OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 12

1 mark · Medium difficulty · Multiple Choice

Determine the conditions of temperature and pressure that produce the greatest equilibrium yield of hydrogen for the given reversible reaction.

Practise this question

Question

Multiple choice question 12 showing a reversible reaction of methane and steam to produce carbon monoxide and hydrogen with an enthalpy change of plus 206 kilojoules per mole. Four options A, B, C, and D provide combinations of low or high temperature and pressure.
Question text

12 Hydrogen gas can be produced as shown below.

CH (g) + H O(g) CO(g) + 3H (g) ∆H = +206 kJ mol−1

42 2

Which conditions produce the greatest equilibrium yield of hydrogen?

A Low temperature and high pressure

B Low temperature and low pressure

C High temperature and high pressure

D High temperature and low pressure

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is option D for question 12, worth 1 mark.

12 D 1 1.1

How to answer it

Predicting Equilibrium Yields Using Le Chatelier's Principle

What this question tests

This question assesses your understanding of Le Chatelier's Principle and how changes in temperature and pressure affect the position of dynamic equilibrium. Specifically, you must interpret enthalpy change signs (AH) and gas mole stoichiometries to maximize the yield of a target product (hydrogen gas, H₂).

Question Part 12

Determining Optimum Equilibrium Conditions

✅ Correct Answer

D (High temperature and low pressure)

Mark Allocation: 1 mark for selecting D.

💡 Key Knowledge

  • Enthalpy Change (AH): AH = +206 kJ mol⁻¹ indicates the forward reaction is endothermic. Therefore, increasing the temperature shifts the equilibrium in the forward direction to absorb the added heat.
  • Gas Moles: Count the stoichiometric coefficients of gases on both sides. Left side: 1(CH₄) + 1(H₂O) = 2 moles of gas. Right side: 1(CO) + 3(H₂) = 4 moles of gas.
  • Pressure Effect: Decreasing the pressure shifts the equilibrium toward the side with more moles of gas (the right side) to increase the pressure.

🧠 Exam Technique

Break the multiple-choice question down into two separate decisions before looking at the options:

  1. Decide temperature: Endothermic wants high temp. Eliminate options A and B.
  2. Decide pressure: More moles on the product side wants low pressure. Eliminate option C.
  3. Match remaining features to find option D.

❌ Common Errors

  • Misinterpreting the positive sign of AH as favoring low temperatures. Remember: Positive = Endothermic = Favoured by high temperature.
  • Forgetting to count reactant states (all species shown are gases, (g) , so all must be included in the mole ratio total of 2 vs 4).

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), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.