Edexcel A-Level Chemistry AS Paper 2, June 2024: Question 7

6 marks · Medium difficulty · Extended Writing

Discuss, with reasons, the effects of increasing temperature and increasing pressure on the yield of methanol and the rate of reaction for the exothermic reaction of carbon dioxide and hydrogen.

Practise this question

Question

Question text

Methanol can be produced by reacting carbon dioxide and hydrogen in the

exothermic reaction shown.

CO2(g) + 3H2(g) CH3OH(g) + H2O(g)

Discuss, with reasons, the effects of changing the conditions on the yield of methanol

and the rate of reaction.

• increasing temperature at constant pressure

• increasing pressure at constant temperature

(6)

… *P76894A02432*

(Total for Question 7 = 6 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

*7 This question assesses a student’s ability to show a coherent and Guidance on how the mark scheme should be applied: (6)

logically structured answer with linkages and fully-sustained

reasoning. The mark for indicative content should be added to the mark for

lines of reasoning.

Marks are awarded for indicative content and for how the answer is

structured and shows lines of reasoning. For example, an answer with five indicative marking points, which is

partially structured with some linkages and lines of reasoning, scores

The following table shows how the marks should be awarded for 4 marks (3 marks for indicative content and 1 mark for partial

indicative content. structure and some linkages and lines of reasoning).

Number of indicative marking Number of marks awarded for If there are no linkages between points, the same five indicative

points seen in answer indicative marking points marking points would yield an overall score of 3 marks (3 marks for

64 indicative content and no marks for linkages).

5–4 3

3–2 2 In general it would be expected

11 that 5 or 6 indicative points would get 2 reasoning marks, and 3 or 4

00 indicative points would get 1 mark

for reasoning, and 0, 1 or 2

The following table shows how the marks should be indicative points would score zero marks for reasoning.

awarded for structure and lines of reasoning.

Reasoning marks may be reduced for extra incorrect chemistry

Number of marks awarded

for structure of answer and

sustained line of reasoning

Answer shows a coherent and 2

logical structure with linkages and

fully sustained lines of reasoning

demonstrated throughout.

Answer is partially structured with 1

some linkages and lines of

reasoning.

Answer has no linkages between 0

points and is unstructured.

Question

Acceptable Answer Additional Guidance Mark

Number

*7 Indicative content: (6)

contd

IP1 A higher temperature increases the rate of reaction and

decreases the yield of methanol

IP2 The reaction is exothermic in the forward direction so the

backward reaction is favoured by a higher temperature

IP3 A greater proportion of the molecules have more energy Ignore increasing frequency of collisions (as a result of

than the activation energy increased kinetic energy)

IP4 An increase in pressure increases the yield of methanol and

would increase the rate of reaction

IP5 The forward reaction yields fewer moles of gas molecules

IP6 The frequency of collisions increases (because there are

more molecules per unit volume) Ignore there are more collisions without a reference to

frequency or time

(Total for Question 7 = 6 marks)

How to answer it

Equilibrium Yield & Reaction Rate in Methanol Synthesis

What This Question Tests

This 6-mark extended-response question assesses your ability to apply Le Chatelier’s Principle and Collision Theory simultaneously to an industrial reversible reaction:

CO₂(g) + 3H₂(g) ⇌ CH₃OH(g) + H₂O(g) [Forward reaction is exothermic]

Equilibrium Yield: Explaining shifts in equilibrium position when temperature and pressure change.
Reaction Kinetics: Explaining changes in reaction rate using activation energy, collision frequency, and molecular energy distributions.
Structure & Reasoning: Linking causes clearly to effects for all 4 distinct aspects (Temp → Yield, Temp → Rate, Pressure → Yield, Pressure → Rate).

Understanding the Level-of-Response Marking Scheme

This is an asterisked question (*7). Marks are awarded as a combination of Indicative Points (IPs) and Logical Structure/Reasoning.

📊 Indicative Points (Max 4 Marks)

IPs Identified	Marks Awarded
6 points	4
4–5 points	3
2–3 points	2
1 point	1

🧠 Line of Reasoning (Max 2 Marks)

2 Marks: Coherent, logically structured response where every effect is paired directly with its correct chemical reasoning.
1 Mark: Partially structured; some links made, but reasoning is incomplete or disjointed.
0 Marks: Isolated facts listed without linkages.

Part 1: Increasing Temperature at Constant Pressure

You must discuss BOTH the effect on Yield and the effect on Rate

✅ What to State (Indicative Points)

IP1 (Observations): Yield of methanol decreases AND rate of reaction increases.
IP2 (Reason for Yield): The forward reaction is exothermic, so increasing temperature favours the backward (endothermic) reaction to oppose the increase.
IP3 (Reason for Rate): A significantly greater proportion of molecules have kinetic energy greater than or equal to the activation energy ( E ≥ Eₐ ).

❌ Common Examiner Traps

Explaining rate by collision frequency: The mark scheme specifically states: "Ignore increasing frequency of collisions as a result of increased kinetic energy". The dominant reason rate increases with temperature is the exponential increase in particles with E ≥ Eₐ.
Failing to address both: Students frequently explain the yield shift and forget to mention what happens to the rate.

Part 2: Increasing Pressure at Constant Temperature

You must discuss BOTH the effect on Yield and the effect on Rate

✅ What to State (Indicative Points)

IP4 (Observations): Yield of methanol increases AND rate of reaction increases.
IP5 (Reason for Yield): There are fewer moles of gas on the right-hand side (4 moles of reactant gas vs. 2 moles of product gas), so the equilibrium shifts right to oppose the pressure increase.
IP6 (Reason for Rate): Gas particles are compressed into a smaller volume, increasing the number of molecules per unit volume, which increases the frequency of collisions.

❌ Common Examiner Traps

"More collisions" (Vague phrasing): Examiners strictly reject "there are more collisions" unless qualified with time or rate (e.g., "more frequent collisions" or "more collisions per unit time").
Forgetting to state state symbols: Always specify fewer moles of gas, not just "fewer moles".

📐 Stoichiometric Gas Analysis (Why Yield Increases)

Compare the gaseous coefficients on either side of the balanced equation:

Left side: 1 CO₂(g) + 3 H₂(g) = 4 moles of gas
Right side: 1 CH₃OH(g) + 1 H₂O(g) = 2 moles of gas
Conclusion: 4 mol(g) → 2 mol(g). Higher pressure shifts equilibrium towards the side with fewer gas moles (forward reaction).

Full Mark Exemplar Response (6 / 6 Marks)

Notice how each condition is clearly separated into two distinct sub-clauses: one for yield (linked to Le Chatelier) and one for rate (linked to collision theory).

Effect of Increasing Temperature:

Increasing the temperature decreases the yield of methanol [IP1]. This is because the forward reaction is exothermic, so the position of equilibrium shifts to the left in the endothermic direction to oppose the increase in temperature [IP2].
However, increasing the temperature increases the rate of reaction [IP1]. This is because the reactant molecules gain kinetic energy, meaning a significantly greater proportion of molecules have energy greater than or equal to the activation energy ( E ≥ Eₐ ), leading to a higher proportion of successful collisions [IP3].

Effect of Increasing Pressure:

Increasing the pressure increases the yield of methanol [IP4]. There are 4 moles of gas on the reactant side (1 mol CO₂ + 3 mol H₂) and only 2 moles of gas on the product side (1 mol CH₃OH + 1 mol H₂O). The position of equilibrium shifts to the right towards the side with fewer moles of gas molecules to oppose the increase in pressure [IP5].
Increasing the pressure also increases the rate of reaction [IP4]. The gas molecules are pushed closer together, meaning there are more molecules per unit volume, which increases the frequency of collisions between reacting particles [IP6].

💡 Top Revision Takeaway

Always structure 6-mark equilibrium questions using a 2×2 grid in your mind:
1. Temperature on Yield (Exo/Endo shift)
2. Temperature on Rate (Fraction with E ≥ Eₐ)
3. Pressure on Yield (Gas mole comparison)
4. Pressure on Rate (Particles per unit volume & collision frequency)

Topics

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

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.