Oxidation and Reduction
Oxidation and reduction refer to ionic (half) equations. An ionic equation where a species loses
electrons is called oxidation and an ionic equation where a species gains electrons is called
reduction. You can remember this using the mnemonic OILRIG - Oxidation Is Loss, Reduction Is Gain.
Another way to remember it is that with reduction, the charge of a species is reduced (made more
negative or less positive).
Examples:
Oxidation
Mg → Mg2+ + 2eNa → Na+ + eCu+ → Cu2+ + e2Cl- → Cl2 + 2eH2 → 2H+ +2e4OH- → O2 +2H2O + 4e-
Reduction
Ag+ + e- → Ag
Al3+ + 3e- → Al
Fe3+ + e- → Fe2+
Cl2 + 2e- → 2ClBr2 + 2e- → 2BrMnO4- +8H+ + 5e- → Mn2+ + 4H2O
Oxidising Agent
An oxidising agent (also called an oxidant) is a substance that can cause other substances to become
oxidised. In such a reaction, the oxidising agent is itself reduced. The classic example of an oxidising
agent is oxygen gas (O2). The ionic equations for the reaction between oxygen gas and calcium metal
would be: Ca → Ca2+ + 2e- and O2 + 4e- → 2O2-. See how the calcium atoms are oxidised (lose
electrons) and the oxygen atoms are reduced (gain electrons).
Reducing Agent
A reducing agent (also called a reductant) is a substance that can cause other substances to become
reduced. In such a reaction, the reducing agent is itself oxidised. The classic example of a reducing
agent is hydrogen gas (H2). The ionic equations for the reaction between hydrogen gas and copper
(II) ions would be: Cu2+ + 2e- → Cu and H2 → 2H+ + 2e-. See how the copper (II) ions are reduced (gain
electrons) and the hydrogen atoms are oxidised (lose electrons).