AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2020: Question 7

💡 Key knowledge

• Equilibrium happens in a sealed system.
• It is dynamic: the forward and reverse reactions still happen.
• At equilibrium, the rate of the forward reaction = rate of the reverse reaction.
• Changing pressure or temperature can move the equilibrium position.

🧠 Exam technique

• Use the exact phrase “same rate” for equilibrium.
• For pressure, compare the number of gas molecules on each side.
• For temperature, link your answer to whether the forward reaction is exothermic.
• Always say which side the equilibrium shifts to.

✅ Correct answer

A reversible reaction reaches equilibrium in a sealed system when the forward and reverse reactions happen at exactly the same rate.

💡 Key knowledge

• Dynamic equilibrium means the reaction has not stopped.
• The concentrations of reactants and products stay constant, but not necessarily equal.
• “Sealed system” matters because substances cannot escape.

❌ Common errors

• Saying the reaction stops at equilibrium.
• Writing only “reactants = products” without mentioning rate.
• Forgetting the idea of a sealed system.

✅ Correct answer

As pressure increases, the equilibrium position shifts to the right-hand side, so the percentage of N₂O₄ increases.

This is because there are fewer moles of gas on the right side: 2 NO₂(g) ⇌ N₂O₄(g) .

💡 Key knowledge

• Left side has 2 moles of gas.
• Right side has 1 mole of gas.
• Higher pressure favours the side with fewer gas molecules.

🧠 Exam technique

• Always mention the equilibrium position and the reason.
• Use the equation to compare molecules on each side.
• “Shifts right” is the clearest phrase for full marks.

❌ Common errors

• Saying pressure “increases the rate” without describing the shift.
• Claiming it shifts to the side with more gas molecules.
• Not linking pressure to the percentage of product.

✅ Correct answer

As temperature increases, the equilibrium position shifts to the left-hand side.

This is because the forward reaction is exothermic, so increasing temperature favours the backward endothermic reaction.

Therefore, the percentage of N₂O₄ decreases, which matches Figure 7.

💡 Key knowledge

• Figure 7 shows that the percentage of N₂O₄ goes down as temperature rises.
• For an exothermic forward reaction, heat is like a product.
• Increasing temperature shifts equilibrium to the side that uses up heat — the endothermic side.

🧠 Exam technique

• Use the graph in your answer: “the percentage of N₂O₄ decreases.”
• Make the cause-and-effect chain clear:
  temperature rises → equilibrium shifts left → less N₂O₄.
• Top answers name the reaction type: exothermic or endothermic.

❌ Common errors

• Saying temperature “increases the rate” only, without discussing equilibrium position.
• Mixing up exothermic and endothermic.
• Ignoring the graph and not stating that the percentage of N₂O₄ decreases.

07.1 model answer

In a sealed system, a reversible reaction reaches dynamic equilibrium when the forward and reverse reactions happen at exactly the same rate.

07.2 model answer

Increasing pressure shifts the equilibrium to the right, because there are fewer moles of gas on the right-hand side. This increases the amount of N₂O₄.

07.3 model answer

Increasing temperature shifts the equilibrium to the left because the forward reaction is exothermic. The percentage of N₂O₄ decreases, as shown in Figure 7.

🧠 How to secure marks

• Use the equation: 2 NO₂(g) ⇌ N₂O₄(g)
• Refer to rate in equilibrium questions.
• For pressure, compare the number of gas molecules.
• For temperature, remember: increase temperature → favours endothermic direction.

❌ Top traps from examiner reports

• Using “equal amounts” instead of “equal rates”.
• Forgetting that equilibrium is in a closed/sealed system.
• Saying “pressure pushes the reaction” without explaining the gas mole difference.
• Not using the figure to support the temperature answer.