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

(i) `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) With the carful application of the concept of electronegativity only we may ifer that sodium is oxidised and hydrogen is reduced.
reaction (iii) chosen here prompts us to think in terms of another way to define redox reactions.