What Is Oxidation and Reduction (in Terms of Electrons; Oxidation State; Oxygen; Hydrogen)?

Oxidation is the loss of electrons, an increase in oxidation state, gain of oxygen, or loss of hydrogen. Reduction is the gain of electrons, a decrease in oxidation state, loss of oxygen, or gain of hydrogen.

These definitions are assessed at both SL and HL in Reactivity 3.2.1. The electron-transfer and oxidation-state definitions are the most general; the oxygen and hydrogen definitions apply only when those elements are involved.

The mnemonic OIL RIG means “Oxidation Is Loss, Reduction Is Gain” of electrons.

Consider this displacement reaction:

$\text{Zn}(s) + \text{Cu}^{2+}(aq) \rightarrow \text{Zn}^{2+}(aq) + \text{Cu}(s)$

Zinc loses two electrons, so it is oxidized:

$\text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq) + 2e^-$

Copper ions gain two electrons, so they are reduced:

$\text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s)$

The oxidation state of zinc increases from 0 to +2, while that of copper decreases from +2 to 0. Oxidation and reduction always occur together because electrons lost by one species must be gained by another.

In the zinc reaction, $\text{Cu}^{2+}$ is the oxidizing agent because it accepts electrons and causes zinc to be oxidized. Zinc is the reducing agent because it donates electrons and causes copper ions to be reduced. The agent itself undergoes the opposite process.

The oxygen and hydrogen definitions can be seen here:

$\text{CuO}(s) + \text{H}_2(g) \rightarrow \text{Cu}(s) + \text{H}_2\text{O}(l)$

Copper(II) oxide loses oxygen, so it is reduced. Hydrogen gains oxygen, so it is oxidized. Equivalently, copper decreases from +2 to 0, while hydrogen increases from 0 to +1.

A common misconception is that oxidation always means gaining oxygen. This is only one possible definition; electron loss or an increase in oxidation state identifies oxidation even when oxygen is absent.

Exam technique: When asked to explain redox, state the relevant oxidation-state changes or write balanced half-equations. Include electrons and charges explicitly, and remember that oxidation occurs at the anode while reduction occurs at the cathode.