AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2022: Question 7

✅ Correct answer

Electrolysis of a molten compound of metal Y.

Alternative accepted answer: displacement by heating with a more reactive metal , such as potassium or magnesium.

💡 Key knowledge

• Metal Y is above carbon in the reactivity series, so it cannot be extracted by reduction with carbon.
• More reactive metals are extracted by electrolysis.
• If a metal is extracted by displacement, a more reactive metal can reduce its compound.
• The mark scheme allows molten or liquid compound.

🧠 Exam technique

• Always link the method to the reactivity series.
• For full marks, name both the method and the condition: molten for electrolysis.
• If using displacement, name the more reactive metal used.

❌ Common errors

• Saying reduction with carbon when the metal is above carbon.
• Just writing electrolysis without saying the compound must be molten.
• Giving a vague answer like “heat it” without the method.

✅ Correct answer

4 Na + TiCl₄ → 4 NaCl + Ti

💡 Key knowledge

• This is a displacement reaction: sodium is more reactive than titanium.
• The chlorine atoms must balance on both sides: 4 chlorines in TiCl₄ means 4 NaCl on the product side.
• Titanium is left as the single element Ti.

🧠 Exam technique

• Check atoms one type at a time.
• Balance metals last if needed.
• Use whole-number coefficients only.

❌ Common errors

• Writing NaCl but forgetting to balance it to 4 NaCl.
• Putting TiCl₄ on the product side.
• Changing the formula of titanium chloride instead of balancing the equation.

✅ Correct answer

Na → Na⁺ + e⁻

💡 Key knowledge

• Oxidation means loss of electrons.
• Sodium atoms lose one electron to form sodium ions.
• State symbols are ignored here.

🧠 Exam technique

• Write the atom on the left and the ion on the right.
• Make sure the charge is correct: Na⁺.
• Only one electron is lost by sodium, so write e⁻ once.

❌ Common errors

• Writing Na + e⁻ → Na⁺ instead of showing oxidation.
• Forgetting the positive charge on the sodium ion.
• Balancing with extra electrons when only one is needed.

📐 Step-by-step method 1: moles

1. Calculate moles of aluminium: 108 ÷ 27 = 4 mol
2. Calculate moles of copper chloride: 1210 ÷ 134.5 = 8.996 mol
3. Compare with the ratio in the equation: 2 Al reacts with 3 CuCl₂.
4. For 4 mol Al, required CuCl₂ = 6 mol.
5. Since 8.996 mol CuCl₂ is available, aluminium is the limiting reactant.
6. From the equation, 2 Al → 3 Cu, so 4 mol Al gives 6 mol Cu.
7. Mass of copper = 6 × 63.5 = 381 g

📐 Step-by-step method 2: mass ratio from the equation

1. Find the reacting masses:
   2 Al = 2 × 27 = 54 g
   3 CuCl₂ = 3 × 134.5 = 403.5 g
   3 Cu = 3 × 63.5 = 190.5 g
2. 108 g Al is twice 54 g, so it would react with 807 g CuCl₂ and produce 381 g Cu.
3. Since there is 1210 g CuCl₂, there is excess copper chloride.
4. Therefore, aluminium is the limiting reactant.

✅ Final answer

Limiting reactant: aluminium

Mass of copper: 381 g

✅ Correct answer

• Sodium metal has delocalised electrons.
• These electrons carry electrical charge through the metal.

💡 Key knowledge

• Metallic bonding contains positive ions in a sea of delocalised electrons.
• The electrons are free to move, so they transfer charge.
• Sodium metal conducts as a solid.

🧠 Exam technique

• Use the exact term delocalised electrons.
• Say they carry charge or transfer charge through the metal.
• You do not need to mention current unless you want to.

❌ Common errors

• Saying only “electrons move” without linking to charge.
• Talking about ions moving in metal — this is not the main reason metals conduct.
• Writing “free electrons” is usually accepted, but delocalised electrons is safer.

✅ Correct answer

Sodium chloride conducts electricity when molten or in aqueous solution because ions are free to move and carry charge.

💡 Key knowledge

• In solid sodium chloride, ions are fixed in place, so it does not conduct.
• When molten or dissolved in water, ions can move.
• Charge is carried by moving ions, not by electrons in the ionic compound.

🧠 Exam technique

• Always mention the correct condition: molten or aqueous.
• Then explain that ions are free to move.
• Finish by linking movement to charge flow or conducting electricity.

❌ Common errors

• Saying sodium chloride conducts when solid.
• Talking about delocalised electrons in sodium chloride — that applies to metals, not ionic compounds.
• Forgetting to explain why the ions matter.

💡 What examiners wanted

• Correct scientific terms: electrolysis, molten, oxidised, delocalised electrons, ions, limiting reactant.
• Balanced equations and correct chemical formulas.
• Clear logic in calculations, showing how the answer is obtained.

🧠 How to secure marks quickly

• Use the reactivity series to choose extraction methods.
• Balance before calculating.
• State the limiting reactant before the final mass.
• Use units at every stage.