AQA GCSE Chemistry Chemistry Paper 1 (Higher), June 2025: Question 6

✅ Mark Scheme Model Answer

• There are strong electrostatic forces of attraction [1 mark]
• between oppositely charged ions (Al³⁺ and O²⁻) [1 mark]
• which act in all directions throughout the giant lattice [1 mark]
• meaning large amounts of energy are required to break them [1 mark].

🧠 Exam Technique: 4-Point Structure

For any 4-mark question explaining the melting/boiling point of an ionic substance, memorize this exact chain of reasoning:

1. Name the force: Strong electrostatic attraction.
2. Identify the particles: Oppositely charged ions.
3. Describe the structure: Giant lattice acting in all directions.
4. Link to energy: Requires lots of thermal energy to overcome.

❌ Common Errors & Examiner Traps

• Mentioning "intermolecular forces": Aluminium oxide is ionic, NOT molecular. Any mention of intermolecular forces will cost you marks!
• Vague answers: Just writing "it has strong bonds" is ignored by examiners unless you specify electrostatic forces between oppositely charged ions.
• Confusing atoms and ions: Saying "forces between atoms" loses the mark.

💡 Key Knowledge

Aluminium oxide (Al₂O₃) forms a giant ionic lattice. Because both Al³⁺ and O²⁻ carry multiple charges, the electrostatic forces holding them together are exceptionally strong, giving it a melting point over 2000 °C.

✅ Model Answers

06.2 [1 mark]: cryolite (Formula Na₃AlF₆ is also accepted)

06.3 [1 mark]: The mixture has a lower melting point than pure aluminium oxide (also allow: to reduce the energy demand / save heating costs).

💡 Why is Cryolite Added?

• Pure Al₂O₃ melts at roughly 2072 °C, which is extremely expensive to maintain.
• Dissolving Al₂O₃ in molten cryolite lowers the operating temperature to around 950 °C.
• This saves massive amounts of energy and substantially lowers industrial production costs.

✅ Balanced Half-Equation

2 O²⁻ → O₂ + 4 e⁻

(Multiples like 4 O²⁻ → 2 O₂ + 8 e⁻ are accepted)

📐 Step-by-Step Balancing Method

1. Identify reactants & products: Oxide ions form diatomic oxygen molecules:
   O²⁻ → O₂
2. Balance the oxygen atoms: Two oxide ions are needed for one O₂ molecule:
   2 O²⁻ → O₂
3. Balance charges with electrons:
   Left side total charge = 2 × (-2) = -4
   Right side total charge must equal -4, so add 4 electrons:
   2 O²⁻ → O₂ + 4 e⁻

❌ Common Equation Mistakes

• Writing single oxygen atoms: O²⁻ → O + 2 e⁻ (Oxygen is diatomic and always forms O₂ gas!).
• Writing electrons on the wrong side: 2 O²⁻ + 4 e⁻ → O₂ (This would make the charge -8 on the left and 0 on the right). Oxidation is the loss of electrons, so electrons must be on the product side.

🧠 Quick Memory Aid: OIL RIG

Oxidation Is Loss of electrons.
At the positive electrode (anode), negative oxide ions lose electrons to become neutral oxygen molecules.

✅ Mark Scheme Model Answer

• The positive electrode (anode) is made of carbon / graphite [1 mark].
• The carbon reacts with the oxygen produced during electrolysis [1 mark].
• To produce carbon dioxide gas (so the electrode burns away) [1 mark].

🧠 Scoring All 3 Marks

This is a 3-mark explanation that requires a 3-step logical chain:

1. State what the electrode is made from (carbon/graphite).
2. State what it reacts with (oxygen gas produced at the anode).
3. State the product formed / outcome (carbon dioxide / wears away).

Chemical equation to remember:
C (s) + O₂ (g) → CO₂ (g)