Suggest a scheme of classification of the following redox reactions:
(a) N2(g) + O2(g) → 2 NO (g)
(b) 2Pb(NO3)2(s) → 2PbO(s) + 2 NO2 (g) + ½ O2 (g)
(c) NaH(s) + H2O(l) → NaOH(aq) + H2 (g)
(d)2NO2(g) + 2OH–(aq) → NO2-(aq) +NO3–(aq)+H2O(l)
(a) It is a combination redox reaction:
0 0 +2 - 2
N2(g) O2(g) 
2NO(g)
because, the compound nitric oxide is formed by the combination of the elemental substances, like nitrogen and oxygen. Since the oxidation number of nitrogen increases from 0 (in N2) to +2(in NO) and that of oxygen decreases from zero (in 02) to -2 (in NO),
hence, it is a combination redox reaction.
(b) It is a decomposition redox reaction,
because on heating lead nitrate decomposes to form a lead oxide, nitrogen dioxide and oxygen. Since the oxidation number of nitrogen decreases from +5 (in lead nitrate) to +4 (in NO2) and that of oxygen increases from -2 (in lead nitrate) to zero (in O2), hence, it is a decomposition redox reaction.
because hydrogen of water has been displaced by hydride ion to produce dihydrogen gas. Also, the oxidation number of hydrogen increases from -1 in NaH) to zero (in H2) while that of hydrogen decreases from +1 (in water) to zero (in H2), hence it is a displacement redox reaction.
(d) It is a disproportionation reaction
Because oxidation state of nitrogen decreases from +4 (In NO2) to +3 (In NO-2 ion), as well as increases from +4 (in NO2) to +5 (In NO3- ion), hence it is disproportionation reaction.
