Extraction Methods Compared

The choice of extraction method is entirely determined by the metal’s position in the reactivity series. This single principle answers every extraction question in the 0620 syllabus and is tested in Papers 2 and 4 regularly.

The principle

Position in reactivity series	Extraction method	Reason	Examples
Above carbon (K, Na, Ca, Mg, Al)	Electrolysis	Too reactive for carbon to reduce; only electrical energy can decompose the ore	Al from Al2O3
Below carbon (Zn, Fe, Sn, Pb, Cu)	Reduction with carbon/coke	Carbon is more reactive and can reduce the metal oxide	Fe from Fe2O3
Very unreactive (Au, Pt, Ag)	Found native / physical separation	So unreactive they exist uncombined	Panning for gold

The reactivity series from most to least reactive: K, Na, Ca, Mg, Al, (C), Zn, Fe, Sn, Pb, (H), Cu, Ag, Au, Pt.

Carbon’s position matters: metals above carbon cannot be reduced by it; metals below carbon can.

Extraction of aluminium by electrolysis

Aluminium is extracted from bauxite ore, which contains aluminium oxide (Al2O3).

Why electrolysis? Aluminium is above carbon in the reactivity series. Carbon cannot reduce Al2O3. Only electrolysis provides enough energy.

The process

1. Aluminium oxide is dissolved in molten cryolite (Na3AlF6) to lower the melting point from 2072 C to about 950 C, reducing energy costs
2. Carbon (graphite) electrodes are used
3. Direct current is passed through the molten mixture

At the cathode (-): Al3+ + 3e- -> Al (aluminium metal collects at the bottom)

At the anode (+): 2O2- -> O2 + 4e- (oxygen gas produced)

The oxygen produced at the anode reacts with the carbon electrodes: C + O2 -> CO2. This means the carbon anodes burn away and must be replaced regularly.

Why is aluminium expensive? Electrolysis uses enormous amounts of electricity. The process runs continuously at high temperature. Replacing carbon anodes adds further cost.

Extraction of iron in the blast furnace

Iron is extracted from haematite ore, which contains iron(III) oxide (Fe2O3).

Why carbon reduction? Iron is below carbon in the reactivity series, so carbon (as coke) can reduce Fe2O3.

The process

Raw materials loaded into the top of the blast furnace:

• Iron ore (Fe2O3)
• Coke (C) as the reducing agent
• Limestone (CaCO3) to remove impurities

Hot air is blasted in at the bottom.

Key reactions

1. Coke burns: C + O2 -> CO2 (provides heat)
2. Carbon dioxide reduced: CO2 + C -> 2CO (produces carbon monoxide)
3. Iron ore reduced by CO: Fe2O3 + 3CO -> 2Fe + 3CO2
4. Limestone decomposes: CaCO3 -> CaO + CO2
5. Lime removes impurities: CaO + SiO2 -> CaSiO3 (slag)

The key equation is reaction 3. Carbon monoxide is the actual reducing agent. Iron is reduced (gains electrons / loses oxygen) and carbon is oxidised (loses electrons / gains oxygen).

Molten iron collects at the bottom of the furnace. Slag (calcium silicate) floats on top and is tapped off separately.

Extraction of copper

Copper can be extracted by reduction with carbon (copper is below carbon in the reactivity series), but high-purity copper is produced by electrolysis.

Purification of copper by electrolysis

• Anode: impure copper (dissolves: Cu -> Cu2+ + 2e-)
• Cathode: pure copper (deposited: Cu2+ + 2e- -> Cu)
• Electrolyte: copper(II) sulfate solution

The impure anode dissolves, releasing Cu2+ ions. These ions are deposited as pure copper on the cathode. Impurities fall to the bottom as anode sludge.

The cathode gains mass. The anode loses mass. The concentration of Cu2+ in solution stays approximately constant.

See electroplating for the related technique.

Recycling metals

Recycling is preferred over extraction because:

Advantage of recycling	Explanation
Saves energy	Extraction requires high temperatures or electrolysis
Conserves finite ore supplies	Metal ores are non-renewable
Reduces mining	Mining damages habitats and landscapes
Reduces landfill	Less waste goes to landfill
Lower CO2 emissions	Less fuel burned for energy

Aluminium recycling saves about 95% of the energy needed compared to extraction from bauxite. This makes economic and environmental sense.

Comparing methods

Feature	Electrolysis	Carbon reduction
Used for	Reactive metals (Al, Na, Mg)	Less reactive metals (Fe, Zn, Cu)
Energy cost	Very high (continuous electricity)	Lower (heat from burning coke)
Product purity	High	May need further purification
Carbon electrodes needed?	Yes (for aluminium extraction)	Carbon is a raw material
CO2 emissions	From burning anodes + electricity generation	From burning coke

Common exam mistakes

1. “Aluminium is extracted using carbon” — aluminium is above carbon in the reactivity series, so carbon reduction does not work. Electrolysis must be used.
2. “Carbon reduces iron in the blast furnace” — technically, carbon monoxide (CO) is the main reducing agent, not carbon directly.
3. “The blast furnace uses electrolysis” — the blast furnace is a chemical reduction, not electrolysis.
4. Confusing the extraction of aluminium with the purification of copper: Both use electrolysis, but with different setups. In aluminium extraction, the electrolyte is molten Al2O3. In copper purification, the anode is impure copper.

Worked exam questions

Explain why aluminium is extracted by electrolysis but iron is extracted by carbon reduction. [4 marks]

• Aluminium is more reactive than carbon [1]
• So carbon cannot reduce aluminium oxide [1]
• Iron is less reactive than carbon [1]
• So carbon/carbon monoxide can reduce iron(III) oxide in the blast furnace [1]

In the electrolysis of aluminium oxide, the carbon anodes need to be replaced regularly. Explain why. [2 marks]

• Oxygen gas is produced at the anode [1]
• The oxygen reacts with the hot carbon anodes, burning them away: C + O2 -> CO2 [1]

Give two reasons why recycling aluminium is better than extracting it from its ore. [2 marks]

• Saves energy / electricity (extraction by electrolysis uses large amounts of energy) [1]
• Conserves aluminium ore / bauxite, which is a finite resource / reduces mining damage [1]