Question

Thiosulphate reacts differently with iodine and bromine in the reactions given below :
$2S_2{O}_3^{2-}+I_2\to S_4{O}_6^{2-}+2I^{-}$
$S_2{O}_3^{2-}+2B{r}_2+5H_{2}O\to 2S{O}_4^{2-}+2B{r}^{-}+10H^{+}$
Which of the following statement(s) justifies the above dual behaviour of thiosulphate?

• A. Bromine is a stronger oxidant than iodine
• B. Bromine is a weaker oxidant than iodine
• C. Thiosulphate undergoes oxidation by bromine and reduction by iodine in these reactions
• D. Bromine undergoes oxidation and iodine undergoes reduction in these reactions

Answer 1

The correct option is A Bromine is a stronger oxidant than iodine

Iodine oxidises $S_2{O}_3^{2-}$ to $S_4{O}_6^{2-}$ in which $S$ has low oxidation state $+2.5$ but bromine oxidises $S_2{O}_3^{2-}$ to ${SO}_4^{2-}$ in which sulphurs has higher oxidation state $+6$. So, Bromine oxidises more and therefore it is stronger oxidising agent than iodine.