AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2018: Question 8

Key skills

• Recall of common reaction products
• Separation methods for mixtures
• Mole ratio calculations from equations
• Reduction as gain of electrons

How marks are awarded

• 1-mark recall answers need the exact correct idea
• Calculation marks are step-by-step: method marks + final answer mark
• In the redox part, you need the species, the reason, and the half-equation

✅ Correct answer

Hydrogen or H₂

💡 Key knowledge

• Metal + acid → salt + hydrogen
• Here, iron reacts with hydrochloric acid to form iron chloride solution and hydrogen

❌ Common errors

• Writing just H instead of H₂
• Giving oxygen or water
• Forgetting that the question asks for the other product

✅ Correct answer

Filtration / use a filter

💡 Key knowledge

• Unreacted iron is a solid
• The product is a solution, so a solid-liquid separation method is needed
• If using filtration, the iron stays as residue in the filter paper

🧠 Exam technique

• Give the method name, not a vague description
• If you say magnet, it works because iron is magnetic
• “Heat it” is not needed and does not score here

❌ Common errors

• Saying evaporation — that would remove the liquid, not separate the iron
• Writing a long explanation but not naming the method
• Thinking the iron is dissolved in the solution

📐 Calculations: step-by-step

1. Find moles of magnesium:
   moles = mass ÷ Ar = 0.120 ÷ 24 = 0.005 mol
2. Use the equation ratio:
   3 Mg : 2 Fe
   So moles of Fe = 0.005 × 2/3 = 0.00333 mol
3. Find mass of iron in grams:
   mass = moles × Ar = 0.00333 × 56 = 0.1866 g
4. Convert to mg:
   0.1866 g = 187 mg

✅ Correct answer

187 mg

• Acceptable range in the mark scheme: 185–190 mg for full marks
• Answers like 0.187 g are mathematically correct but the question asks for mg

💡 Key knowledge

• Equation: 3 Mg + 2 FeCl₃ → 2 Fe + 3 MgCl₂
• Use the balanced equation to get the reacting ratio
• Relative atomic masses: Mg = 24, Fe = 56

🧠 Exam technique

• Always show each stage clearly for method marks
• Use the ratio from the balanced equation, not just the masses
• Keep units consistent: g at the start, then convert to mg at the end
• Round sensibly: 0.1866 g → 0.187 g → 187 mg

❌ Common errors

• Ignoring the 3:2 ratio and using a 1:1 ratio
• Using 24 and 56 as if they were the ratio directly
• Forgetting to convert grams to milligrams
• Writing 184 mg from a wrong calculation path — the mark scheme gives 0 marks for this

✅ Correct answer

Fe³⁺ is reduced.

Reduction is gain of electrons:

Fe³⁺ + 3e⁻ → Fe

💡 Key knowledge

• Reduction means gain of electrons
• Fe³⁺ gains 3 electrons to become Fe
• Oxidation state changes from +3 to 0

🧠 Exam technique

• Name the ion: Fe³⁺, not just “iron”
• Link the answer to electron gain
• Write the half-equation exactly and balance charge

❌ Common errors

• Saying magnesium is reduced — magnesium is actually oxidised
• Writing Fe → Fe³⁺ which is oxidation, not reduction
• Forgetting the 3 electrons in the half-equation

Essential facts

• Iron + acid → salt + hydrogen
• Unreacted iron can be separated by filtration or a magnet
• Use balanced equations for mole ratios
• Reduction = gain of electrons

Top-mark habits

• Use the equation ratio, not guesswork
• Show units throughout
• Write the half-equation neatly and balanced
• Convert g to mg at the end when asked