AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2020: Question 4

✅ Mark-Winning Response

• Chromate ions (CrO₄²⁻) move to the positive electrode [1 mark]
• Because opposite charges attract (or negative ions are attracted to the positive electrode) [1 mark]

💡 Key Knowledge

• Anions are negatively charged; they migrate to the anode (positive electrode).
• The prompt states: CrO₄²⁻ ions are yellow. Their concentration increases near the positive electrode, causing the yellow colour.

🧠 Exam Technique

Two-mark "explain" questions require a what and a why:

1. What moves? Chromate / CrO₄²⁻ / yellow ions.
2. Why does it move? Electrostatic attraction (opposite charges attract).

❌ Common Errors

• Saying "the solution turned yellow because it reacted" without naming the migrating ion.
• Confusing the charges and claiming positive copper ions went to the positive electrode.

✅ Mark-Winning Response

Water [1 mark]

💡 Key Knowledge

In any aqueous solution, water molecules naturally self-ionise in small amounts:

H₂O(l) ⇌ H⁺(aq) + OH⁻(aq)

This provides both H⁺ and OH⁻ ions alongside the dissolved salt ions.

🧠 Exam Technique

Look at the prompt carefully: "Name the substance in the solution...". It asks for the parent chemical compound, not the ion itself or the full mixture name.

❌ Common Errors

• Writing "copper chromate solution" (the mark scheme explicitly says ignore this).
• Writing "oxygen" or "air" instead of water.

✅ Mark-Winning Response

Option A (Word explanation):

• Copper ions gain two electrons [2 marks]
• To form copper atoms [1 mark]

Option B (Half-equation):

Writing this balanced equation scores all 3 marks directly!

💡 Key Knowledge

• Cathode rule: Reduction happens at the negative electrode (gain of electrons: OIL RIG).
• Cu²⁺ has a 2+ charge, so each ion must gain exactly 2 electrons to become a neutral copper atom (the solid coating).

🧠 Exam Technique

Writing the half-equation is the quickest, most foolproof way to earn all 3 marks. If explaining in words, remember: ions gain electrons to become atoms.

❌ Common Errors

• Saying "copper ions gain electrons" without specifying two electrons (scores only 1 mark).
• Saying copper ions are "oxidised" (automatically loses electron-gain marks).
• Confusing copper atoms and copper ions (e.g. stating "copper atoms gain electrons").

✅ Mark-Winning Response

• Negative electrode: Hydrogen (or H₂) [1 mark]
• Positive electrode: Iodine (or I₂) [1 mark]

💡 Aqueous Electrolysis Rules

• At the Negative Electrode (Cathode):
  Compare the metal to Hydrogen:
  Potassium is more reactive than hydrogen → Hydrogen gas (H₂) is discharged.
• At the Positive Electrode (Anode):
  Check for halide ions (Cl⁻, Br⁻, I⁻):
  Halide present (I⁻) → Halogen is formed: Iodine (I₂).

🧠 Exam Technique

Always write down the 4 ions present before choosing:

K⁺, I⁻, H⁺, OH⁻

Then apply the discharge hierarchy rules step by step.

❌ Common Errors

• Writing potassium at the negative electrode (a classic trap! Potassium never forms in aqueous solution because it is much more reactive than hydrogen).
• Writing oxygen at the positive electrode (forgetting that halide ions take priority over OH⁻).
• Writing "iodide" instead of the neutral product iodine.