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REDOX REACTIONS 245 Solution reaction (c), hydrogen of water has been In reaction (a), the compound nitric displaced by hydride ion into dihydrogen oxide is formed by the combination of gas. Therefore, this may be called as the elemental substances, nitrogen and displacement redox reaction. The reaction oxygen; therefore, this is an example (d) involves disproportionation of NO 2 of combination redox reactions. The (+4 state) into NO (+3 state) and NO – reaction (b) involves the breaking down 2 3 of lead nitrate into three components; (+5 state). Therefore reaction (d) is an example of disproportionation redox therefore, this is categorised under decomposition redox reaction. In reaction. 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: C3O 2where oxidation number of carbon is (4/3)], Br3O 8where oxidation number of bromine is (16/3)] and Na 2S4O 6where oxidation number of sulphur is 2.5). 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 3O2, Br3O 8nd S O4 6–reveal the following bonding situations: +2 0 +2 O = C = C*= C = O Structure of C O 3 2 (carbon suboxide) Structure of Br3O 8tribromooctaoxide) Structure of S 4 6–(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 3 2 two carbon atoms are present in +2 oxidation state each, whereas the third one is present in zero oxidation state and the average is 4/3. However, the realistic picture is +2 for two terminal carbons and zero for the middle carbon. Likewise in Br O , each 3 8 of the two terminal bromine atoms are present in +6 oxidation state and the middle bromine is present in +4 oxidation state. Once again the average, that is different from reality, is 2– 16/3. In the same fashion, in the species S4O 6 each of the two extreme sulphurs exhibits oxidation state of +5 and the two middle sulphurs as zero. The average of four oxidation numbers of sulphurs of the S O 2–is 2.5, whereas the reality being + 5,0,0 and +5 oxidation number respectively for 4 6 each sulphur. We may thus, in general, conclude that the idea of fractional oxidation state should be taken with care and the reality is revealed by the structures only. Further, whenever we come across with fractional oxidation state of any particular element in any species, we must understand that this is the average oxidation number only. In reality (revealed by structures only), the element in that particular species is present in more than one whole number oxidation states. Fe O , Mn O , 3 4 3 4 Pb3O 4re some of the other examples of the compounds, which are mixed oxides, where we come across with fractional oxidation states of the metal atom. However, the oxidation states may be in fraction as in O+ and O – ere it is + and – res ectivel 2 2 Reprint 2026-27