AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), June 2025: Question 4

11 marks · Standard Demand difficulty · Short Answer

Explain metal extraction methods including reduction by carbon and electrolysis of aluminium oxide, write a half equation, compare extraction costs, and explain why alloys are hard.

Practise this question

Question

Question 4 consists of seven parts about metals: 04.1 asks why iron can be extracted by reduction with carbon in terms of reactivity (1 mark). 04.2 shows Figure 4, a diagram of an electrolysis cell for aluminium extraction containing carbon positive electrodes, a negative electrode lining, molten aluminium, and a molten mixture of aluminium oxide and cryolite, asking why cryolite is used (2 marks). 04.3 asks for a balanced half equation for the production of oxygen gas at the positive electrode (2 marks). 04.4 gives the displacement equation Cr2O3 + 2Al -> Al2O3 + 2Cr and asks which substance is reduced (1 mark). Table 3 provides relative costs of extracting equal masses: aluminium is 2, chromium is 20, and iron is 1. 04.5 asks for a reason for the cost difference between aluminium and iron (1 mark). 04.6 asks for a reason for the cost difference between aluminium and chromium (1 mark). 04.7 asks to explain why alloys are harder than pure metals (3 marks).
Question text

04 This question is about metals.

04.1 Iron is extracted from iron oxide by reduction with carbon.

Why can iron be extracted by reduction with carbon?

Answer in terms of reactivity.

[1 mark]

Aluminium is extracted by the electrolysis of aluminium oxide.

Figure 4 shows the apparatus used to extract aluminium.

Figure 4

04.2 Explain why a mixture of aluminium oxide and cryolite is used as the electrolyte

instead of only aluminium oxide.

[2 marks]

04.3 Oxygen gas is produced at the positive electrode.

Write a balanced half equation for the production of oxygen at the positive electrode.

[2 marks]

→ 16 +

Chromium is extracted by reacting chromium oxide with aluminium.

The equation for the reaction is:

Cr2O3 + 2Al → Al2O3 + 2Cr

04.4 Which substance in the equation is reduced?

[1 mark]

Table 3 shows the relative cost of extracting equal masses of aluminium, chromium

and iron.

Table 3

Relative cost of

Metal

extraction

Aluminium 2

Chromium 20

Iron 1

04.5 Give one reason for the difference in the cost of extracting aluminium and iron.

Use Table 3.

[1 mark]

04.6 Give one reason for the difference in the cost of extracting aluminium and chromium.

Use Table 3.

[1 mark]

04.7 Metals are mixed together to form alloys.

Explain why alloys are harder than pure metals.

[3 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 4: 04.1 requires 'iron is less reactive than carbon' (1 mark, AO1). 04.2 gives 1 mark for 'the mixture has a lower melting point (than aluminium oxide)' and 1 mark for '(so) less energy is used (to melt the electrolyte)' (AO1). 04.3 awards 2 marks for 2 O2- -> O2 + 4 e- (allow multiples; 1 mark for correct species unbalances) (AO2). 04.4 awards 1 mark for Cr2O3 or chromium oxide (allow Cr3+ ions) (AO2). 04.5 gives 1 mark for 'extracting aluminium uses more energy (so costs more)' (AO3). 04.6 gives 1 mark for '(extracting chromium costs more because) aluminium is used in the extraction of chromium' (AO3). 04.7 awards 3 marks: alloys contain different sized atoms (1 mark), so the layers are distorted (1 mark), meaning layers slide less easily (1 mark) (AO1). Total marks = 11.

Question 4

AO /

Question Answers Extra information Mark

Spec. Ref.

04.1 iron is less reactive than carbon 1 AO1

5.4.1.3

AO /

Spec. Ref.

04.2 the mixture has a lower melting 1 AO1

point (than aluminium oxide) 5.4.3.3

(so) less energy is used (to melt 1

the electrolyte)

AO /

Spec. Ref.

04.3 2 O2− → O + 4 e− allow multiples 2 AO2

5.4.3.3

5.4.3.5

allow 1 mark for O2−→ O + e−

with no / incorrect balancing

numbers

AO /

Spec. Ref.

04.4 Cr O allow Cr3+ ions 1 AO2

or 5.4.1.3

chromium oxide

AO /

Spec. Ref.

04.5 extracting aluminium uses more 1 AO3

energy (so costs more) 5.4.1.3

– – 8464/C/1H – 5.4.3.3

AO /

Spec. Ref.

04.6 (extracting chromium costs 1 AO3

more because) 5.4.1.3

aluminium is used in the 5.4.3.3

extraction of chromium

AO /

Spec. Ref.

04.7 alloys contain different sized allow alloys contain different 1 AO1

atoms sized (positive) ions 5.2.2.7

(so the) layers are distorted 1

(so the) layers (of atoms) slide allow (so the) atoms cannot 1

less easily slide over each other

Total Question 4 11

How to answer it

Extracting Metals, Electrolysis of Aluminium & Alloys

📋 What This Question Tests

This question assesses your understanding of extraction methods, electrolysis, redox reactions, and material structures from AQA GCSE Combined Science:

  • Reactivity series: Using carbon displacement to extract metals.
  • Aluminium electrolysis: The role of cryolite, energy costs, and writing balanced ionic half equations.
  • Redox: Identifying substances reduced in terms of oxygen loss or electron gain.
  • Cost evaluation: Comparing industrial costs based on energy and raw materials.
  • Metallic bonding & structure: Explaining why alloys are harder than pure metals.
Question 04.1 • 1 Mark

Extraction of Iron Using Carbon

Explaining extraction feasibility using reactivity

✅ Correct Answer

Iron is less reactive than carbon (or carbon is more reactive than iron).

1 Mark: Direct comparison between iron and carbon reactivity.

🧠 Exam Technique

Always state both elements in your comparative statement. Simply saying "iron is unreactive" gains 0 marks because displacement depends on relative position in the reactivity series.

Question 04.2 • 2 Marks

Electrolyte Composition: Cryolite and Aluminium Oxide

Explaining why cryolite is added

✅ Correct Answer

  • The mixture has a lower melting point (than pure aluminium oxide) [1 mark]
  • (So) less energy is needed to melt the electrolyte / lowers energy costs [1 mark]

💡 Key Knowledge

Pure aluminium oxide (Al₂O₃) has an extremely high melting point (over 2000 °C) due to strong ionic bonds. Dissolving it in molten cryolite lowers the melting point to around 950 °C, making the process commercially viable.

❌ Common Errors

  • Saying "cryolite lowers the boiling point" (it lowers the melting point).
  • Claiming cryolite acts as an electrical conductor rather than a solvent.
Question 04.3 • 2 Marks

Half Equation at the Positive Electrode (Anode)

Oxidation of oxide ions to produce oxygen gas

✅ Correct Answer

2 O²⁻ → O₂ + 4 e⁻

2 Marks: Fully correct balanced half equation.
(1 mark awarded if species are correct: O²⁻ → O₂ + e⁻ but balancing numbers are omitted or incorrect).

🧠 Step-by-Step Construction

  1. Reactant ion: Oxide is O²⁻ .
  2. Product molecule: Oxygen is diatomic O₂ .
  3. Balance atoms: Two oxide ions are needed: 2 O²⁻ → O₂ .
  4. Balance charges: Left side charge is −4. Right side needs 4 e⁻ added to equal −4: 2 O²⁻ → O₂ + 4 e⁻ .
Question 04.4 • 1 Mark

Identifying the Reduced Substance

Equation: Cr₂O₃ + 2 Al → Al₂O₃ + 2 Cr

✅ Correct Answer

Chromium oxide (or Cr₂O₃ / Cr³⁺ ions).

1 Mark: Correctly naming the reactant that loses oxygen.

💡 Key Knowledge

Reduction can be defined in two ways:

  • In terms of oxygen: Reduction is the loss of oxygen. Cr₂O₃ loses oxygen to become Cr.
  • In terms of electrons: Chromium ions gain electrons ( Cr³⁺ + 3 e⁻ → Cr ).

❌ Common Errors

Writing just chromium ( Cr ). Reduction happens to a reactant ( Cr₂O₃ ), not the product formed!

Questions 04.5 & 04.6 • 2 Marks Total

Comparing Extraction Costs

Data: Aluminium (2), Chromium (20), Iron (1)

04.5: Cost Difference: Aluminium vs Iron [1 mark]

Extracting aluminium uses electrolysis which requires large amounts of electricity/energy, making it more expensive than heating iron oxide with carbon.

04.6: Cost Difference: Aluminium vs Chromium [1 mark]

Chromium is extracted using aluminium (which has already been expensively extracted first) / it is a multi-step process.

🧠 Exam Technique

Look at the chemical equation provided in the stem! The equation showed that Aluminium (Al) is a reactant needed to make Chromium. If aluminium is already expensive to produce, using it as a fuel/reducing agent will make chromium even more expensive.

Question 04.7 • 3 Marks

Why Alloys are Harder Than Pure Metals

Explaining structural resistance to force

✅ Perfect 3-Mark Model Answer

  1. Alloys contain atoms of different sizes. [1 mark]
  2. This distorts the regular layers of atoms. [1 mark]
  3. Therefore, the layers cannot slide over each other as easily. [1 mark]

💡 Visualising the Structure

Pure Metal: Identical spherical atoms arranged in neat, uniform rows. When a force is applied, layers slide easily (malleable/soft).

Alloy: Larger or smaller solute atoms break up the regular pattern. The disrupted rows lock the structure in place, making it harder.

❌ Common Errors

  • Talking about "stronger bonds" or "intermolecular forces" — hardness here is strictly about layers sliding!
  • Omitting the word "layers" (saying just "atoms can't slide" misses Mark 2 or 3).

Topics

Chemistry · C2: Bonding, Structure and the Properties of Matter · C4: Chemical Changes

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.