AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2020: Question 6

✅ Mark Scheme Answer

The minimum energy needed for particles to react.

(Also accepted: the minimum energy needed for a reaction to occur / to start a reaction.)

🧠 Exam Technique

The word minimum is essential. Saying just "the energy needed to start a reaction" will lose the mark on many exam series because it does not state that it is a threshold barrier.

❌ Common Errors

• Omitting the word minimum (e.g. writing "the heat energy taken in to react").
• Confusing activation energy with overall energy change (ΔH).

💡 Key Knowledge

• Particles must collide with energy equal to or greater than the activation energy to have a successful collision.
• High activation energy means strong bonds must be broken before new bonds form, so the mixture often requires a spark or flame to initiate.

Step 1: Calculate the formula mass (Mᵣ) of Fe₂O₃
Mᵣ(Fe₂O₃) = (2 × 56) + (3 × 16) = 112 + 48 = 160 [Mark 1]

Step 2: Convert kg to g and find moles of each reactant
• Moles of Fe₂O₃ = 3000 g / 160 = 18.75 mol [Mark 2]
• Moles of Al = 1000 g / 27 = 37.0 mol (or 37.04 mol) [Mark 3]

Step 3: Compare with the balanced molar ratio
From the equation: 2 moles of Al react with 1 mole of Fe₂O₃.
• To react all 18.75 mol of Fe₂O₃, required Al = 18.75 × 2 = 37.5 mol.
• We only have 37.0 mol of Al available (37.0 < 37.5).
Conclusion: Aluminium is the limiting reactant because there is not enough aluminium to react with all the iron oxide. [Mark 4]

❌ Common Calculation Traps

• Unit conversion: Forgetting that 1 kg = 1000 g. If using 1.00 and 3.00, units are kmol, but students often forget this and divide 1 / 27 directly without keeping units consistent.
• Ignoring the 2:1 ratio: Simply comparing 37.0 mol Al to 18.75 mol Fe₂O₃ and incorrectly claiming Fe₂O₃ is limiting because 18.75 is a smaller number. You must apply the reaction stoichiometry!
• Mᵣ errors: Calculating Mᵣ(Fe₂O₃) as (56 + 16) or using atomic numbers instead of mass numbers.

🧠 Alternative Method (Mass-Based)

You can also show Al is limiting by mass:

• 18.75 mol Fe₂O₃ needs 37.5 mol Al.
• Required mass of Al = 37.5 mol × 27 g/mol = 1012.5 g (1.0125 kg).
• Since 1.0125 kg is needed and only 1.00 kg is provided, aluminium runs out first and is the limiting reactant.

✅ Correct Completed Equation

Mg(s) + Zn²⁺(aq) → Mg²⁺(aq) + Zn(s)

Products can be written in either order: Mg²⁺(aq) + Zn(s) or Zn(s) + Mg²⁺(aq) .

💡 Key Knowledge: Why do spectator ions vanish?

In full aqueous solution:

Mg(s) + Zn²⁺(aq) + SO₄²⁻(aq) → Mg²⁺(aq) + SO₄²⁻(aq) + Zn(s)

Sulfate ions (SO₄²⁻) do not change state or charge; they are spectator ions and cancel out on both sides.

❌ Common Errors

• Missing out state symbols or writing incorrect ones (e.g., writing Mg²⁺(s) or Zn(aq)).
• Writing incorrect charges such as Mg⁺ or Zn⁺. Both magnesium and zinc form 2+ ions.
• Re-introducing the sulfate ion (SO₄²⁻) into an ionic equation.

🧠 Exam Technique

Read the question carefully: "You should include state symbols." In AQA mark schemes, this is worth an independent mark. Even if your formulae are correct, omitting state symbols caps your score at 1/2.

✅ Mark Scheme Answer

• Oxidation: Magnesium (atoms) are oxidised because they lose electrons. [Mark 1]
• Reduction: Zinc (ions) are reduced because they gain electrons. [Mark 2]

💡 The Golden Rule: OIL RIG

• Oxidation Is Loss of electrons:
  Mg → Mg²⁺ + 2e⁻
• Reduction Is Gain of electrons:
  Zn²⁺ + 2e⁻ → Zn

❌ Common Errors & Examiner Commentary

• Vague species identification: Saying "zinc is oxidised" instead of specifying zinc ions (Zn²⁺). In the reactant mixture, it is the Zn²⁺ ion gaining electrons, not zinc metal.
• Reverting to oxygen definitions: Explaining the reaction as "magnesium gains oxygen" when there is no oxygen in the ionic equation!
• Swapping the terms: Stating oxidation is gain of electrons. Always double-check with OIL RIG before writing.

🧠 Exam Technique

Structure your response into two distinct bullet points naming the exact species:

1. "Magnesium atoms lose 2 electrons to form Mg²⁺, so magnesium is oxidised."

2. "Zinc ions gain 2 electrons to form Zn atoms, so zinc ions are reduced."