Carbon reduction of metal oxides

Some metals can be obtained from their oxides by heating the oxide with carbon. The carbon takes oxygen from the metal oxide, leaving the metal behind. Removing oxygen from a substance is called reduction, so the metal oxide is reduced to the metal.

This method works for metals less reactive than carbon: carbon can take oxygen from their oxides. Carbon reduction is not suitable for extracting metals that are more reactive than carbon.

For example, copper is less reactive than carbon. One reaction in which carbon reduces copper oxide is:

Here, copper oxide, $$\ce{CuO}$$, loses oxygen and forms copper, $$\ce{Cu}$$. Carbon gains oxygen and forms carbon dioxide, $$\ce{CO2}$$.

When interpreting a given extraction process, identify the metal oxide and check whether the metal is less reactive than carbon. Then use the stated reactants and products to determine whether carbon removes oxygen from the oxide. Any further judgement about the process should be based on the information provided.

Oxidation and reduction by oxygen transfer

In an oxygen-transfer reaction, oxidation means gaining oxygen and reduction means losing oxygen. To identify each change, track where the oxygen starts and where it ends up.

For example, carbon can extract copper from copper oxide:

The copper oxide, $$\ce{CuO}$$, loses oxygen and becomes copper, $$\ce{Cu}$$. The copper oxide is therefore reduced.

The carbon, $$\ce{C}$$, gains oxygen and becomes carbon dioxide, $$\ce{CO2}$$. The carbon is therefore oxidised.

Both changes happen in the same reaction: oxygen is removed from the metal oxide and gained by carbon.

Reactivity and how metals occur

A metal’s reactivity affects whether it is found in the Earth as the metal itself or combined with other elements. A reactive metal readily takes part in chemical reactions, while an unreactive metal reacts less easily.

Very unreactive metals, such as gold, can be found as the metal itself because they do not readily react with other substances. This is known as occurring native. Native metal can be obtained without chemically separating it from a compound.

Most metals are more reactive and are found combined with other elements in compounds. To obtain the pure metal from a compound, a chemical reaction is needed to separate the metal from the elements it is combined with. This is called extraction.

Reactivity therefore helps explain the difference: some unreactive metals can occur as metals, whereas metals found in compounds must be chemically extracted.