In the reactions given below, identify the species undergoing oxidation and reduction :
(i) `H_(2)S (g) + Cl_(2) (g) rarr HCl (g) + S (s)`
(ii) `3Fe_(3)O_(4) (s) + 8 Al (s) rarr 9 Fe(s) + 4Al_(2)O_(3) (s)`
(iii) `2Na (s) + H_(2) (g) rarr 2 NaH (s)`

Explanation

`H_(2)S` is oxidised because a more electronegative element, chlorine is added to hydrogen (or a more electropositive element, hydrogen has been removed from S). Chlorine is reduced due to addition of hydrogen to it.
(ii) Aluminium is oxidised because oxygen is added to it. Ferrous ferric oxide `(Fe_(3)O_(4))` is reduced because oxygen has been removed from it.
(iii) This reaction is very intersting. It may be said from the above definitions that the reaction is only a reduction reaction as it involves the addition of sodium (electropositive metal) or hydrogen whereas sodium undergoes oxidation and hydrogen undergoes redicton it is to say that the above definitions for oxidation and reduction can not explain this and they have. limitations like this, Therefore a new concept has to be considered for the oxidation and the reduction.