AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2021: Question 7

✅ Model Answer

• Electrolysis: Uses electricity (electrical energy) to produce a chemical reaction (decomposing an electrolyte). [1 mark]
• Chemical cells: Use a chemical reaction to produce electricity (voltage / electrical current). [1 mark]

🧠 Exam Technique

Always state the direction of energy change for both systems clearly. Use reciprocal phrasing: "Electrolysis uses electricity to cause a reaction; a cell uses a reaction to produce electricity."

✅ Model Answer

2Br⁻ → Br₂ + 2e⁻

or: 2Br⁻ - 2e⁻ → Br₂

💡 Key Knowledge

Bromide ions (Br⁻) are negative anions. They are attracted to the positive electrode (anode) where they lose electrons (oxidation) to form diatomic bromine molecules (Br₂).

❌ Common Errors

• Writing Br instead of diatomic Br₂ .
• Balancing with a single electron: 2Br⁻ → Br₂ + e⁻ (charge must balance!).
• Putting electrons on the wrong side (e.g. 2Br⁻ + 2e⁻ → Br₂ ).

✅ Model Answer

• Copper nitrate: Product at positive electrode = oxygen [1 mark]
• Potassium iodide: Product at positive electrode = iodine [1 mark]
• Potassium iodide: Product at negative electrode = hydrogen [1 mark]

💡 Rules for Aqueous Solutions

• Negative electrode (Cathode): Hydrogen gas is produced unless the metal is less reactive than hydrogen (Cu, Ag, Au). Copper is less reactive, so copper forms. Potassium is more reactive, so hydrogen forms.
• Positive electrode (Anode): Halide ions (Cl⁻, Br⁻, I⁻) form halogens. If no halide is present (e.g. nitrate NO₃⁻, sulfate SO₄²⁻), hydroxide ions (OH⁻) are discharged to form oxygen.

✅ Model Answer (4-Step Method)

1. Filter the mixture/solution from the beaker to collect the fallen copper pieces. [1 mark]
2. Wash and dry the copper residue collected on the filter paper. [1 mark]
3. Weigh the collected dry copper (e.g. using a balance). [1 mark]
4. Add this mass to the increase in mass of the negative electrode. [1 mark]

🧠 Exam Technique

Questions asking to find a total mass when precipitate or solid has fallen off always follow standard filtration and gravimetric steps. Don't forget that wet copper contains water, so drying is essential before weighing!

✅ 07.5: Proportional to Time [1 mark]

Answer: The line of best fit is a straight line passing through the origin (0,0) (for a given current).

✅ 07.6: Proportional to Current [1 mark]

Answer: At any fixed time, when the current doubles, the mass produced doubles.

Supporting data example (at 30 minutes):

• At 0.3 A, mass = 0.18 g
• At 0.6 A, mass = 0.36 g (0.18 × 2 = 0.36 g)

✅ Model Answer

Copper ions (Cu²⁺) are discharged / removed from the solution (they gain electrons at the cathode to form copper atoms, so the concentration of copper ions decreases).

❌ Common Errors

• Saying "copper is being used up" without specifying copper ions or the solution.
• Claiming the water evaporates or dilutes the solution.

❌ Common Calculation Traps

• Reading the wrong curve: Reading the 0.3 A or 0.9 A line instead of 0.6 A.
• Premature rounding: Rounding to 0.0038 mol too early can lead to rounding discrepancies. Keep calculator values until the final step.
• Significant figures penalty: Forgetting the instruction to quote the answer to 3 significant figures loses the final mark.

🧠 Exam Technique: Error Carried Forward (ECF)

If you misread the graph (e.g. reading 0.25 g instead of 0.24 g), you can still receive the remaining 2 marks if your mole calculation and multiplication by Avogadro's constant are executed correctly to 3 sig figs.