Chemistry of Metal Extraction and Corrosion

Metals can occur naturally in pure form or as compounds within ores. Reactive metals are found in compound form while less reactive metals can be extracted from their ores.
Reduction, the removal of oxygen, is essential for extracting metals from oxides.
Metals are placed in a reactivity series based on their tendency to form positive ions. The more reactive metals are harder to reduce.
Carbon is used as a reducing agent for metals less reactive than itself; more reactive metals require extraction through electrolysis.

For metals above carbon in the reactivity series, electrolysis is used. Metal ions gain electrons at the cathode, converting the compound into a pure metal.

A practical experiment involves heating copper(II) oxide with carbon to demonstrate reduction and validate carbon's position in the reactivity series.

The primary ore of iron is hematite (
Fe2O3
), containing 60% iron, and is extracted via a blast furnace using a reduction process with carbon (coke).
The blast furnace operates at high temperatures where carbon dioxide reacts with more carbon to form carbon monoxide, a crucial reducing agent. The iron(III) oxide is reduced to iron:
ext{Fe}2 ext{O}3 + 3 ext{CO}
ightarrow 2 ext{Fe} + 3 ext{CO}_2

Limestone is added to remove impurities like silicon(IV) oxide, forming calcium silicate slag:
ext{CaCO}3 ightarrow ext{CaO} + ext{CO}2

Aluminium, which is high in reactivity, requires extraction via electrolysis after being purified from bauxite, which is treated with sodium hydroxide to obtain alumina.
The Hall-Héroult process involves dissolving alumina in cryolite to lower the melting point for electrolysis, making it more energy-efficient.
Aluminium is discharged at the cathode:
ext{Al}^{3+} + 3e^-
ightarrow ext{Al}

Corrosion affects metals when they react with substances such as air and water. Iron rusting is a significant form of corrosion, resulting in hydrated iron(III) oxide.
Unlike iron, aluminium forms a protective oxide layer that prevents further corrosion.

Several methods exist to protect iron from rusting:
- Barrier Methods: Painting, oiling, and using plastic coatings prevent contact with moisture and air.
- Galvanising: Coating iron with zinc provides protection even when scratched.
- Sacrificial Protection: Attaching more reactive metals like zinc to iron structures aids in preventing rusting by corroding in place of iron.