OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2024: Question 8

1 mark · Easy difficulty · Multiple Choice

Identify how a catalyst affects the activation energy and the proportion of molecules with energy greater than the activation energy.

Practise this question

Question

A multiple choice question numbered 8 asking which row in a table explains how a catalyst affects activation energy and the proportion of molecules with energy greater than the activation energy. The table has columns for 'How the activation energy changes' and 'Proportion of molecules with energy > Ea', with four rows A, B, C, D giving combinations of 'decreases' and 'increases'.
Question text

8 Which row in the table explains how a catalyst affects the activation energy (Ea) and the

proportion of molecules with energy > Ea?

How the activation Proportion of molecules

energy changes with energy > Ea

A decreases decreases

B decreases increases

C increases decreases

D increases increases

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme shows the correct answer for question 8 is B, worth 1 mark.

8 B 1

How to answer it

Effect of Catalysts on Activation Energy and Molecular Proportions

What this question tests

This multiple-choice question assesses your fundamental understanding of how a catalyst functions during a chemical reaction. It specifically targets your knowledge of activation energy (Ea) and how lowering Ea shifts the threshold on a Maxwell-Boltzmann distribution curve, directly altering the proportion of molecules possessing sufficient energy to react.

Question 8 Analysis

Exam Part: Multiple Choice [1 Mark]

✅ Correct Answer: B

Activation energy: decreases

Proportion of molecules with energy > Ea: increases

Mark Awarded: 1 / 1 for selecting option B.

💡 Key Knowledge

  • A catalyst provides an alternative reaction pathway requiring a lower activation energy ( Ea ).
  • On a Maxwell-Boltzmann distribution curve, lowering the activation energy shifts the Ea marker line to the left.
  • Moving the activation energy line to the left encompasses a larger total area under the curve beyond that point, meaning a greater proportion of molecules now have energy > Ea.

🧠 Exam Technique

In multiple-choice questions involving two variables, evaluate them one at a time to quickly eliminate incorrect options:

  • Step 1: Ask yourself what a catalyst does to activation energy. It speeds up reactions by decreasing Ea. This immediately eliminates options C and D.
  • Step 2: Look at remaining options (A and B). If Ea is lower, more particles can react. Therefore, the proportion must increase, leaving only option B.

❌ Common Errors

  • Confusing temperature with catalysts: While both increase the rate of reaction, a higher temperature shifts the entire Maxwell-Boltzmann curve shape and increases the peak energy, whereas a catalyst physically changes the position of the activation energy boundary ( Ea ) itself.
  • Rushing the second column: Students who correctly identify that a catalyst decreases Ea sometimes misread the second column and assume fewer molecules are needed, choosing option A.

Topics

Module 3: Periodic table and energy · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.