Redox Reactions (Chemistry grade 12 notes)

Definitions
Oxidation is the addition of oxygen to a substance.
Oxidation involves the loss of electrons.
Oxidation involves an increase in the oxidation number of an element.
There are other definitions but for now these will suffice.
Reduction is the removal of oxygen from a substance.
Reduction involves the gaining of electrons.
Reduction involves a decrease in the oxidation number of a substance.
Lets take this reaction:
2Mg + O2 = 2MgO
Mg gained O2 so it is oxidised.
When Mg combines with O2 each Mg atom releases 2 electrons to form Mg2+ ions.
Mg = Mg2+ + 2e-
Mg loses electrons so it is oxidised.
The oxidation number of Mg changes from 0 to 2+ so it has increased.
Each O atom gains 2e- to form O2-
O2 + 4e- = 2O2-
The oxidation number of oxygen changed from 0 to 2- thus satisfying definitions 2 and 3 of reduction.
The equations showing loss and gaining of electrons are called half equations, the two equations can be put together to write a net redox equation. Lets put together the two equations:
2Mg + O2 + 4e- = 2Mg2+ + 4e- + 2O2-
Eliminate the electrons and the reaction remains as:
2Mg + O2 = 2Mg2+ + 2O2-
The sum of the oxidation numbers on both sides of the equation must be zero. The Mg2+ ions and the O2- ions attract each other forming the compound MgO.
Lets continue in the next session.