Extraction of Metals and Reduction, and Oxidation and Reduction in Terms of Electrons [HT for electrons]

SPEC 4.4.1.3 – 4.4.1.4 • MODULE 4 • HT FOR ELECTRONS

Extraction of Metals & Redox

Where the reactivity series gets used industrially. Carbon reduction vs electrolysis. And — at Higher Tier — a second definition of redox in terms of electrons that AQA tests as carefully as the oxygen one.

Why This Lesson Matters

Lesson 4.01 gave you the reactivity series. This lesson is where you apply it industrially — and (for HT) where redox gets a second, electron-based definition.

10%

Scored Both Marks On Q07.2

2

Definitions of Redox

⭐

Priority Lesson

Spec Scope (Important)

The spec says: "Knowledge and understanding are limited to the reduction of oxides using carbon" and "knowledge of the details of processes used in the extraction of metals is not required". So we cover the concept of carbon reduction and electrolysis — not blast-furnace stages or Bayer process details.

Key Definitions

Ore

A rock that contains enough of a metal (or metal compound) to make extraction economically worthwhile.

Reduction (oxygen def.)

The loss of oxygen by a substance.

Oxidation (oxygen def.)

The gain of oxygen by a substance.

HT only — second definition (4.4.1.4)At Higher Tier, redox gets a more precise definition in terms of electrons: OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). We'll cover this from slide 11 onwards.

How Metals Are Found & Extracted

Spec 4.4.1.3: "Unreactive metals such as gold are found in the Earth as the metal itself but most metals are found as compounds that require chemical reactions to extract the metal."

Three Categories

Position relative to carbon in the reactivity series determines the extraction method (or whether one is needed at all).

CategorySpec metalsMethodCost

More reactive than CK, Na, Li, Ca, Mg (+ Al)Electrolysis of molten compoundHigh (lots of electrical energy)

Less reactive than CZn, Fe, CuReduction with carbon (heat oxide with coke)Low (carbon cheap from coal)

Unreactive (found native)Au (gold), often AgNo chemical extraction needed — found as the metal itselfMining cost only

The Logic

Carbon can only pull oxygen off metals that are less reactive than itself. Anything more reactive than C must be ripped apart by electricity. The most unreactive metals never combined with anything in the first place — they sit in the ground as nuggets of pure metal. Position vs C tells you the method.

Carbon Reduction — Iron from Iron Oxide

The classic GCSE example. Iron is less reactive than carbon, so iron oxide can be reduced by heating with coke.

It's a REDOX Reaction

Both halves happen at once — one species is reduced, another is oxidised. Reduction + oxidation = redox. This is the same reaction we used in 4.01 for the iron oxide trap (Q09.1).

Why Aluminium Needs Electrolysis

Aluminium is the most-tested extraction example. The argument has three linked steps — and most students miss the comparison.

The 3-Step Chain

Aluminium is more reactive than carbon — so carbon cannot displace it from Al₂O₃.

The only way to reduce Al₂O₃ is by electrolysis — using electrical energy.

Electrolysis requires large amounts of energy, which makes it expensive compared to carbon reduction (carbon comes cheaply from coal).

Why The Comparison Is Critical

A 2-mark "explain why aluminium is more expensive" question requires BOTH sides of the comparison. Mark scheme verbatim: "more energy is needed (to obtain aluminium)" + "(because) aluminium is obtained by electrolysis". Carbon being cheap is the implicit comparison — but you must mention electrolysis explicitly.

Trap 1 — The Cost Comparison

P1 Jun 2022 Q07.2: Carbon is used to reduce silicon dioxide because carbon is cheaper than aluminium. Carbon comes from heating coal; aluminium comes from aluminium oxide. Explain why aluminium is more expensive than carbon.2 MARKS

"Only 10% of students were awarded both marks... Many students realised that aluminium is extracted by electrolysis, although some stated that aluminium would be extracted by the use of a more reactive metal. Although many mentioned the high energy cost of electrolysis, few compared electrolysis to the lower energy cost of heating coal to obtain carbon." — AQA Examiner Report, P1 Jun 2022 Q07.2

✗ Misses Marks

"Aluminium is too reactive to be extracted by carbon."

True but not the question. Doesn't mention electrolysis or cost.

✓ Scores Both

"Aluminium is extracted by electrolysis, which needs more energy than heating coal — so aluminium is more expensive."

Trap 2 — Separate, Not Decompose

P1 Jun 2022 Q07.3: Why are the products difficult to separate if magnesium is used to reduce silicon dioxide? Reaction: 2Mg + SiO₂ → Si + 2MgO.1 MARK

"Many students realised that the products both being solids would make their physical separation difficult. Some appeared to misunderstand what 'separate' means, and described the difficulty of decomposing magnesium oxide; this did not gain credit. Another common incorrect approach was to say that magnesium is high in the reactivity series. Whilst true, this was not relevant to the question set." — AQA Examiner Report, P1 Jun 2022 Q07.3

✗ Loses Mark

"Magnesium oxide is hard to decompose."

"Decompose" ≠ "separate". Wrong concept entirely.

✓ Scores

"Both products are solid, so they are difficult to separate (physically)."

Discipline

Read carefully — "separate" is a physical process (filtering, picking apart). "Decompose" is a chemical process (breaking compounds). Don't substitute one for the other.

Evaluating Extraction Methods

Spec skill: "Interpret or evaluate specific metal extraction processes when given appropriate information." A typical question gives you 2–3 methods and asks you to compare them — usually 4 marks (level-marked).

Comparison Framework

Cost of reactants: carbon & iron are cheap; hydrogen and reactive metals are expensive

Energy requirement: electrolysis >> carbon reduction

Separation of products: two solids are hard to separate; gas products escape easily

Purity: some methods leave the metal contaminated with reactant residues

Yield / waste: some metal may be lost as a side-product (e.g. tungsten carbide forming when extracting tungsten with carbon)

Source: P1 Nov 2021 Q03.4

A 4-mark level-of-response question on three methods of extracting tungsten. Level 2 (3–4 marks) = "logically linked reasons". Level 1 (1–2 marks) = "relevant points but not linked". Always link cost → energy → separation → conclude.

Higher Tier — A Second Definition of Redox

HT only — Spec 4.4.1.4

So far we've defined oxidation as gain of oxygen, reduction as loss of oxygen. At Higher Tier, AQA introduces a more precise definition based on electrons.

Why The Second Definition?

The oxygen definition only works for reactions involving oxygen. The electron definition works for all redox reactions — including ones with no oxygen at all (like Zn + Cu²⁺ → Zn²⁺ + Cu, the displacement reaction).

Both Definitions Are Tested

Foundation tier: oxygen definition only. Higher tier: both. The mark scheme will tell you which to use — if it says "in terms of oxygen", use that. If it says "in terms of electrons", use OIL RIG. Never mix them.

OIL RIG — The Electron Definitions

HT only

OIL — Oxidation Is Loss

Oxidation = LOSS of electronsMg → Mg²⁺ + 2e⁻Mg atom loses 2 electrons → becomes a 2⁺ ion. Mg is OXIDISED.

RIG — Reduction Is Gain

Reduction = GAIN of electronsCu²⁺ + 2e⁻ → CuCu²⁺ ion gains 2 electrons → becomes a Cu atom. Cu²⁺ is REDUCED.

Charge Logic — Don't Get Tricked

If a species becomes more positive (e.g. Mg → Mg²⁺), it has lost electrons → OXIDISED.

If a species becomes less positive (e.g. Cu²⁺ → Cu), it has gained electrons → REDUCED.

Track the charge change — that tells you the direction.

Worked Example — Zn + CuSO₄

HT only

Q: Zinc is added to copper sulfate solution. Identify what is oxidised and what is reduced, in terms of electrons.2 MARKS

Step 1 — Write the Half-Equations

Zn → Zn²⁺ + 2e⁻ (zinc atom loses 2 electrons)Cu²⁺ + 2e⁻ → Cu (copper ion gains 2 electrons)

Step 2 — Apply OIL RIG

OILZn loses 2 electrons → Zn is OXIDISED.RIGCu²⁺ gains 2 electrons → Cu²⁺ is REDUCED.

Mark-Scheme Note (P1 Jun 2022 Q04.6)

When the question asks "in terms of electrons", the mark scheme literally states: "do not accept references to oxygen". There's no oxygen in this reaction anyway — the only valid explanation is electron transfer.

Ionic Equations for Displacement

HT only — Spec 4.4.1.4

An ionic equation shows only the species that actually change. Spectator ions (which sit unchanged on both sides) are removed.

From Full Equation to Ionic Equation

Full equation:Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)Identify spectator ions: SO₄²⁻ appears unchanged on both sides — it's a spectator. Cancel it out.

Ionic equation (with state symbols, mark scheme P1 Jun 2024 Q04.4):Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

Examiner Note (P1 Jun 2024 Q04.4)

"A common incorrect formula was Zn²⁻" — wrong sign. Zinc is a metal — when it loses electrons, it becomes a positive ion (Zn²⁺). Negatives are for non-metals gaining electrons. Mark scheme allows 1 mark for "Zn²⁺ + Cu" without state symbols; full 2 marks needs the state symbols.

The Charge Change Method

HT only

P1 Jun 2023 Q09.3 (multiple choice): The ionic equation for a displacement reaction is A + 2B⁺ → 2B + A²⁺. Why is this a redox reaction?1 MARK

"Over three-quarters of students could correctly apply their knowledge of redox in terms of electrons. By far the most common incorrect response was 'A gains electrons and B⁺ loses electrons'." — AQA Examiner Report, P1 Jun 2023 Q09.3

Tick One Box

☐ A gains electrons and B⁺ loses electrons. ← the only common wrong answer☑ A loses electrons and B⁺ gains electrons. ← correct☐ Both A and B⁺ gain electrons.☐ Both A and B⁺ lose electrons.

Working It Out — Track The Charge

A goes from neutral (charge 0) to A²⁺ (charge +2). To become more positive, A must lose 2 electrons → A is OXIDISED.

B goes from B⁺ (charge +1) to neutral B (charge 0). To become less positive, B⁺ must gain 1 electron → B⁺ is REDUCED.

Most Students Score This — But Don't Guess

75%+ of students get this right. The single common wrong answer is the opposite of the truth — students who muddle OIL/RIG flip oxidation and reduction. Track the charge change methodically: more positive = lost e⁻; less positive = gained e⁻. Don't rely on memory of "OIL" or "RIG" alone — derive it from the charges.

Trap 4 — Don't Mix the Two Definitions

The mark scheme rule from P1 Jun 2022 Q04.6 (HT, in terms of electrons): "do not accept references to oxygen".

✗ Wrong Tool

Question: "Explain in terms of electrons why zinc is oxidised."

Wrong: "Zinc is oxidised because it gains oxygen."

The question asked about electrons. The MS explicitly rejects oxygen references — even if the answer is otherwise correct.

✓ Correct Tool

Question: "Explain in terms of electrons why zinc is oxidised."

Right: "Zinc atoms lose 2 electrons to form Zn²⁺."

The Inverse Trap (from 4.01)

The opposite of this also catches students out. P1 Jun 2023 Q09.1 said "answer in terms of oxygen" but many HT students reached for OIL RIG. Mark scheme: "ignore Fe³⁺ gains electrons".

Read the question. Use the right definition.

Mark-Scheme Phrases to Memorise

Verbatim language from real Paper 1 mark schemes. Use these phrases — don't paraphrase.

ConceptMark-scoring phraseDon't write

Why Al is more expensive than C"more energy is needed... aluminium is obtained by electrolysis "; carbon comes cheaply (from coal)"Al is too reactive" alone (no comparison)

Mg + C product separation"both products are solid ""hard to decompose"

Why electrolysis (not C) for Al"Al is more reactive than carbon ""a more reactive metal is used"

Oxidation (HT, electrons)" loses electrons" / "loses N electrons"any reference to oxygen

Reduction (HT, electrons)" gains electrons" / "gains N electrons"any reference to oxygen

Ionic equation (Zn + Cu²⁺)" Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) "Zn²⁻ (wrong sign)

Practice 1 — Foundation

Magnesium is extracted by electrolysis. Iron is extracted by reduction with carbon.

(a) Use the reactivity series to explain why iron is extracted by reduction with carbon, but magnesium is not.2 MARKS

(b) Suggest one reason why extracting magnesium is more expensive than extracting iron.1 MARK

a / 1

Iron is less reactive than carbon, so carbon can reduce iron oxide.

a / 2

Magnesium is more reactive than carbon, so carbon cannot reduce magnesium oxide / magnesium must be extracted by electrolysis.

b / 1

Electrolysis requires (a lot of) electrical energy / "more energy is needed than for carbon reduction".

⚠️

Watch the comparison — naming reactivity alone (e.g. "Mg is too reactive") is not enough. Tie it to the method: electrolysis vs carbon reduction.

Practice 2 — HT

Magnesium reacts with copper sulfate solution. The full equation is:

Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)(a) Write the ionic equation for this reaction. Include state symbols.2 MARKS

(b) Identify the species that is oxidised. Explain your answer in terms of electrons.2 MARKS

a / 1

Correct ions identified — magnesium becomes Mg²⁺; copper ion becomes Cu metal. SO₄²⁻ is a spectator ion and is cancelled.

a / 2

Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)(Common wrong answer per Jun 2024 Q04.4 ER: "Mg²⁻" — wrong sign. Allow 1 mark for correct ions without state symbols.)

b / 1

Magnesium (or Mg) is oxidised.

b / 2

Magnesium atoms lose 2 electrons (to form Mg²⁺). Mark scheme verbatim from Jun 2022 Q04.6: "loses (2) electrons".

⚠️

Do NOT mention oxygen for part (b) — the question said "in terms of electrons". MS rule (Jun 2022 Q04.6): "do not accept references to oxygen".

Summary

Extraction — Three Categories

Unreactive (Au, often Ag) → found in the Earth as the metal itself; no extraction needed

Less reactive than C (Zn, Fe, Cu) → reduction with carbon — cheap (heating with coke). Principle applies to any metal below C, e.g. tin.

More reactive than C (K, Na, Li, Ca, Mg, Al) → electrolysis of molten compound — expensive (electrical energy)

Detailed industrial processes are not required per spec

Redox by Oxygen (Foundation + HT)

Oxidation = gain of oxygen · Reduction = loss of oxygen

Carbon reduction example: Fe₂O₃ + 3C → 2Fe + 3CO — Fe₂O₃ reduced; C oxidised

Redox by Electrons — OIL RIG (HT only)

OIL: Oxidation Is Loss of electrons · RIG: Reduction Is Gain of electrons

Spec requires: write ionic equations for displacement; identify oxidised/reduced in given equations or half-equations

Ionic equation: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) (cancel spectator ion SO₄²⁻)

Track the charge change to identify oxidation/reduction

Three Examiner Pitfalls + One Method

Trap 1 (Al cost — only 10% scored both): must compare electrolysis (expensive) vs carbon (cheap from coal). Naming one side alone scores half.

Trap 2 (Mg + C): products are solid — hard to separate , NOT decompose

Method (Q09.3 — 75%+ scored): A → A²⁺ = lost electrons = oxidation. Track the charge change methodically; don't guess from memory.

Trap 3 (definitions): "in terms of electrons" forbids oxygen references; "in terms of oxygen" forbids electron references — both rules verbatim from MS

Exam Tip

For every redox question, your first move is to read the wording: "in terms of oxygen" or "in terms of electrons"? That single phrase tells you which definition to use — and using the wrong one is an instant mark loss even if the rest of your answer is right.