Iron(III) Oxide

Iron(III) oxide is the same compound at both ends of iron’s life: it is the ore fed into the blast furnace and the rust that eats iron away afterwards. That double role is why Fe2O3 shows up in extraction, corrosion and redox questions alike.

Key facts

Property	Detail
Formula	Fe2O3
Relative formula mass (Mr)	160 (Fe 56, O 16)
Bonding / structure	Giant ionic lattice (Fe3+ and O2- ions)
Appearance	Red-brown solid
Classification	Basic oxide
Main exam roles	Iron ore (haematite), rust, redox example

Where this compound appears in 0620

• Extraction of metals (Topic 10): the reduction of Fe2O3 by carbon monoxide in the blast furnace.
• Rusting and prevention (Topic 10): rust is hydrated iron(III) oxide.
• Redox and oxidation numbers (Topic 7): a textbook oxidation-and-reduction pair.
• Tests for ions (Topic 11): identifying the Fe3+ ion.

Reactions you must know

In the blast furnace:

Fe2O3(s) + 3CO(g) -> 2Fe(l) + 3CO2(g)

A redox reaction: Fe2O3 is reduced (iron goes from +3 to 0) and CO is oxidised (carbon from +2 to +4), so CO is the reducing agent. Iron sits below carbon in the reactivity series, which is why reduction with carbon works; at the hottest part of the furnace carbon can reduce the oxide directly:

2Fe2O3(s) + 3C(s) -> 4Fe(l) + 3CO2(g)

Rusting:

4Fe(s) + 3O2(g) + 2xH2O(l) -> 2Fe2O3.xH2O(s)

Both oxygen and water are needed. The hydrated iron(III) oxide is flaky and non-protective, so fresh iron is continually exposed.

With acids (basic oxide):

Fe2O3(s) + 6HCl(aq) -> 2FeCl3(aq) + 3H2O(l)

This gives iron(III) salts — Fe3+ — such as yellow-brown iron(III) chloride.

Identifying Fe3+: adding sodium hydroxide solution gives a red-brown precipitate of Fe(OH)3.

Worked exam question

Rust is hydrated iron(III) oxide. A student sets up three boiling tubes to find out what makes iron nails rust: Tube A holds a nail in ordinary water open to the air; Tube B holds a nail in boiled water sealed under a layer of oil; Tube C holds a nail in dry air over a drying agent.

(a) Name the two substances needed for iron to rust. (1 mark)

(b) State which tube shows rusting and explain why the other two do not. (2 marks)

(c) Galvanising coats iron with zinc. Explain how the zinc still protects the iron if the coating is scratched. (2 marks)

Mark scheme

• (a) Water and oxygen (both needed) [1]
• (b) Only Tube A rusts [1]; it is the only tube with both water and oxygen — Tube B has no oxygen (boiled out and sealed by oil) and Tube C has no water (removed by the drying agent) [1]
• (c) Zinc is more reactive than iron [1]; so the zinc loses electrons / corrodes in preference to the iron (sacrificial protection), protecting the iron even where it is exposed [1]

Examiner note: in (a) both substances are needed for the single mark — naming only one scores zero. In (c), the reasoning must be that zinc is more reactive and is oxidised first; “the zinc covers the iron” does not explain protection at a scratch.

Common exam mistakes

• Producing iron(II) salts from Fe2O3. Iron(III) oxide contains Fe3+, so it makes iron(III) salts (FeCl3), not iron(II) salts (FeCl2).
• Saying carbon monoxide is reduced in the furnace. CO is oxidised to CO2; it is the iron oxide that is reduced.
• Writing rust as Fe2O3. Rust is hydrated iron(III) oxide, Fe2O3.xH2O — both water and oxygen must be present for it to form.
• Comparing it wrongly with aluminium. Rust flakes off and offers no protection, unlike the coherent oxide layer on aluminium.