(s) → 2PbO(s) + NO (g) + O (g) (c) NaH(s) + H O(l) → NaOH(aq) + H (g) (d) 2NO (g) + 2OH – (aq) → NO – (aq) + NO – (aq)+H O(l) 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 The Paradox of Fractional Oxidation Number Sometimes, we come across with certain compounds in which the oxidation number of a particular element in the compound is in fraction. Examples are: C O [where oxidation number of carbon is ( / )], Br O [where oxidation number of bromine is ( / )] and Na S O (where oxidation number of sulphur is .
). We know that the idea of fractional oxidation number is unconvincing to us, because electrons are never shared/transferred in fraction. Actually this fractional oxidation state is the average oxidation state of the element under examination and the structural parameters reveal that the element for whom fractional oxidation state is realised is present in different oxidation states. Structure of the species C O , Br O and S O – reveal the following bonding situations: + + O = C = C*= C = O Structure of C O (carbon suboxide) Structure of Br O (tribromooctaoxide) Structure of S O – (tetrathionate ion) The element marked with asterisk in each species is exhibiting the different oxidation state (oxidation number) from rest of the atoms of the same element in each of the species.
This reveals that in C O , two carbon atoms are present in + oxidation state each, whereas the third one is present in zero oxidation state and the average is / . However, the realistic picture is + for two terminal carbons