Edexcel A-Level Chemistry AS Paper 2, June 2019: Question 4

7 marks · Medium difficulty · Short Open Response

Discuss the characteristics of dynamic equilibrium, the effects of temperature and pressure on the yield of methanol, and the role of a catalyst in its industrial manufacture.

Practise this question

Question

Exam question about the industrial manufacture of methanol from carbon monoxide and hydrogen according to the equation CO(g) + 2H2(g) ⇌ CH3OH(g) with ΔH = -90.8 kJ mol⁻¹. Part (a) asks for two characteristics of all reactions at equilibrium (2 marks). Part (b)(i) is a multiple-choice question with a table showing options A to D for how equilibrium yield changes with temperature and pressure (1 mark), and part (b)(ii) asks to explain this answer (2 marks). Part (c) asks to explain why a catalyst is used in the industrial process (2 marks).
Question text

4 Methanol is manufactured from a mixture of carbon monoxide and hydrogen.

CO(g) + 2H (g) U CH OH(g) ¨H = –90.8 kJ mol–1

(a) Give two characteristics of all reactions at equilibrium.

(2)

(b) (i) How does the equilibrium yield of methanol change if the temperature is increased

at constant pressure or the pressure increased at constant temperature?

(1)

Equilibrium yield when Equilibrium yield when

temperature is increased pressure is increased

A decrease decrease

B decrease increase

C increase decrease

D Increase increase

(ii) Explain your answer to (b)(i).

(2)

(c) Explain why, in the industrial process involving this reaction, a catalyst is used.*P55611A01028*

(2)

(Total for Question 4 = 7 marks)

Mark scheme

Show the mark scheme Mark scheme providing acceptable answers and additional guidance for parts 4(a), 4(b)(i), 4(b)(ii), and 4(c), allocating a total of 7 marks for the question.

How to answer it

Equilibrium and Industrial Processes

Edexcel AS Level Chemistry • Physical Chemistry

What this question tests

This question assesses your understanding of dynamic equilibrium characteristics, Le Chatelier's Principle (predicting and explaining the effects of changing temperature and pressure on position of equilibrium), and the role of catalysts in industrial processes.

Part (a) — Characteristics of Equilibrium

Give two characteristics of all reactions at equilibrium. (2 marks)

✅ Correct Answer

  • The concentration (or amount) of all reactants and products remains constant.
  • The rate of the forward reaction is equal to the rate of the backward reaction.

❌ Common Errors

  • Saying concentrations/amounts are equal instead of constant .
  • Stating that the rates of reaction are equal without specifying forward and backward rates.
Mark breakdown: 1 mark for constant concentrations/amounts + 1 mark for equal forward and backward rates.

Part (b)(i) — Multiple Choice: Temperature and Pressure Effects

How does the equilibrium yield of methanol change with temperature and pressure? (1 mark)

✅ Correct Answer

Option B: decrease (temperature) / increase (pressure)

  • Temperature increase: Yield decreases because the forward reaction is exothermic ( ΔH = -90.8 kJ mol⁻¹ ), driving the equilibrium in the endothermic backward direction.
  • Pressure increase: Yield increases because there are fewer moles of gas on the right (1 mole) compared to the left (3 moles), shifting equilibrium to the right.

🧠 Exam Technique

Always inspect the sign of ΔH for temperature questions ( - means exothermic forward) and count total gas moles on each side ( 1 CO + 2 H₂ = 3 moles vs 1 CH₃OH = 1 mole ) for pressure questions.

Mark breakdown: 1 mark for selecting option B.

Part (b)(ii) — Explaining Equilibrium Shifts

Explain your answer to (b)(i). (2 marks)

💡 Key Knowledge (Le Chatelier's Principle)

  • Point 1: An increase in temperature shifts the position of equilibrium in the endothermic direction (backward), reducing the yield.
  • Point 2: An increase in pressure shifts the position of equilibrium to the side with fewer moles of gas molecules, increasing the yield.

❌ Common Errors

  • Forgetting to link the temperature change explicitly to the endothermic/exothermic direction.
  • Referring to "molecules" vaguely without mentioning moles of gas .
Mark breakdown: 1 mark for temperature reasoning (shifts endothermic/left) + 1 mark for pressure reasoning (shifts to fewer moles of gas).

Part (c) — The Role of Catalysts

Explain why, in the industrial process involving this reaction, a catalyst is used. (2 marks)

✅ Correct Answer

  • A catalyst increases the rate at which equilibrium is reached (or decreases the time taken to reach a particular yield) by providing an alternative reaction pathway with a lower activation energy.
  • It allows milder conditions (lower temperatures/pressures) to be used, saving energy and reducing industrial costs.

❌ Common Errors

  • Simply stating "a catalyst increases the rate of reaction" without mentioning rate of attainment of equilibrium .
  • Vaguely stating "it saves money" without explaining that it enables milder operating conditions (lower temperature/pressure).
Mark breakdown: 1 mark for lowering activation energy / increasing rate to reach equilibrium + 1 mark for enabling milder operating conditions / lower costs.

Topics

Physical Chemistry · Topic 10: Equilibrium I · Topic 9: Kinetics I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.