AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2021: Question 6

✅ Correct answer

Bubble the gas through limewater or calcium hydroxide solution. If carbon dioxide is not present, the limewater stays colourless / does not turn milky (cloudy).

💡 Key knowledge

• Carbon dioxide is identified using limewater.
• The positive test is a milky/cloudy appearance.
• No colour change means carbon dioxide is not present.

🧠 Exam technique

For 2 marks, give the test and the result. A complete answer might be: Use limewater. It stays colourless / does not turn milky.

❌ Common errors

• Writing only “test it” without naming limewater.
• Saying “it turns cloudy” — that is the result for carbon dioxide present.
• Confusing carbon dioxide with oxygen tests.

✅ Correct answer

A catalyst provides an alternative pathway with a lower activation energy.

💡 Key knowledge

• Lower activation energy means more particles can react.
• The catalyst is not used up.
• It speeds up both the forward and reverse reactions.

🧠 Exam technique

This is a classic 2-mark definition. To get both marks, include both ideas: alternative pathway + lower activation energy.

❌ Common errors

• Saying “it makes particles move faster” — that is not how a catalyst works.
• Writing only “speeds up the reaction” without explaining why.
• Suggesting the catalyst changes the products.

✅ Correct answer

In a closed system, the apparatus prevents the escape of reactants and products. Equilibrium is reached when the forward and reverse reactions happen at the same rate.

💡 Key knowledge

• Equilibrium only happens in a closed system.
• Reactants and products must not escape.
• At equilibrium, concentrations stay constant because the two rates are equal.

🧠 Exam technique

Marks are for the closed system idea and the equal rates idea. You do not need to say the amounts are equal — only the rates are equal.

❌ Common errors

• Saying “the reactions stop” — they do not; it is dynamic.
• Not mentioning the system is closed.
• Confusing equilibrium with the point where there are equal amounts of reactants and products.

✅ Correct answer

The equilibrium position stays the same / has no effect on the equilibrium position, because the catalyst increases the rate of the forward and reverse reactions by the same amount.

💡 Key knowledge

• A catalyst does not change the final equilibrium position.
• It only helps equilibrium be reached faster.
• Forward and reverse reactions are both sped up equally.

🧠 Exam technique

A 2-mark answer needs: no change to equilibrium position + same effect on forward and reverse rates.

❌ Common errors

• Thinking a catalyst increases yield.
• Writing that it shifts equilibrium to the right.
• Using temperature language instead of catalyst language.

✅ Correct answer

The yield of ammonia increases.

💡 Key knowledge

• For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), there are 4 moles of gas on the left and 2 moles of gas on the right.
• Increasing pressure favours the side with fewer gas molecules.
• That means equilibrium shifts to the right, making more ammonia.

🧠 Exam technique

The question asks for the effect, so the single best answer is: the yield of ammonia increases . If you can, link it to fewer gas molecules for secure understanding.

❌ Common errors

• Choosing “stays the same”.
• Mixing up pressure with temperature.
• Forgetting that ammonia is on the side with fewer gas moles.

✅ Correct answer

The yield of ammonia decreases because the system shifts in the endothermic direction to counteract the increase in temperature.

💡 Key knowledge

• The forward reaction is exothermic, so heat is a product.
• Increasing temperature favours the endothermic reverse reaction.
• This shifts equilibrium to the left, so less ammonia is made.

🧠 Exam technique

To get both marks, state the change in yield and the reason linked to equilibrium. A strong answer says the system shifts to the endothermic direction to counteract the change.

❌ Common errors

• Saying the yield increases — that is wrong for an exothermic forward reaction.
• Just saying “it speeds up” without mentioning equilibrium shift.
• Confusing endothermic and exothermic directions.

✅ Short model answers

• 06.1: Use limewater. It stays colourless / does not turn milky.
• 06.2: It provides an alternative pathway with lower activation energy.
• 06.3: In a closed system, equilibrium is when the forward and reverse reactions happen at the same rate.
• 06.4: No effect on equilibrium position because it speeds up both reactions equally.
• 06.5: The yield of ammonia increases.
• 06.6: The yield decreases because the system shifts to the endothermic direction.

🧠 How marks were awarded

• One mark for naming the correct test or prediction.
• One mark for the correct science explanation.
• In longer answers, top responses used precise equilibrium language: closed system, same rate, no effect, endothermic direction.

❌ Biggest traps in this question

• Thinking catalysts change yield or equilibrium position.
• Confusing the effect of pressure and temperature.
• Forgetting that equilibrium is dynamic, not stopped.
• Not using the exact test result for carbon dioxide.