OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 12

1 mark · Easy difficulty · Multiple Choice

Identify the effect of increasing pressure on the rate of reaction and equilibrium yield of HI for the reaction H2(g) + I2(g) ⇌ 2HI(g).

Practise this question

Question

Question 12 presents a reversible reaction: H2(g) + I2(g) ⇌ 2HI(g). It asks which row is correct when pressure is increased. A table lists four options: Row A shows Rate of reaction increases, Equilibrium yield of HI increases; Row B shows Rate of reaction increases, Equilibrium yield of HI has no effect; Row C shows Rate of reaction increases, Equilibrium yield of HI decreases; Row D shows Rate of reaction decreases, Equilibrium yield of HI has no effect.
Question text

12 Hydrogen and iodine react in the reaction below.

H2(g) + I2(g) 2HI(g)

Which row is correct when pressure is increased?

Rate of reaction Equilibrium yield of HI

A Increases Increases

B Increases No effect

C Increases Decreases

D Decreases No effect

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 12 shows the correct answer is B, awarded 1 mark.

12 B 1

How to answer it

OCR AS Chemistry • Module 3: Physical Chemistry

Effect of Pressure on Reaction Rate and Equilibrium Position

📋 What this question tests

This multiple-choice question assesses your ability to decouple and evaluate the dual effects of changing pressure on:

  • Reaction kinetics: How increased pressure influences the frequency of successful collisions between gas particles.
  • Dynamic equilibria (Le Chatelier's Principle): How changing system pressure affects the position of equilibrium by comparing moles of gaseous reactants and products.
Question 12 • 1 Mark

Analysis of Question 12

Reaction: H₂(g) + I₂(g) ⇌ 2HI(g)

H₂(g) + I₂(g) ⇌ 2HI(g)

✅ Correct Answer

B: Increases | No effect

  • Rate of reaction: Increases
  • Equilibrium yield of HI: No effect
Mark Scheme: Award 1 mark for letter B.

💡 Key Knowledge

  • Rate: Increasing pressure compresses gas into a smaller volume. Particles are closer together (higher concentration), leading to more frequent collisions per second. Rate always increases when pressure rises.
  • Equilibrium Position: Increasing pressure shifts equilibrium toward the side with fewer moles of gas.
  • Count moles of gas:
    • Left side: 1 mol H₂ + 1 mol I₂ = 2 moles of gas
    • Right side: 2 mol HI = 2 moles of gas
  • Since moles of gas are equal on both sides ( 2 = 2 ), a pressure change has no effect on the equilibrium position or yield.

📐 Step-by-Step Decision Process

  1. Assess Column 1 (Rate of reaction):
    Increasing pressure increases collision frequency.
    → Rate increases (eliminates row D).
  2. Assess Column 2 (Equilibrium yield):
    Compare gaseous stoichiometry:
    Left: 1 + 1 = 2 mol(g)
    Right: 2 mol(g)
    → Moles of gas are identical on both sides.
    → No shift in equilibrium position.
    → Yield has no effect (eliminates rows A and C).
  3. Select Row: Only row B matches both criteria.

🧠 Exam Technique & Strategy

  • Split the table: Solve one column at a time to immediately rule out distractor options.
  • Never confuse rate and equilibrium: Rate is about how fast collisions occur; equilibrium yield is about where the final balance lies.
  • Remember: Catalysts and pressure (when Δn = 0) increase the rate of both forward and reverse reactions by the same factor, leaving the equilibrium position unaltered.

❌ Common Errors & Pitfalls

  • Confusing the effect on rate: Assuming that because the equilibrium position doesn't shift, the rate must also have "no effect". Rate depends on collision frequency, which always rises with pressure.
  • Assuming higher pressure always favours products: Students frequently forget to count the stoichiometric coefficients of gaseous species.
  • Misreading states: Always verify state symbols. Only gaseous moles (g) count when applying Le Chatelier's principle to pressure changes (though here all species are gases).

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 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.