Edexcel A-Level Chemistry AS Paper 2, November 2021: Question 3

1 mark · Easy difficulty · Multiple Choice

Identify which Maxwell-Boltzmann distribution shows the effect of adding a catalyst to a reaction at the same temperature and pressure.

Practise this question

Question

Multiple-choice question 3 featuring an initial Maxwell-Boltzmann distribution curve showing number of molecules against kinetic energy for an uncatalysed reaction. Below are four options labelled A, B, C, and D, each showing a different Maxwell-Boltzmann curve with varying peak heights and distributions.
Question text

3 The Maxwell‑Boltzmann distribution of molecular energies for the reactant molecules

in an uncatalysed reaction is shown.

Number of

molecules with

energy E

Kinetic energy E

Which of these Maxwell‑Boltzmann distributions would you expect for the same

molecules in the presence of a catalyst at the same temperature and pressure?

All diagrams are drawn to the same scale.

A B

Number of Number of

molecules with molecules with

energy E energy E

Kinetic energy E Kinetic energy E

C D

Number of Number of

molecules with molecules with

energy E energy E

Kinetic energy E Kinetic energy E

(Total for Question 3 = 1 mark)

Mark scheme

Show the mark scheme The mark scheme indicates that the only correct answer is A, with explanatory notes for why B, C, and D are incorrect based on the effects of temperature and catalysts on Maxwell-Boltzmann distributions.

Question

Answer Mark

Number

3 The only correct answer is A (1)

B is not correct because a catalyst does not affect the Maxwell-Boltzmann distribution of molecular energies at the same

T and P. This curve would be expected from a decrease in temperature.

C is not correct because a catalyst does not affect the Maxwell-Boltzmann distribution of molecular energies at the

same T and P. This curve would be expected from an increase in temperature.

D is not correct because the area under the curve has increased, so the number of molecules has increased.

(Total for Question 3 = 1 mark)

How to answer it

Maxwell-Boltzmann Distributions with Catalysts

What this question tests

This question assesses your understanding of how a catalyst affects the Maxwell-Boltzmann distribution of molecular energies in a system at a constant temperature and pressure. It tests your ability to distinguish between the effects of adding a catalyst versus altering temperature or changing the total number of reactant molecules.

Multiple-Choice Question 3

Analysis and Complete Breakdown

Total Marks: 1

✅ Correct Answer: A

Option A is identical to the original uncatalysed Maxwell-Boltzmann distribution curve.

Reasoning: A catalyst provides an alternative reaction pathway with a lower activation energy, but it does not alter the kinetic energy distribution of the reactant molecules at a given temperature and pressure.

💡 Key Knowledge

  • Catalysts: Lower the activation energy (Eₐ) by providing an alternative route. They do not get used up and do not change molecular speeds or kinetic energy distributions at constant temperature.
  • Total Area: The total area under a Maxwell-Boltzmann curve represents the total number of molecules and must remain constant if no extra reactant is added.

🧠 Exam Technique

When evaluating graphical multiple-choice questions, check variables systematically:

  • Does temperature change? (If no, peak height and most probable energy must stay the same).
  • Does the number of particles change? (If no, the total area under the curve must be identical).

❌ Common Errors & Distractor Analysis

  • B is incorrect: Shows a taller, narrower peak shifted left, which represents a decrease in temperature.
  • C is incorrect: Shows a flattened, broader curve shifted right, representing an increase in temperature.
  • D is incorrect: Shows a significantly larger curve with an increased total area, implying the total number of molecules increased.
Examiner Insight: Many students mistakenly believe that because a catalyst changes reaction rate, it must physically alter the shape of the Maxwell-Boltzmann distribution curve. Remember: catalysts change where the activation energy threshold sits on the graph, not the energy distribution of the molecules themselves!

Topics

Physical Chemistry · Topic 9: Kinetics I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.