Sulphur shows a large number of oxidation states in its compounds such as `H_2S, SO_2, H_2SO_3,Na_2S_2O_3, H_2SO_4, H_2S_2O_7, H_2S_2O_4` and elemental S . In the reactions of sulphur compounds , the oxidation number of sulphur increases and decreases in different reactions and the compounds act as reducing and oxidising agents.
(i) `H_2S +HNO_3 rarr NO+S+H_2O`
(ii) `2Na_2S_2O_3+I_2rarr Na_2S_4O_6+2NaI`
(iii) `H_2SO_4+2HBr rarr 2H_2O+Br_2+SO_2`
(iv) `S_8+12OH^(-) rarr 4S^(2-) +2S_2O_3^(2-) +6H_2O`
Name the substance which gets (a) reduced and (b) oxidised in reaction (iii)

Answer

(a) `H_2SO_4` (b) HBr