Oxidation of Metals

Rusting, Burning, and Extracting

Leave a shiny iron nail outside, and it slowly turns into flaky, orange-brown rust. This familiar process is a chemical reaction with the air around it. When metals react with oxygen, they produce compounds called metal oxides.

During these reactions, the mass of the solid actually increases. This happens because oxygen atoms from the air are added to the metal to form the solid product.

Defining Oxidation and Reduction

We can define these chemical changes strictly in terms of oxygen transfer:

• Oxidation is the gain of oxygen by a substance.
• Reduction is the loss of oxygen from a substance.

If you heat a metal like magnesium in a crucible, it undergoes oxidation. It burns with a bright white flame to produce a white powder, magnesium oxide. The oxidation is complete when the substance is heated to a constant mass, meaning no more oxygen is reacting with it.

$$2Mg(s) + O_2(g) \rightarrow 2MgO(s)$$

The Mechanism of Redox Reactions

In many industrial processes, such as extracting metals from an ore, both oxidation and reduction happen at the same time. This is called a redox reaction.

We can demonstrate the causal mechanism of oxygen transfer using the reaction between copper(II) oxide and carbon. If you strongly heat black copper(II) oxide powder with black carbon powder, they react to form a pink/red-brown solid (copper) and a gas (carbon dioxide) that turns limewater cloudy.

$$2CuO(s) + C(s) \rightarrow 2Cu(s) + CO_2(g)$$

Here is exactly how the oxygen transfer works:

1. The carbon acts as a reducing agent. Because carbon has a higher affinity for oxygen than copper, it removes or "steals" the oxygen from the copper compound.
2. The $C$ gains oxygen to become $CO_2$. Therefore, the carbon is oxidised.
3. The $CuO$ loses oxygen to become pure $Cu$. Therefore, the copper(II) oxide acts as an oxidising agent and is reduced.

This method of extracting metals using carbon only works if the metal is less reactive than carbon. Highly reactive metals hold onto their oxygen too tightly and must be extracted using electrolysis instead.