Reduction of a metal oxide by carbon

Reducing a metal oxide with carbon is how moderately reactive and unreactive metals are extracted from their ores. It sits in the Form 5 Redox chapter and joins two big ideas: redox as the loss and gain of oxygen, and the reactivity series as the guide to the correct extraction method. This makes it a very common structured-question topic.

The idea: carbon as a reducing agent

When carbon is heated strongly with a metal oxide, the carbon takes the oxygen away from the metal. The carbon is oxidised (it gains oxygen to become carbon dioxide) and the metal oxide is reduced (it loses oxygen to become the metal). Carbon is therefore the reducing agent and the metal oxide is the oxidising agent. This only works if the metal is less reactive than carbon, because a more reactive metal holds its oxygen too strongly for carbon to remove.

Balanced equations

Lead(II) oxide

2PbO(s) + C(s) → 2Pb(l) + CO₂(g)

Copper(II) oxide

2CuO(s) + C(s) → 2Cu(s) + CO₂(g)

Tin(IV) oxide

SnO₂(s) + C(s) → Sn(l) + CO₂(g)

Iron(III) oxide (as in the blast furnace, where carbon first makes carbon monoxide, the actual reducing agent)

2C(s) + O₂(g) → 2CO(g)

Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g)

Zinc oxide

2ZnO(s) + C(s) → 2Zn(s) + CO₂(g)

Notice that in each equation the oxygen is transferred from the metal to the carbon, and every equation is balanced for each element.

Which metals can be extracted this way

Only metals below carbon in the reactivity series, zinc, iron, tin, lead and copper, can be extracted by carbon reduction. Metals above carbon, potassium, sodium, calcium, magnesium and aluminium, are too reactive; their oxides are not reduced by carbon and they are extracted by electrolysis of the molten compound instead. Knowing where carbon sits in the series is the key that unlocks these questions.

Conditions required

Strong, sustained heating is required, usually in a furnace, because the reaction needs a high temperature to proceed. A supply of carbon (coke) is mixed with the ore.

Observations

The mixture glows on heating. For copper(II) oxide, the black solid changes to brown copper. A colourless gas is released that turns limewater milky, confirming carbon dioxide. For metals with low melting points, such as lead and tin, a bead of molten metal forms.

Common mistakes to avoid

Do not try to extract aluminium or magnesium with carbon, a frequent error. Do not forget the state symbols; freed lead and tin are molten, (l), at the high extraction temperature, while copper is often written as solid. When asked to identify the reducing agent, name carbon (or carbon monoxide in the blast furnace), not the metal.

How it appears in the SPM exam

In Paper 2 (4541/2) you may be asked to write a balanced equation, to name the oxidising and reducing agents, or to explain why a named metal is or is not extracted by carbon using the reactivity series. In Paper 1 (4541/1), objective items often test the order of the reactivity series or ask which extraction method suits a given metal. A favourite structured question compares two metals, one above and one below carbon, and asks you to predict which is extracted by carbon and to give a reason. Tie every answer back to oxygen transfer and the position of the metal relative to carbon, and remember that “reduction” here means loss of oxygen, so the metal oxide is what is reduced while the carbon is what is oxidised. Getting these definitions the right way round is where marks are most often won or lost.