Oxidation and reduction

A black coating and a brown colour

Heat a piece of shiny copper wire in a flame. It becomes covered with a black layer. The copper has gained oxygen.

Now pass hydrogen gas over the hot black copper oxide. The black colour changes back to brown, the colour of copper, and drops of water form. The copper oxide has lost oxygen.

In the first reaction, copper gained oxygen. In the second, copper oxide lost oxygen. Chemists have names for these two opposite changes.

Oxidation and reduction

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

In $\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}$:

• $\text{CuO}$ loses oxygen, so it is reduced.
• $\text{H}_2$ gains oxygen, becoming $\text{H}_2\text{O}$, so it is oxidised.

The two happen together. A substance can lose oxygen only if another substance takes it. So one substance cannot be oxidised unless another is reduced. A reaction in which both take place is a redox reaction (reduction and oxidation).

Oxidising and reducing agents

• The substance that gives oxygen (or takes hydrogen) is the oxidising agent. It is itself reduced. Here it is $\text{CuO}$.
• The substance that takes oxygen (or gives hydrogen) is the reducing agent. It is itself oxidised. Here it is $\text{H}_2$.

A good way to remember: the agent does the job to another substance, and the opposite happens to itself. The oxidising agent oxidises the other, and is reduced itself.

You should find that a fifth of the mass always goes, the oxygen, and four fifths stay as copper.

Redox in daily life

• Burning. Burning of a fuel in oxygen is oxidation. When methane burns, carbon and hydrogen gain oxygen: $\text{CH}_4 + 2\,\text{O}_2 \rightarrow \text{CO}_2 + 2\,\text{H}_2\text{O}$.
• Rusting. Iron gains oxygen from the air and water, and forms a brown flaky solid.
• A cut apple turning brown. Substances in the apple gain oxygen from the air. Lemon juice slows it, because it holds back the oxidation.
• Respiration. Our body oxidises glucose to give energy: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O}$.
• Getting a metal from its ore. Iron ore is reduced by carbon monoxide: $\text{Fe}_2\text{O}_3 + 3\,\text{CO} \rightarrow 2\,\text{Fe} + 3\,\text{CO}_2$. The iron oxide loses oxygen (it is reduced), and the carbon monoxide gains oxygen (it is oxidised).
• Bleaching. Bleach gives oxygen to coloured substances and destroys the colour.

More examples

• $\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}$. Zinc oxide loses oxygen (reduced). Carbon gains oxygen (oxidised).
• $\text{MnO}_2 + 4\,\text{HCl} \rightarrow \text{MnCl}_2 + 2\,\text{H}_2\text{O} + \text{Cl}_2$. Manganese dioxide loses oxygen (reduced). Hydrogen chloride loses hydrogen (oxidised).
• $\text{H}_2 + \text{Cl}_2 \rightarrow 2\,\text{HCl}$. There is no oxygen here. In the full study of redox, this is also oxidation and reduction, using the idea of electrons, which you will meet in Class 11. For now, we use the oxygen and hydrogen definitions.

Practise with new numbers

Try it in Python

This program lists what happens to the two reactants of $\text{CuO} + \text{H}_2$.

Common mistakes

• Thinking that oxidation can happen alone. Each oxidation comes with a reduction.
• Mixing up the agent and the changed substance. The oxidising agent is the one that is reduced.
• Thinking that oxidation always needs oxygen gas. It can also be a loss of hydrogen.
• Forgetting that rusting is oxidation. It is slow burning of the metal.

Remember these

• Oxidation: gain of oxygen or loss of hydrogen. Reduction: loss of oxygen or gain of hydrogen.
• They happen together. Such a reaction is a redox reaction.
• The oxidising agent is reduced. The reducing agent is oxidised.
• Burning, rusting, respiration and the extraction of metals are redox reactions.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In $\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}$, which substance is reduced?

2. Is the burning of carbon in oxygen an oxidation or a reduction?

3. Can a reduction happen without an oxidation?

4. Why does lemon juice keep a cut apple from turning brown?