AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2024: Question 7
8 marks · Standard Demand difficulty · Short Answer
This question covers the testing for sulfate ions, the effect of temperature and pressure on the equilibrium yield of sulfur trioxide in the Contact process, and the identification of the catalyst used.
Practise this questionQuestion
Question text
07 This question is about sulfuric acid.
07.1 Sulfuric acid contains sulfate ions.
Describe the test for the presence of sulfate ions in sulfuric acid.
Give the result of the test.
[2 marks]
Test
Result
One stage in the industrial production of sulfuric acid is the reaction of sulfur dioxide
with oxygen to produce sulfur trioxide.
This reversible reaction reaches dynamic equilibrium.
Figure 7 shows the percentage yield of sulfur trioxide in this reaction at different
temperatures.
Figure 7
07.2 Which statement about the forward reaction is correct?
Use Figure 7.
*26* [1 mark]
Tick ( ) one box.
The yield is greater at higher temperatures because the reaction is
exothermic.
The yield is greater at higher temperatures because the reaction is
endothermic.
The yield is smaller at higher temperatures because the reaction is
exothermic.
The yield is smaller at higher temperatures because the reaction is
endothermic.
The equation for the reaction is:
2 SO2(g) + O2(g) ⇌ 2 SO3(g)
07.3 Explain why the percentage yield of sulfur trioxide in this reaction is greater if the
pressure is higher.
[2 marks]
07.4 In industry, the reaction is done at 450 °C and atmospheric pressure.
Under these conditions the yield of sulfur trioxide is 86%.
Suggest two reasons why a higher pressure is not used.
[2 marks]
07.5 This reaction uses a catalyst to increase the rate of the reaction.
*27* The catalyst is a metal oxide.
Which is the most likely metal in the metal oxide catalyst?
Use the periodic table.
[1 mark]
Tick ( ) one box.
Aluminium (Al)
Barium (Ba)
Potassium (K)
Vanadium (V)
Mark scheme
Show the mark scheme
Question 7
AO /
Question Answers Extra information Mark
Spec. Ref.
07.1 ignore (hydrochloric / nitric / AO1
sulfuric) acid 4.8.3.5
(test) add barium chloride allow add barium nitrate 1
(solution) (solution)
(result) white precipitate 1
MP2 is dependent upon MP1
being awarded
AO /
Spec. Ref.
07.2 the yield is smaller at higher 1 AO2
temperatures because the 4.6.2.6
reaction is exothermic
AO /
Spec. Ref.
07.3 allow converse AO2
4.6.2.7
there are more moles / ignore particles 1
molecules (of gas) on the left
(so the position of) equilibrium – – 1 –
shifts to the right
AO /
Spec. Ref.
07.4 any two from: 2 AO3
• the yield is already high 4.6.2.7
• more energy required
• risk of explosion is increased
• higher income from increased
yield is outweighed by the
extra expenditure
• increased cost of safety allow requires stronger vessels
precautions allow requires thicker walls 23
AO /
Spec. Ref.
07.5 vanadium (V) 1 AO3
4.1.3.2
Total Question 7 8
How to answer it
Sulfuric Acid, Ion Tests and Equilibrium
What this question tests
This question assesses your ability to recall chemical tests for negative ions (anions), interpret graphical data regarding reversible reactions, and apply Le Chatelier's Principle to explain how changes in temperature and pressure affect the yield of a product in an industrial process.
Testing for Sulfate Ions
💡 Key Knowledge
- Sulfate ions (SO₄²⁻) are identified using Barium Chloride solution.
- The solution must be acidified (usually with Hydrochloric acid) to remove any carbonate ions that might give a false positive.
✅ Correct Answer
Test: Add barium chloride (solution).
Result: White precipitate.
❌ Common Errors
Students often confuse the test for sulfates with the test for halides (Silver Nitrate). Remember: Sulfate uses Barium (S-B). Also, you cannot get the mark for the "white precipitate" if you name the wrong reagent first!
Temperature and Yield
🧠 Exam Technique: Reading the Graph
Look at Figure 7. As the temperature increases (moving right on the x-axis), the percentage yield goes down (moving down on the y-axis).
In equilibrium, if increasing the temperature decreases the yield, the forward reaction must be exothermic because the system is trying to oppose the heating by shifting in the endothermic (reverse) direction.
✅ Correct Answer
Tick the third box: "The yield is smaller at higher temperatures because the reaction is exothermic."
Pressure and Equilibrium
2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g)
📐 Step-by-Step Logic
- Count the gas molecules on the left: 2 (from SO₂) + 1 (from O₂) = 3 moles.
- Count the gas molecules on the right: 2 moles (from SO₃).
- Apply the rule: Increasing pressure shifts the equilibrium to the side with fewer molecules to reduce the pressure.
- Conclusion: Equilibrium shifts to the right, increasing the yield of SO₃.
✅ Correct Answer
- There are more moles/molecules (of gas) on the left hand side.
- So the position of equilibrium shifts to the right.
Industrial Considerations
🧠 Exam Technique: "Why NOT?" Questions
When an exam asks why a "better" condition (like higher pressure) isn't used in a factory, think about Money and Safety.
✅ Correct Answer (Any two from:)
- The yield is already high enough (86%).
- Higher pressure is dangerous / increased risk of explosion.
- It is expensive to build thick-walled pipes/vessels to withstand high pressure.
- High energy costs to maintain high pressure.
Catalysts in the Contact Process
💡 Key Knowledge
The production of sulfuric acid is known as the Contact Process. The specific catalyst used is Vanadium(V) Oxide (V₂O₅).
✅ Correct Answer
Tick the box for: Vanadium (V)
🧠 Examiner Insight
Even if you didn't remember the specific process, catalysts are almost always transition metals. Looking at the Periodic Table, Vanadium is the only transition metal in the list provided (Aluminium is Group 3, Barium is Group 2, Potassium is Group 1).
Topics
Chemistry
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.