Originally, the term oxidation was used to describe the addition of oxygen to an element or a compound.
Because of the presence of dioxygen in the atmosphere (~20%), many elements combine with it and this is the principal reason why they commonly occur on the earth in the form of their oxides.
The following reactions represent oxidation processes according to the limited definition of oxidation:
2Mg (s) + O2 (g)à 2 MgO (s)
S (s) + O2 (g) à SO2 (g)
In above reactions the elements magnesium and sulphur are oxidised on account of addition of oxygen to them. Similarly, methane is oxidised owing to the addition of oxygen to it.
CH4 (g) + 2O2 (g) à CO2 (g) + 2H2O (l)
A careful examination of above reaction in which hydrogen has been replaced by oxygen prompted chemists to reinterpret oxidation in terms of removal of hydrogen from it and, therefore, the scope of term oxidation was broadened to include the removal of hydrogen from a substance.
The following illustration is another reaction where removal of hydrogen can also be cited as an oxidation reaction.
2 H2S (g) + O2 (g) à 2 S (s) + 2H2O (l)
As knowledge of chemists grew, it was natural to extend the term oxidation for reactions similar to above reactions, which do not involve oxygen but other electronegative elements.
The oxidation of magnesium with fluorine, chlorine and sulphur etc. occurs according to the following reactions:
Mg (s) + F2 (g) → MgF2 (s)
Mg (s) + Cl2 (g) → MgCl2 (s)
Mg (s) + S(s) → MgS (s)
Incorporating the above reactions within the fold of oxidation reactions encouraged chemists to consider not only the removal of hydrogen as oxidation, but also the removal of electropositive elements as oxidation. Thus the reaction:
2K4[Fe(CN)6](aq) + H2O (aq) → 2K3[Fe(CN)6](aq) + 2 KOH (aq)
is interpreted as oxidation due to the removal of electropositive element potassium from potassium ferrocyanide before it changes to potassium ferricyanide.
To summarise, the term “oxidation” is defined as the addition of oxygen/electronegative element to a substance or removal of hydrogen/ electropositive element from a substance.
In the beginning, reduction was considered as removal of oxygen from a compound.
However, the term reduction has been broadened these days to include removal of oxygen/electronegative element from a substance or addition of hydrogen/ electropositive element to a substance.
According to the definition given above, the following are the examples of reduction processes:
2 HgO (s) → 2 Hg (l) + O2 (g)
(removal of oxygen from mercuric oxide )
2 FeCl3 (aq) + H2 (g) → 2 FeCl2 (aq) + 2 HCl(aq)
(removal of electronegative element, chlorine from ferric chloride)
CH2 = CH2 (g) + H2 (g) → H3C – CH3 (g)
(addition of hydrogen)
2 HgCl2 (aq) + SnCl2 (aq) → Hg2Cl2 (s) + SnCl4 (aq)
(addition of mercury to mercuric chloride)
In the above reaction simultaneous oxidation of stannous chloride to stannic chloride is also occurring because of the addition of electronegative element chlorine to it.
It was soon realised that oxidation and reduction always occur simultaneously (as will be apparent by re-examining all the equations given above), hence, the word “redox” was coined for this class of chemical reactions.