OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 12

1 mark · Easy difficulty · Multiple Choice

Identify which prediction can be made using Le Chatelier's principle from the given choices.

Practise this question

Question

Multiple-choice question 12 asks: 'Which prediction can be made using le Chatelier’s principle?'. The options are: A 'The effect of a catalyst on the reaction rate', B 'The effect of a catalyst on the equilibrium position', C 'The effect of temperature on the reaction rate', and D 'The effect of concentration on the equilibrium position'. Below is an answer box and an indicator for 1 mark.
Question text

12 Which prediction can be made using le Chatelier’s principle?

A The effect of a catalyst on the reaction rate.

B The effect of a catalyst on the equilibrium position.

C The effect of temperature on the reaction rate.

D The effect of concentration on the equilibrium position.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 12 showing the correct answer as 'D' worth 1 mark, mapped to assessment objective AO1.1.

12 D 1 AO1.1

How to answer it

Scope and Application of Le Chatelier’s Principle

📋 What this question tests

Core specification coverage: Module 3 – Physical Chemistry & Transition Elements (Chemical Equilibrium)

  • Understanding the definition and strict scope of Le Chatelier’s principle.
  • Distinguishing clearly between factors influencing rate of reaction (kinetics) versus factors influencing equilibrium position (thermodynamics).
  • Recognising how changes in concentration, temperature, pressure, and the addition of a catalyst affect dynamic equilibrium systems.
Question 12 • Multiple Choice • 1 Mark

Predicting Shifts in Dynamic Equilibrium

Identifying which chemical prediction relies on Le Chatelier's principle

✅ Correct Answer

D — The effect of concentration on the equilibrium position.

Mark Scheme Breakdown:
• D [1 mark] (AO1.1 – Recall and basic conceptual understanding)

💡 Key Knowledge

  • Le Chatelier's Principle: When a system in dynamic equilibrium is subjected to an external change in conditions, the system shifts its equilibrium position to counteract that change.
  • Applicable variables: Only applies to shifts in the position of equilibrium caused by changes in:
    • Concentration of reactants or products
    • Pressure (in gaseous equilibria with unequal gas moles)
    • Temperature
  • Catalysts: Do not alter the position of equilibrium; they only speed up both forward and reverse reactions equally, reducing the time taken to reach equilibrium.

🧠 Exam Technique: Elimination Strategy

Read the question carefully: it asks what prediction is made using Le Chatelier's principle.

  • A & C (Reaction Rate): Le Chatelier's principle never explains or predicts rates of reaction. Rates are governed by collision theory and activation energy. Eliminate A and C immediately.
  • B (Catalyst on Equilibrium): A catalyst has no effect on the equilibrium position, so no such prediction is derived from Le Chatelier's principle. Eliminate B.
  • D (Concentration on Equilibrium): Increasing reactant concentration shifts equilibrium to the right; decreasing it shifts it to the left. This direct causal shift is predicted using Le Chatelier's principle. D is correct.

❌ Common Misconceptions & Traps

  • Confusing Kinetics with Equilibria: Students often conflate rate (how fast) with yield/position (how far). Phrases like "higher temperature increases reaction rate" describe collision theory, not Le Chatelier's principle.
  • Misunderstanding Catalysts: Thinking a catalyst causes a shift toward products because it increases yield per unit time. A catalyst increases the rate of reaching equilibrium, but the equilibrium concentrations remain identical.

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), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.