We have already learnt that the reactions
2Na(s) + Cl2(g) → 2NaCl (s) ------(1)
4Na(s) + O2(g) → 2Na2O (s) ------(2)
2Na(s) + S(s) → Na2S (s) ------(3)
are redox reactions because in each of these reactions sodium is oxidised due to the addition of either oxygen or more electronegative element to sodium. Simultaneously, chlorine, oxygen and sulphur are reduced because to each of these, the electropositive element sodium has been added.
From our knowledge of chemical bonding we also know that sodium chloride, sodium oxide and sodium sulphide are ionic compounds and perhaps better written as Na+Cl– (s), (Na+)2O2–(s), and (Na+)2 S2–(s).
Development of charges on the species produced suggests us to rewrite few reactions in the following manner :
For convenience, each of the above processes can be considered as two separate steps, one involving the loss of electrons and the other the gain of electrons. As an illustration, we may further elaborate one of these, say, the formation of sodium chloride.
2 Na → 2 Na+ (g) + 2e-
Cl2 (g) + 2e- → 2 Cl-
Each of the above steps is called a half reaction, which explicitly shows involvement of electrons. Sum of the half reactions gives the overall reaction :
2 Na + Cl2 (g) → 2 Na+ Cl- (s)
Reactions (1) to (3) suggest that half reactions that involve loss of electrons are called oxidation reactions. Similarly, the half reactions that involve gain of electrons are called reduction reactions.
It may not be out of context to mention here that the new way of defining oxidation and reduction has been achieved only by establishing a correlation between the behaviour of species as per the classical idea and their interplay in electron-transfer change.
In reactions (8.12 to 8.14) sodium, which is oxidised, acts as a reducing agent because it donates electron to each of the elements interacting with it and thus helps in reducing them.
Chlorine, oxygen and sulphur are reduced and act as oxidising agents because these accept electrons from sodium. To summarise, we may mention that
Oxidation: Loss of electron(s) by any species.
Reduction: Gain of electron(s) by any species.
Oxidising agent: Acceptor of electron(s).
Reducing agent: Donor of electron(s).