4.9 Oxidation-Reduction (Redox) Reactions

What is a redox reaction?

In an oxidation-reduction (redox) reaction, electrons transfer between species, changing oxidation numbers. Oxidation = loss of electrons (oxidation number increases). Reduction = gain of electrons (oxidation number decreases). Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain. These reactions include combustion, corrosion, and many biological processes.

Assigning oxidation numbers

Rules: free elements = 0; monatomic ions = ionic charge; oxygen is usually −2; hydrogen is usually +1; the sum of oxidation numbers in a neutral compound equals 0, and in a polyatomic ion equals its charge. Comparing oxidation numbers before and after a reaction identifies which atoms were oxidized or reduced.

Oxidizing and reducing agents

The species that is reduced (gains electrons) is the oxidizing agent, because it causes another species to be oxidized. The species that is oxidized (loses electrons) is the reducing agent. In a balanced redox reaction, electrons lost by the reducing agent equal electrons gained by the oxidizing agent.

Key terms

Oxidation

Loss of electrons; oxidation number increases.

Reduction

Gain of electrons; oxidation number decreases.

OIL RIG

Mnemonic: Oxidation Is Loss, Reduction Is Gain (of electrons).

Oxidizing agent

The species that is reduced (gains electrons), causing another species to be oxidized.

Reducing agent

The species that is oxidized (loses electrons), causing another species to be reduced.

Try an AP-style question

What is the oxidation state of S in SO₄²⁻?

1. +4
2. −2
3. +8
4. +6