Getting the metal out
Because most metals are combined chemically in their ores, we must break those bonds to obtain the pure metal. This process is called extraction. The method used depends on the metal's position in the reactivity series: the more reactive the metal, the more energy is needed to free it.
Key idea
Metals below carbon in reactivity (e.g. iron, zinc, lead) can be extracted by heating their oxide with carbon. Metals above carbon (e.g. aluminium, magnesium) must be extracted by electrolysis.
Reduction with carbon
For a metal such as iron, the oxide ore is heated strongly with carbon (in the form of coke) in a furnace. Carbon is more reactive than iron, so it takes the oxygen away from the iron. Removing oxygen is called reduction, and the reaction is: iron(III) oxide + carbon → iron + carbon dioxide.
Example
In a blast furnace, iron(III) oxide is reduced to molten iron while carbon is oxidised to carbon dioxide. The carbon is oxidised, and the iron oxide is reduced, in the same reaction.
Very reactive metals
Carbon cannot remove oxygen from a very reactive metal such as aluminium, because aluminium holds its oxygen too strongly. Instead, the molten ore is split using electricity in a process called electrolysis. This needs a great deal of electrical energy, which is why aluminium is expensive to produce.
Remember
- Reduction = removal of oxygen from a metal oxide.
- Below carbon: reduce the oxide with carbon (cheaper).
- Above carbon: use electrolysis (needs lots of energy).