OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 12

1 mark · Easy difficulty · Multiple Choice

Identify which statement about dynamic equilibrium is not correct from four multiple-choice options.

Practise this question

Question

Multiple choice question 12 asking which statement about dynamic equilibrium is not correct, with options A to D. Option A: A catalyst increases the rate of both forward and reverse reactions by the same amount. Option B: Dynamic equilibrium exists only in a closed system. Option C: The concentrations of the reactants and products are equal. Option D: The rate of the forward reaction is equal to the rate of the reverse reaction.
Question text

12 Which statement about dynamic equilibrium is not correct?

A A catalyst increases the rate of both forward and reverse reactions by the same amount.

B Dynamic equilibrium exists only in a closed system.

C The concentrations of the reactants and products are equal.

D The rate of the forward reaction is equal to the rate of the reverse reaction.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 12 indicating the correct answer is C, worth 1 mark.

12 C 1 AO1.1

How to answer it

Understanding Dynamic Equilibrium

What this question tests

This question assesses your foundational understanding of the core criteria that define a dynamic equilibrium system in physical chemistry (OCR AO1.1 knowledge recall and comprehension).

Question 12: Multiple Choice Analysis

Identifying statements about dynamic equilibrium

✅ Correct Answer: C

Statement C ("The concentrations of the reactants and products are equal") is not correct, making it the right choice to select. At equilibrium, concentrations remain constant (unchanging over time), but they are rarely equal to one another.

💡 Key Knowledge

  • Dynamic Equilibrium: The rate of the forward reaction equals the rate of the reverse reaction.
  • Constant Concentration: Macroscopic properties (like colour and concentration) stay constant because the two opposing rates are balanced.
  • Closed System: Required so no substances can enter or escape.
  • Catalysts: Speed up both forward and reverse rates equally without changing the position of equilibrium.

🧠 Exam Technique

Read multiple-choice questions extremely carefully, especially when negative phrasing like not correct is used. Evaluate every option (A through D) rather than stopping at the first plausible statement.

❌ Common Errors

Students frequently confuse constant concentration with equal concentration. While some reactions at equilibrium might happen to have similar amounts of reactants and products, equality is not a defining rule of equilibrium.

Mark Allocation: [1] mark total for identifying option C. Tested under OCR Assessment Objective AO1.1 (Demonstrate knowledge and understanding of scientific ideas).

Topics

Module 3: Periodic table and energy · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.