Consider the reactions
`2S_(2)O_(3)^(2-)(aq)+I_(2)(s)toS_(4)O_(6)^(2-)(aq)+2I^(-)(aq)`
`S_(2)O_(3)^(2-)(aq)+2Br_(2)(l)+5H_(2)O(l)to2SO_(4)^(2-)(aq)+4Br^(-)(aq)+10H^(+)(aq)`
Why does the same reductant, thiosulphate react differently with iodine and bromine ?

Explanation

Iodine is a weak oxidising agent while bromine is stronger oxidising reaction. So the oxidation of `S_(2)O_(3)^(2-)` with iodine will take place until the oxidation state of sulphur +2 in `S_(2)O_(3)^(2-)` changes to 2.5 in `S_(4)O_(6)^(2-)` only. But bromine being stronger oxidising agent can oxidise the sulphur ion `S_(2)O_(8)^(2-)` to its highest oxidation state +6 in `SO_(4)^(2-)`.