Explain the different types of redox reactions.

Explanation

A chemical reaction in which both oxidation and reduction reactions are involved is called an oxidation - reduction reaction or simply a redox reaction.
Ex.
In this reaction, Zn loses two electrons to form `Zn^(+2)` and undergoes oxidation. `Cu^(+2)` gains two electrons to form Cu and thus undergoes reduction.
Most of the chemical reactions are redox reactions. These are mainly four types of redox reactions. They are
(a) Chemical combination reactions
(b) Chemical decomposition reactions
( c) Chemical displacement reactions and
(d) Chemical disproportionation reactions
a) Chemical combination reactions :
Ex : Burning of coal in air.
`overset(0)(C_((s)))+overset(0)(O_(2(g))) to overset(+4)(C)overset(-2)(O_(2(g)))`
In this reaction the oxidation numbers of carbon and oxygen are zero. In `CO_(2)`, the oxidation number of 'C' is +4 and that of oxygen is -2. As the oxidation number of carbon increase from 0 to +4, we say that carbon undergoes oxidation. Similarly the oxidation number of oxygen decreases from 0 to -2. Hence the oxygen undergoes reduction. Since this reaction involves both oxidation and reduction, we can infer that the above chemical combination reaction is a redox reaction.
b) Chemical decomposition reactions :
Ex. : Thermal decomposition of mercuric oxide.

In HgO, the oxidation number of Hg is +2 and that of oxygen is -2. The oxidation numbers of free metallic mercury and elemental oxygen are zero. In this reaction Hg undergoes reduction from +2 to 0 and oxygen undergoes oxidation from -2 to 0. The decomposition of HgO involves both oxidation and reduction. Hence, we can infer that the above decomposition reaction is a redox reaction.
c) Chemical displacement reactions :
Ex. Zinc displaces copper from aqueous copper sulphate solution.

In this reaction, the oxidation numbers of elemental zinc and elemental copper are each zero, and the oxidation numbers of Cu and Zn in their aq. solutions are each +2. In this reaction the conversion of Zn into `ZnSO_(4)` is oxidation and the conversion of `CuSO_(4)` into Cu reduction. Hence, we can infer that the above displacement reaction is a redox reaction.
d) Chemical disproportionation reactions :
Ex. : Chlorine is passed into cold and dilute solution of NaOH.

In this reaction the oxidation number of elemental chlorine is zero. The oxidation number of Cl in NaOCl is (+1) and in NaCl is (-1). In this reaction the same element chlorine has undergone both oxidation and reduction. Here this is a redox reaction.