Identify the species undergoing oxidation and reduction.
a. `H_(2)S(g)+Cl_(2)(g)rarr2HCl(g)+S(s)`
b. `3Fe_(3)O_(4)(s)+8Al(s)rarr9Fe(s)+4Al_(2)O_(3)(s)`
c. `2Na(s)+H_(2)(g)rarr2NaH(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 beem removed from it .
(iii) With the careful application of the concept of electronegativity only we may infer 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 .