Extraction of Metals (Iron and Aluminium)

Extraction is where the Metals topic turns into a structured-question goldmine: five to eight marks at a time on Papers 3 and 4, built from equations that are entirely predictable. The blast furnace sequence and the aluminium cell have appeared, between them, on nearly every recent series. Yet examiner reports keep recording half-known equations and “carbon reduces the iron oxide” where the reducing agent is carbon monoxide.

Reactivity decides the method

The unifying principle: a metal’s position relative to carbon in the reactivity series decides how it is extracted.

Position	Method	Examples
Above carbon	Electrolysis of the molten compound	Potassium, sodium, calcium, magnesium, aluminium
Below carbon	Reduction of the oxide with carbon	Zinc, iron
Very unreactive	Found native (uncombined)	Silver, gold

Cost is the second scoring idea: electrolysis uses large amounts of electricity and is expensive, so it is used only where carbon reduction is chemically impossible. Gold needs no extraction chemistry at all. It occurs as the element because it is too unreactive to form compounds easily.

Iron: the blast furnace

Raw materials in: hematite (mainly iron(III) oxide, Fe2O3), coke (carbon), limestone (calcium carbonate), and a blast of hot air. The four-step equation sequence is the question, so learn it as a story:

1. Coke burns in the hot air, releasing the heat that drives the furnace: C + O2 → CO2 (exothermic).
2. Carbon monoxide forms when the carbon dioxide rises through more hot coke: CO2 + C → 2CO.
3. The reduction. Carbon monoxide reduces the iron(III) oxide: Fe2O3 + 3CO → 2Fe + 3CO2. The reducing agent is carbon monoxide. Name it precisely; “carbon” in this equation loses the mark.
4. Slag forms. Limestone decomposes in the heat (CaCO3 → CaO + CO2), and the calcium oxide, a basic oxide, reacts with the acidic silicon dioxide impurity in the ore: CaO + SiO2 → CaSiO3. The calcium silicate (slag) is molten, floats on the denser molten iron, and is tapped off separately.

Definitions ride along: reduction is loss of oxygen (or gain of electrons, Supplement). Fe2O3 is reduced; CO is oxidised to CO2.

Aluminium: electrolysis

Aluminium sits above carbon, so its oxide cannot be reduced by carbon. The ore bauxite is purified to aluminium oxide (alumina, Al2O3), which is then dissolved in molten cryolite. The cryolite mark is precise: it lowers the operating temperature (well below the 2000+ °C needed to melt pure alumina), reducing energy costs.

The cell has carbon (graphite) electrodes. The molten mixture contains free Al3+ and O2− ions:

• Cathode (negative): aluminium ions are reduced: (Supplement half-equation) Al3+ + 3e− → Al. Molten aluminium collects at the bottom and is tapped off.
• Anode (positive): oxide ions are oxidised: (Supplement half-equation) 2O2− → O2 + 4e−.

The oxygen produced attacks the hot carbon anodes, forming carbon dioxide, so the anodes burn away and must be replaced regularly, a two-mark favourite. The electrode logic is shared with electrochemistry, and revising the two pages together halves the workload.

Worked exam question

The blast furnace extracts iron from hematite. (a) Write the equation for the reduction of iron(III) oxide and name the reducing agent. [3] (b) Explain the purpose of the limestone, including one equation. [3] (c) Explain why this method cannot be used to extract aluminium. [2]

Model answer: (a) Fe2O3 + 3CO → 2Fe + 3CO2: formulae (1), balancing (1); the reducing agent is carbon monoxide (1). (b) Limestone decomposes to calcium oxide: CaCO3 → CaO + CO2 (1); the calcium oxide reacts with the silicon dioxide (sand) impurity (1) to form calcium silicate, slag, which is removed: CaO + SiO2 → CaSiO3 (1). (c) Aluminium is above carbon in the reactivity series (1), so carbon/carbon monoxide cannot reduce aluminium oxide (1).

Mark-by-mark: (a) “carbon” as the reducing agent contradicts the equation just written and loses M3. (b) the impurity must be named (silicon dioxide/silica/sand) and the slag product identified. (c) needs the explicit comparison with carbon; “aluminium is too reactive” without the reference point is one mark at best.

The mistakes that cost marks

1. “Carbon reduces the iron oxide.” In the main reduction the reducing agent is carbon monoxide. Equation 3 is written with CO for a reason.
2. Limestone “removes impurities” with no chemistry. The marks are in the detail: CaCO3 decomposes, CaO neutralises the acidic SiO2, CaSiO3 (slag) floats on the iron.
3. Cryolite as a solvent only. “It dissolves the aluminium oxide” misses the scoring idea: lower operating temperature, lower energy cost.
4. Electrode products swapped. Aluminium forms at the cathode (negative electrode); oxygen at the anode. Reversing them voids the half-equations too.
5. Forgetting why anodes are replaced. Oxygen + hot carbon → carbon dioxide; the anodes burn away. Two easy marks, frequently left blank.

How examiners want it phrased

Student wording	Mark-scheme wording
”Aluminium is too reactive for the blast furnace"	"Aluminium is above carbon in the reactivity series, so carbon cannot reduce aluminium oxide"
"Cryolite dissolves the ore"	"Dissolving the oxide in molten cryolite lowers the operating temperature, reducing energy costs"
"Limestone cleans the iron"	"Calcium oxide reacts with the silicon dioxide impurity to form calcium silicate (slag)"
"The anodes wear out"	"Oxygen produced at the anode reacts with the hot carbon anode to form carbon dioxide, so the anodes burn away”

Write out both processes from memory once a week until the exam. The equations do not change, and neither do the questions. To see whether your version would actually score, bring it to a free trial lesson and we will mark it line by line against real schemes.

Test yourself

Place each metal against carbon in the series first, then click each answer.

Q1 (3 marks). State how each metal is obtained, giving the reason from the reactivity series: (a) sodium; (b) zinc; (c) gold.

Show answer

• (a) electrolysis of the molten compound: sodium is above carbon, so carbon cannot reduce its oxide [1] • (b) reduction of the oxide with carbon: zinc is below carbon [1] • (c) gold is found native (uncombined): it is too unreactive to form compounds easily [1]

Q2 (2 marks). In the blast furnace, write the equations for (a) the burning of the coke and (b) the formation of carbon monoxide.

Show answer

• (a) C + O2 → CO2 (exothermic: this heats the furnace) [1] • (b) CO2 + C → 2CO [1]

Q3 (3 marks). In the extraction of aluminium: (a) state why the aluminium oxide is dissolved in molten cryolite; (b) (Extended) write the half-equation at the cathode; (c) explain why the carbon anodes must be replaced regularly.

Show answer

• (a) cryolite lowers the operating temperature, reducing energy costs [1] • (b) Al3+ + 3e− → Al [1] • (c) the oxygen formed at the anode reacts with the hot carbon to form carbon dioxide, so the anodes burn away [1]