Always a pair

The idea

Reactions in which one reactant gets oxidised while the other gets reduced are oxidation-reduction reactions, or redox reactions.

Who takes the oxygen?

Hydrogen is passed over hot black copper(II) oxide. The black coat turns brown: copper is back. The oxygen has left the copper oxide.

Where did that oxygen go?

• To the hydrogen, which became water
• It vanished
• It stayed in the copper

Find the pair

Run each reaction. For every substance tap oxidised or reduced. Do you ever find just one of them alone?

Oxidation and reduction go together

Reactions in which one reactant gets oxidised while the other gets reduced are oxidation-reduction reactions, or redox reactions.

The oxygen that one substance loses is taken by another substance.

• Reaction | Oxidised | Reduced
• CuO + H₂ → Cu + H₂O | H₂ | CuO
• ZnO + C → Zn + CO | C | ZnO
• MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ | HCl | MnO₂

Notes

Reactions in which one reactant gets oxidised while the other gets reduced are oxidation-reduction reactions, or redox reactions.

Check yourself

In ZnO + C → Zn + CO, which pair is right?

Why is CuO + H₂ → Cu + H₂O a redox reaction?

In MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, HCl is oxidised to Cl₂. So MnO₂ must be …

• Carbon is oxidised and ZnO is reduced — correct. Yes! Carbon gains oxygen, ZnO loses it.
• Carbon is reduced and ZnO is oxidised. It is the other way round: carbon gains oxygen (oxidised) and ZnO loses it (reduced).
• Both are oxidised. In a redox reaction one is oxidised and the other is reduced.
• Because it makes water. Making water is not what makes a reaction redox.
• One reactant is oxidised while the other is reduced — correct. Yes! That is the meaning of redox.
• Because it needs heat. Heat is not the test. We look at who is oxidised and who is reduced.
• oxidised too. The pair must be one oxidised and one reduced.
• not involved. MnO₂ lost its oxygen: it is reduced.
• reduced — correct. Yes! It loses oxygen. In a redox reaction one is oxidised and the other reduced.