AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2021: Question 9

✅ Correct Answer

The delivery tube is dipping into the sulfuric acid (it is below the surface of the liquid).

💡 Key Knowledge

The delivery tube must remain above the surface of the acid. If it sits in the liquid, the gas cannot escape into the syringe freely; instead, building pressure will force liquid up into the tube and block gas collection.

🧠 Exam Technique

Whenever you are asked to identify an error in a gas-collection diagram, check three critical failure points:

• Is the delivery tube pushed down into the liquid?
• Is there a stopper sealing the conical flask?
• Is the gas syringe clamped or open?

❌ Common Errors

Vague answers like "the tube is too long" or "the flask has no lid" score 0. You must specify that the tube is submerged in the acid/solution.

✅ Mark Scheme Breakdown

• Mark 1: The reaction has stopped / finished (allow: no more gas is produced). [1 mark]
• Mark 2: A reactant has been completely used up (allow: zinc or sulfuric acid has run out). [1 mark]

🧠 Exam Technique: "Explain why..."

A 2-mark "explain" question requires two distinct ideas:

1. State what physical event has happened (the reaction stopped).
2. State why at a particle/chemical level (a reactant has run out).

❌ Common Errors

Saying only "all the gas has been collected" or "it reached 60 cm³" does not explain the chemistry. You must state that the reaction has stopped because a reactant is used up.

💡 Key Knowledge

When one reactant is fully consumed (the limiting reactant), collisions between reactants drop to zero and the volume of gas levels off completely (gradient = 0).

✅ Acceptable Answers (Any One)

• The curve for 0.05 mol/dm³ acid is less steep / has a shallower gradient.
• It produces less gas in a given/fixed time (e.g. at 50 s, it has made ~40 cm³ compared to ~60 cm³).
• The reaction takes longer to finish (takes ~200 s vs ~150 s).

❌ Critical Trap: Total Gas vs Rate

Do NOT accept: "It produces less gas in total."

Total gas produced depends on the amount (moles) of limiting reactant, not how fast the reaction occurs. Rate relates only to steepness / gas produced per unit time.

Place a ruler against the solid line at exactly 80 seconds so it just touches the curve without crossing it. Draw a long, straight line extending across the grid.

Pick two points far apart on your tangent line to calculate:
• Vertical change: Δy = y₂ - y₁ (volume in cm³)
• Horizontal change: Δx = x₂ - x₁ (time in s)
Tolerance allowed by examiner: ± 0.5 of a small square.

Rate = gradient = Δy / Δx = (change in volume) / (change in time)

Example reading from a well-drawn tangent at 80 seconds:
Tangent passes through (0 s, 26 cm³) and (160 s, 60 cm³):
Rate = (60 - 26) / (160 - 0) = 34 / 160 = 0.2125 cm³/s
(Accepted ranges typically lie around 0.18 to 0.25 cm³/s depending on ruler placement).

0.2125 cm³/s rounded to 2 s.f. = 0.21 cm³/s

❌ Common Mistakes to Avoid

• Wrong curve: Using the dashed line (0.10 mol/dm³) instead of the solid line (0.05 mol/dm³).
• Small tangent triangle: Drawing a tiny tangent leads to large reading errors. Always draw the tangent line across at least half the grid.
• Forgetting 2 s.f.: Leaving the answer as 0.2125 or 0.2 loses the final mark automatically!

🧠 Exam Technique: Tangent Checklist

Make sure your working is visibly drawn directly on Figure 9. Even if your final number varies slightly, examiners award up to 4 marks for correct tangent lines, correct formula, and rounding.

✅ Correct Answer

Tick: Cu²⁺

💡 Key Knowledge

• Catalysts speed up reactions by providing an alternative pathway with a lower activation energy.
• Transition metals and their ions (such as Copper, Cu²⁺) frequently act as catalysts for redox and acid-metal reactions.
• Al³⁺, Ca²⁺, and Na⁺ are main group metals (Groups 1, 2, and 3) and do not catalyze this reaction.