AQA A-Level Chemistry AS Paper 1, November 2021: Question 9
6 marks · Medium difficulty · State/Explain/Describe
Explain, in terms of equilibrium yield and cost, why specific conditions of temperature, pressure, and a nickel catalyst are used in the industrial preparation of hydrogen from methane and steam.
Practise this questionQuestion
Question text
09 Hydrogen can be prepared on an industrial scale using the reversible reaction
between methane and steam.
CH (g) + H O(g) ⇌ CO(g) + 3 H (g) ΔH = +206 kJ mol−1
42 2
The reaction is done at a temperature of 800 °C and a low pressure of 300 kPa in the
presence of a nickel catalyst.
Explain, in terms of equilibrium yield and cost, why these conditions are used.
[6 marks]
Mark scheme
Show the mark scheme
Question Marking guidance Additional Comments/Guidelines Mark
This question is marked using levels of response. Refer to the Mark Indicative chemistry content
Scheme Instructions for Examiners for guidance on how to mark this
question. Stage 1: Temperature
Level 3: 1a. The reaction is endothermic (so equilibrium
All stages are covered and the explanation of each stage is shifts to RHS to reduce the temperature)
generally correct and virtually complete. 1b. So, higher temperature increases the yield
1c. High temperatures are costly (so
Answer is well structured with no repetition or irrelevant 5-6
compromise temperature used)
points.
Accurate and clear expression of ideas with no errors in use Stage 2: Pressure
of technical terms. 2a. More moles of gas on the right hand side,
Level 2: (so equilibrium shifts to RHS to increase the
All stages are covered but the explanation of each stage yield)
may be incomplete or may contain inaccuracies OR two 2b. So, lower pressure increases the yield
stages are covered and the explanations are generally 2c. A low pressure means a low cost
09 correct and virtually complete. 3-4 6
Stage 3: Catalyst
Answer shows some attempt at structure.
3a. Catalyst has no effect on yield
Ideas are expressed with reasonable clarity with, perhaps,
some repetition or some irrelevant points. 3b. Adding a catalyst allows a lower
Some minor errors in use of technical terms. temperature to be used
3c. So, this lowers the cost
Level 1:
Two stages are covered but the explanation of each stage
may be incomplete or may contain inaccuracies, OR only
one stage is covered but the explanation is generally correct
and virtually complete.
1-2
Answer includes isolated statements but these are not
presented in a logical order or show some confusion.
Answer may contain valid points which are not clearly linked
to an argument structure. Errors in the use of technical
terms.
– MISTRY – – JUNE 2021
Level 0
Insufficient correct chemistry to gain a mark.
How to answer it
Industrial Production of Hydrogen (Le Chatelier's Principle & Economics)
What this question tests
This is a 6-mark extended-response (levels of response) question assessing your understanding of chemical equilibria, Le Chatelier's principle, and the crucial economic compromises required in industrial chemical processes (Haber/Contact process style reasoning applied to steam reforming).
Equilibrium Yield, Cost, and Compromise Conditions
💡 Key Knowledge: Temperature
- The forward reaction is endothermic ( ΔH = +206 kJ mol⁻¹ ).
- By Le Chatelier's principle, increasing temperature shifts the position of equilibrium to the right (RHS) to absorb added heat.
- This increases the equilibrium yield of hydrogen.
- Economic factor: High temperatures require substantial energy input (fuel/heating costs), so a compromise temperature (800 °C) is used.
💡 Key Knowledge: Pressure
- ">
- Count moles of gas: 2 moles on the left ( CH₄ + H₂O ) vs 4 moles on the right ( CO + 3 H₂ ).
- Decreasing pressure shifts equilibrium to the side with more moles of gas (RHS).
- Therefore, low pressure increases the equilibrium yield.
- Economic factor: Low pressures reduce equipment/pumping costs and safety risks, but slow down the rate of reaction.
💡 Key Knowledge: Catalyst
- Adding a nickel catalyst has no effect on the position of equilibrium or the equilibrium yield.
- Economic factor: It speeds up the rate of reaction, allowing the process to run at a lower temperature (saving energy and financial cost) while achieving an acceptable rate.
🧠 Exam Technique (Levels of Response)
- To hit Level 3 (5–6 marks), you must cover all three stages (Temperature, Pressure, Catalyst) and explicitly link each condition to both equilibrium yield AND cost/rate.
- Avoid isolated bullet points; structure your response logically with clear chemical terminology.
✅ Model Answer Structure
Stage 1 (Temperature): The forward reaction is endothermic, so a higher temperature shifts equilibrium to the right, increasing yield. However, high temperatures are costly, so an 800 °C compromise is used.
Stage 2 (Pressure): There are more moles of gas on the right (4 vs 2), so a lower pressure shifts equilibrium to the right, increasing yield and lowering equipment costs.
Stage 3 (Catalyst): The nickel catalyst does not change the yield, but it increases the rate of reaction, enabling a lower temperature to be used which reduces overall costs.
❌ Common Errors & Pitfalls
- Confusing endothermic/exothermic: Stating the reaction is exothermic loses credit for the temperature argument.
- Molar volume mix-ups: Incorrectly counting gas moles or stating that high pressure favors the side with more moles.
- Missing the economic link: Describing *what* happens to equilibrium without mentioning *why* industrial plants choose these specific operating costs.
• Level 3 (5–6 marks): All stages covered with generally correct and complete explanations of yield and cost.
• Level 2 (3–4 marks): All stages covered with minor omissions/inaccuracies, OR two stages fully correct.
• Level 1 (1–2 marks): Two stages covered superficially or only one stage fully explained.
• Level 0 (0 marks): Insufficient correct chemistry.
Topics
Physical Chemistry · 3.1.5 Kinetics · 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, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.