-axis).
  • The specific shift of the peak ( Emp - most probable energy) when temperature increases.
Question 09 • Multiple Choice • [1 Mark]

Analysis & Correct Answer

Identifying the correct statement about gas molecules at an elevated temperature

✅ Correct Answer: Option D

"There is an increase in the most probable energy of the molecules."

Mark Scheme Breakdown:
• D scores 1 mark (AO1 recall & understanding).

💡 Key Knowledge

  • Most probable energy ( Emp ): The energy corresponding to the absolute peak of the curve.
  • At higher temperature (T₂ > T₁):
    • The peak shifts to the right (higher energy value along the x-axis).
    • The peak becomes flatter / lower (fewer molecules possess exactly this most probable energy).
    • The total area under the curve remains constant (total number of molecules has not changed).
    • The curve widens out and has a higher proportion of molecules with energy greater than the activation energy ( E ≥ Ea ).

❌ Breakdown of Incorrect Options

  • Option A: "There are more molecules with any given energy."
    False: The total number of molecules is fixed (area is constant). Because the curve flattens, there are actually fewer molecules with low to intermediate energies.
  • Option B: "There are more molecules with the mean energy."
    False: The distribution spreads out, lowering the height of the curve across the central region, meaning fewer molecules have the mean energy.
  • Option C: "There are more molecules with the most probable energy."
    False: The peak drops in height; therefore, there are fewer molecules with the most probable energy, not more.

🧠 Exam Technique: X vs Y Axis Trap

Students regularly conflate the position of the peak on the x-axis with the height of the peak on the y-axis:

  • X-axis: Represents Energy ( E ). As temperature increases, the peak shifts to the right → Emp increases.
  • Y-axis: Represents Number / Fraction of molecules. As temperature increases, the peak height decreases → fewer molecules have the peak energy.
  • Tip: Always read carefully whether the option refers to the energy value or the number of molecules!

📐 Mental Visualisation (Maxwell-Boltzmann Curves)

When sketching or visualising this change for exams:

  • Start both curves at the origin (0,0) .
  • The higher-temperature curve must have its peak down and to the right compared to the lower-temperature curve.
  • The two curves cross only once.
  • At high energies, the higher-temperature curve remains above the lower-temperature curve, neither touching the x-axis (asymptotic).

Topics

Physical Chemistry · 3.1.5 Kinetics

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.