AQA A-Level Chemistry Paper 3, 2019: Question 12

1 mark · Medium difficulty · Multiple Choice

Identify which change increases the equilibrium yield of ammonia in the Haber process without affecting the value of the equilibrium constant Kp.

Practise this question

Question

Multiple-choice question 12: Nitrogen reacts with hydrogen in the exothermic reaction N2(g) + 3H2(g) equilibrium arrow 2NH3(g). Which change increases the equilibrium yield of ammonia but has no effect on the value of the equilibrium constant Kp? Option A: Add a catalyst; Option B: Increase the partial pressure of nitrogen; Option C: Decrease the temperature; Option D: Decrease the total pressure.
Question text

12 Nitrogen reacts with hydrogen in this exothermic reaction

N2(g) + 3H2(g) ⇌ 2NH3(g)

Which change increases the equilibrium yield of ammonia but has no effect on the

value of the equilibrium constant Kp?

[1 mark]

A Add a catalyst

B Increase the partial pressure of nitrogen

C Decrease the temperature

D Decrease the total pressure

Mark scheme

Show the mark scheme Mark scheme for question 12 showing that the correct option is B, worth 1 mark.

12 B 1

How to answer it

Equilibrium Yield vs. Equilibrium Constant (Kp)

What this question tests

This multiple-choice question tests your understanding of Le Chatelier’s Principle and the fundamental factors that influence the equilibrium constant (Kp). Specifically, you must distinguish between changes that shift the position of equilibrium (altering product yield) and the sole factor (temperature) that changes the numerical value of Kp.

Question 12

Effect of Conditions on Haber Process Equilibrium

Reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)  |  ΔH < 0 (Exothermic)

✅ Correct Answer: B

Increase the partial pressure of nitrogen

  • Yield of NH₃: Adding more reactant increases p(N₂). By Le Chatelier’s Principle, the system opposes this increase by shifting to the right (forward direction), increasing the yield of NH₃.
  • Value of Kp: Temperature remains unchanged, so the value of Kp does not change. The partial pressures adjust to restore the ratio to the constant value of Kp.

💡 Key Knowledge

  • The Golden Rule: The value of the equilibrium constant (Kc or Kp) is affected ONLY by temperature.
  • Concentration, partial pressure, total pressure, and catalysts have NO effect on the value of Kp.
  • Catalysts: Increase the rate of both forward and backward reactions equally. They speed up the approach to equilibrium but have no effect on position of equilibrium or yield.

🧠 Step-by-Step Option Elimination

  • Option A (Add a catalyst): Increases rate only. Has no effect on yield of NH₃ and no effect on Kp. ❌ Incorrect (yield does not increase).
  • Option B (Increase partial pressure of nitrogen): Shifts equilibrium to the right, increasing the yield of NH₃. Temperature is constant, so Kp is unchanged. ✅ Correct!
  • Option C (Decrease the temperature): Since the forward reaction is exothermic, decreasing temperature shifts equilibrium to the right (increasing yield). HOWEVER, changing temperature alters the numerical value of Kp (Kp increases). ❌ Incorrect (it affects Kp).
  • Option D (Decrease the total pressure): The left side has 4 gas moles, the right has 2 gas moles. Decreasing pressure shifts the equilibrium to the side with more moles (left), which decreases the yield of NH₃. ❌ Incorrect.

❌ Common Student Traps

  • Falling for Option C: Many students instantly select "decrease temperature" because they know it increases the yield of an exothermic reaction, forgetting the second half of the question stem: "has no effect on the value of Kp".
  • Believing pressure changes Kp: A common misconception is that changing pressure alters Kp because partial pressures change. In reality, the ratio of partial pressures shifts precisely to maintain the constant value of Kp.
Mark Scheme: Option B — 1 Mark (AO1 / AO2 recall and application).

Topics

Physical Chemistry · 3.1.6 Chemical Equilibria, Le Chatelier's Principle and Kc · 3.1.10 Equilibrium Constant Kp

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.