AQA A-Level Chemistry Paper 3, June 2018: Question 34

1 mark · Easy difficulty · Multiple Choice

Determine which combination of temperature and pressure gives the greatest equilibrium yield of products for an endothermic gas-phase reaction.

Practise this question

Question

Question 34 asks: For this reaction at equilibrium, which combination of temperature and pressure would give the greatest equilibrium yield of products? The given equation is W(g) + X(g) reaches dynamic equilibrium with 2Y(g) + Z(g), with an enthalpy change delta H of positive 47 kilojoules per mole. Four options are provided: A (High pressure and high temperature), B (High pressure and low temperature), C (Low pressure and high temperature), and D (Low pressure and low temperature).
Question text

34 For this reaction at equilibrium, which combination of temperature and pressure would give

the greatest equilibrium yield of products?

W(g) + X(g) ⇌ 2Y(g) + Z(g) ∆H = +47 kJ mol–1

[1 mark]

A High pressure and high temperature

B High pressure and low temperature

C Low pressure and high temperature

D Low pressure and low temperature

Mark scheme

Show the mark scheme Mark scheme table shows that for question number 34, the correct answer is C.

34 C

How to answer it

Equilibrium Yield & Le Chatelier's Principle

📌 What this question tests

This multiple-choice question assesses your ability to apply Le Chatelier's Principle to predict how varying temperature and pressure affects the position of dynamic equilibrium and the yield of gaseous products.

  • Counting gaseous moles on each side of a balanced reversible equation.
  • Relating changes in pressure to shifts toward fewer or more gaseous moles.
  • Using the sign of standard enthalpy change (ΔH) to deduce whether the forward reaction is endothermic or exothermic.
  • Applying temperature changes to shift equilibrium to favor the desired products.

Question 34 Analysis

W(g) + X(g) ⇌ 2Y(g) + Z(g)   ΔH = +47 kJ mol⁻¹

✅ Correct Answer

C: Low pressure and high temperature

Mark Scheme: 1 mark for selecting option C.

📐 Step-by-Step Deduction

  1. Effect of Pressure:
    • Reactant gas moles = 1 + 1 = 2 mol
    • Product gas moles = 2 + 1 = 3 mol
    To maximize product yield, we need equilibrium to shift right (towards the side with more moles of gas).
    ➔ Low pressure shifts equilibrium to the side with more moles.
  2. Effect of Temperature:
    • ΔH = +47 kJ mol⁻¹ (positive = endothermic).
    To maximize product yield, we must favor the forward endothermic reaction.
    ➔ High temperature supplies heat, shifting equilibrium to the endothermic side.

💡 Key Knowledge

  • Le Chatelier's Principle: If a system at equilibrium is subjected to a change in conditions, the position of equilibrium moves to oppose that change.
  • Pressure: An increase in pressure favors the side with fewer moles of gas. A decrease in pressure favors the side with more moles of gas.
  • Temperature: An increase in temperature favors the endothermic reaction (absorbs heat). A decrease in temperature favors the exothermic reaction (releases heat).

🧠 Exam Technique: 2-Step Elimination

  • Step 1: Check Pressure first. Count gas molecules: 2 moles on left vs 3 moles on right. Yield increases at low pressure. Cross out A and B immediately.
  • Step 2: Check Temperature. Look at the sign of ΔH (+ means endothermic forwards). Higher temperature shifts right. Cross out D.
  • This systematic split avoids mixing up the two independent variables!

❌ Common Errors & Pitfalls

  • Confusing Rate with Yield: Students often choose "High pressure and high temperature" (Option A) thinking that industrial conditions always require high values to be fast. The question asks strictly about equilibrium yield, not rate of reaction.
  • Misreading the stoichiometric coefficients: Forgetting that 2Y(g) + Z(g) equals 3 moles total of gas, not 2.
  • Sign convention of ΔH: Forgetting that a positive ΔH indicates an endothermic forward reaction, which requires an increase in temperature to shift right.

Topics

Physical Chemistry · 3.1.6 Chemical Equilibria, Le Chatelier's Principle and Kc

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.