AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2018: Question 6

✅ Correct answers

Use damp or moist litmus paper.

Chlorine bleaches the paper, so it goes white.

💡 Key knowledge

• Chlorine is a bleaching agent.
• The paper must be damp/moist because chlorine needs water to show its bleaching effect properly.
• Ignore the initial colour of the litmus paper — the key result is that it turns white.

🧠 Exam technique

• Write both the test and the result.
• Say “damp litmus paper”, not just “litmus paper”.
• For full marks, include the word bleaches or goes white.

❌ Common errors

• Saying chlorine turns litmus red — that is not the key test here.
• Forgetting the paper must be damp.
• Giving only the test or only the result.

✅ Correct answer

The forward and reverse reaction rates become equal because it is a closed system, so no reactants or products can escape.

💡 Key knowledge

• At equilibrium, the reaction is still happening, but it is dynamic.
• Forward rate = reverse rate.
• A sealed flask is a closed system.

🧠 Exam technique

• Use the phrase “rates are equal” for one mark.
• Add “in a closed system” for the second mark.
• If you say “the reaction stops”, that loses credit — equilibrium is dynamic.

❌ Common errors

• Saying the reaction is finished.
• Only saying “it’s sealed” without linking to no escape of reactants/products.
• Mixing up equilibrium with same amounts — the key idea is equal rates, not equal amounts.

✅ Correct answer

The equilibrium shifts to the right-hand side to produce more products and/or use up reactants.

A new equilibrium is then established.

💡 Key knowledge

• Equation: NaClO(aq) + 2HCl(aq) ⇌ NaCl(aq) + H₂O(l) + Cl₂(g)
• Adding HCl increases the concentration of a reactant.
• Le Chatelier: the system responds to counteract the change.

🧠 Exam technique

• Explain the answer in a chain: more HCl added → equilibrium shifts right → more chlorine produced → new equilibrium.
• The mark scheme accepts “in favour of the forward reaction”.
• Use the exact language of equilibrium if possible: “shifts”, not “moves faster” alone.

❌ Common errors

• Saying the equilibrium shifts left.
• Only stating that “more chlorine is made” without explaining why.
• Forgetting to mention that a new equilibrium is reached.

✅ Correct answer

The amount of chlorine gas increases.

The system shifts in the endothermic direction to counteract the temperature increase.

💡 Key knowledge

• Forward reaction: 2HCl(g) ⇌ H₂(g) + Cl₂(g)
• The question says the forward reaction is endothermic.
• Increasing temperature favours the endothermic direction.

🧠 Exam technique

• State the effect first: “more chlorine is produced”.
• Then explain with Le Chatelier: the system shifts to take in energy.
• This is a classic 2-mark explain question: one mark for prediction, one for reasoning.

❌ Common errors

• Saying temperature increase always shifts equilibrium right — it depends on whether the forward reaction is endothermic or exothermic.
• Writing only “more products” without naming chlorine.
• Confusing temperature with pressure.

✅ Correct answer

No change.

This is because there are equal numbers of gas molecules on each side of the equation.

💡 Key knowledge

• In 2HCl(g) ⇌ H₂(g) + Cl₂(g), there are 2 moles of gas on the left and 2 moles of gas on the right.
• Pressure only shifts equilibrium if the number of gas moles is different on each side.

🧠 Exam technique

• Count the gas molecules before you answer.
• Say “equal numbers of moles of gas” for the mark.
• Because both sides are equal, increasing pressure does not favour either side.

❌ Common errors

• Thinking higher pressure always means “more products”.
• Forgetting to count only gases — ignore solids and liquids.
• Saying pressure changes the position even when both sides have the same number of gas particles.

📐 How to secure the marks

• 06.1: name the test + result.
• 06.2: equal forward and reverse rates + closed system.
• 06.3: shift right + more products / less reactants + new equilibrium.
• 06.4: more chlorine + shifts to absorb heat.
• 06.5: no change + equal gas moles on both sides.

🧠 Final exam tip

For equilibrium questions, always use the language of shifts, rates, and closed systems. The examiner is looking for the exact ideas, not long explanations.