AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2024: Question 5

Key facts to remember

• Equilibrium means the forward and reverse reaction rates are equal.
• Changing conditions can shift the position of equilibrium.
• Catalysts speed up both directions equally and do not change equilibrium position.

How marks are awarded

• Most 1-mark parts need the exact direction of shift.
• For explanation questions, give the reason as well as the effect.
• For graph questions, use the data carefully and explain the plateau.

✅ Correct answer

198 kJ/mol

1 mark for the correct value.

💡 Key knowledge

• If the forward reaction releases 198 kJ/mol, the reverse reaction takes in the same amount.
• The magnitude stays the same; only the direction changes.

❌ Common errors

• Choosing < 198 or > 198 .
• Writing the wrong sign and losing the mark in a multiple-choice style question.

✅ Correct answer

Equilibrium position shifts to the right

1 mark for the correct direction.

💡 Key knowledge

• Increasing the concentration of a reactant makes the system use up that reactant.
• Here, more O₂ means the forward reaction is favoured, producing more SO₃.

🧠 Exam technique

Use Le Chatelier’s principle: the equilibrium shifts to oppose the change. More O₂ added → shift to the side that removes O₂.

❌ Common errors

• Saying “no change” when a reactant concentration has changed.
• Mixing up left and right.

✅ Correct answer

Equilibrium position shifts to the left

1 mark for the correct direction.

💡 Key knowledge

• At lower pressure, equilibrium shifts to the side with more gas molecules.
• Left side has 3 moles of gas: 2SO₂ + O₂ .
• Right side has 2 moles of gas: 2SO₃ .

🧠 Exam technique

Count the total number of moles of gas on each side before deciding the direction.

❌ Common errors

• Thinking lower pressure always means “towards fewer moles” — it is actually towards more moles.
• Ignoring the coefficients in the equation.

✅ Correct answer

Equilibrium position shifts to the left

1 mark for the correct direction.

💡 Key knowledge

• The forward reaction is exothermic, so heat is treated like a product.
• Increasing temperature favours the endothermic direction, which is the reverse reaction.
• So equilibrium shifts left.

🧠 Exam technique

A good short explanation is: “Temperature increases, so the endothermic reverse reaction is favoured.”

❌ Common errors

• Assuming higher temperature always means “more product”.
• Forgetting that the forward reaction is exothermic.

✅ Correct answer

Effect: equilibrium position does not change

Reason: it increases the rate of the forward reaction and reverse reaction equally

2 marks: 1 for the effect, 1 for the reason.

💡 Key knowledge

• A catalyst lowers activation energy.
• It helps both directions more quickly.
• It does not change the equilibrium position or the equilibrium constant.

🧠 Exam technique

The examiner wanted both parts: the equilibrium position stays the same, and the catalyst speeds up both directions equally.

❌ Common errors

• Saying a catalyst shifts equilibrium to one side.
• Only saying “it speeds up the reaction” without explaining both directions.

✅ Correct answer

Time: 210 s
Accepted range: 205 to 210 s

3 marks: time + lines level/horizontal + explanation.

💡 Key knowledge

• At equilibrium, the amounts of all substances stay constant.
• On a graph, this means the lines become horizontal/level.
• The point where all three lines flatten is around 210 s.

🧠 Exam technique

• Quote the time from the graph first.
• Then state that the lines become level.
• Finish with the equilibrium reason: forward rate = reverse rate, or the number of moles stays constant.

❌ Common errors

• Giving the time when one line starts to change more slowly, instead of when all lines level off.
• Forgetting the explanation.
• Stating “reaction stops” — it does not; equilibrium is dynamic.

📐 Calculations — step by step

1. Draw a tangent to the curve at 60 s .
2. Choose two clear points on the tangent.
3. Find the change in y divided by the change in x:

rate = change in number of moles ÷ change in time

Units should be mol/s .

Awarding marks: tangent drawn, correct coordinates from tangent, correct calculation, correct units.

✅ Correct answer

Rate = 0.04 mol/s approximately

Any sensible value from a good tangent is acceptable.

💡 Key knowledge

• Rate on a graph = gradient of the tangent.
• Use a large triangle on the tangent for accuracy.
• Read coordinates carefully from the grid.

🧠 Exam technique

The mark scheme allows the rate to be found from any correctly read tangent values. The exact answer can vary slightly because different tangents and points may be used.

❌ Common errors

• Using the curve itself instead of a tangent at 60 s.
• Using a secant line across two points on the curve.
• Forgetting units or giving mol instead of mol/s .
• Reversing the calculation and doing x/y instead of y/x .

Top-level features

• Correct use of equilibrium language: “shifts to the left/right”.
• Clear reasons using concentration, pressure, temperature, and catalyst ideas.
• For graph questions, students quoted the graph and explained the plateau properly.

How to secure full marks

• Learn the gas-mole rule for pressure changes.
• Remember: catalysts do not change equilibrium position.
• For rates, use a tangent and show your working clearly.