Suggest a scheme of classification of the following redox reactions
(a) `N_(2) (g) + O_(2) (g) rarr 2 NO (g)`
(b) `2 Pb (NO_(3))_(2) (s) rarr 2 PbO (s) + 4 NO_(2) (g) + O_(2) (g)`
(c) `NaH(s) + H_(2) O (l) rarr NaOH (aq) + H_(2) (g)`
(d) `2NO_(2) (g) + 2OH^(-) (aq) rarr NO_(2)^(-) (aq) + NO_(3)^(-) (aq) + H_(2) O (l)`

Explanation

In reaction (a), the compound nitric oxide is formed by the combination of the elemental substances, nitrogen and oxygen, therefore, this is an example of combination redox reactions. The reaction (b) involves the breaking down of lead nitrate into three components, therefore, this is categorised under decomposition redox reaction. In reaction (c), hydrogen of water has been displaced by hydride ion into dihydrogen gas. Therefore, this may be called as displacement redox reaction. the reaction (d) involves disproportionation of `NO_(2)` 4 state) into `NO_(2)^(-)` (+3 state) and `NO_(3)^(-)` (+5 state). Therefore reaction (d) is an example of disproportionation redox reaction.