AQA A-Level Chemistry AS Paper 1, June 2025: Question 15
1 mark · Easy difficulty · Multiple Choice
Identify which equilibrium reaction results in a lower yield of product when the pressure of the system is increased at constant temperature.
Practise this questionQuestion
Question text
15 Which equilibrium has a lower yield of product when the pressure in the system is
increased at constant temperature?
[1 mark]
A 2NOCl(g) ⇌ 2NO(g) + Cl2(g)
B 2SO2(g) + O2(g) ⇌ 2SO3(g)
C I2(g) + H2(g) ⇌ 2HI(g)
D N2(g) + 3H2(g) ⇌ 2NH3(g)
Mark scheme
Show the mark scheme
15 A 1 (AO2) 2NOCl(g) ⇌ 2NO(g) + Cl2(g)
How to answer it
Equilibria: Effect of Pressure on Product Yield
This question assesses your ability to apply Le Chatelier’s principle to gaseous systems. Specifically, it tests whether you can determine how changing the total pressure influences the position of dynamic equilibrium and hence the yield of product, based on the stoichiometric moles of gaseous reactants versus gaseous products.
Question 15 (Multiple Choice)
Identifying an equilibrium where an increase in pressure reduces product yield
✅ Correct Answer
A — 2 NOCl(g) ⇌ 2 NO(g) + Cl₂(g)
• 1 mark (AO2): Selecting option A.
💡 Key Knowledge
- Le Chatelier’s Principle: If a change in conditions is imposed on a system in equilibrium, the position of equilibrium shifts to oppose that change.
- Effect of Increasing Pressure: Causes the equilibrium to shift toward the side with fewer moles of gas to reduce pressure.
- Lower Yield of Product: For yield of product to decrease, the backward reaction must be favored (i.e. equilibrium shifts to the left). This occurs when the left-hand side has fewer moles of gas than the right-hand side.
📐 Step-by-Step Option Analysis
Count the moles of gas on each side of the equilibrium to predict the shift upon increasing pressure:
| Option | Equation | Moles of Gas (Reactants : Products) | Effect of Increasing Pressure | Yield of Product |
|---|---|---|---|---|
| A | 2 NOCl(g) ⇌ 2 NO(g) + Cl₂(g) | 2 moles : 3 moles (2 + 1) | Shifts left (fewer gas moles on LHS) | Decreases (Lower yield) ✅ |
| B | 2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g) | 3 moles (2 + 1) : 2 moles | Shifts right (fewer gas moles on RHS) | Increases |
| C | I₂(g) + H₂(g) ⇌ 2 HI(g) | 2 moles (1 + 1) : 2 moles | No change in equilibrium position | Unchanged |
| D | N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g) | 4 moles (1 + 3) : 2 moles | Shifts right (fewer gas moles on RHS) | Increases |
🧠 Exam Technique
- Underline key words: Highlight "lower yield" and "pressure is increased". Students often misread "lower yield" as "higher yield".
- Quick annotation: Write the total gas stoichiometric coefficients directly above the reactants and products on your exam paper (e.g. 2 ⇌ 3 for option A).
- Check state symbols: Always confirm all species are gaseous. Solids and liquids do not count toward gas mole totals.
❌ Common Errors & Pitfalls
- Confusing Option B & D: Mistakenly picking familiar industrial processes (Contact Process or Haber Process) where pressure is increased to increase product yield.
- Forgetting to count single coefficients: Overlooking that Cl₂(g) or O₂(g) represents 1 mole of gas when no explicit number precedes it.
- Confusing Option C: Assuming equal moles of gas still result in a shift; in C, equal gas moles mean a pressure change has no effect on equilibrium position or yield.
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, AS Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.