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

`to` Thio sulphate ion is not a strong reducing agent.
`to` `I_(2)` is not strong oxidising agent.
`to` The reaction between `I_(2)` and thio sulphate form tetrathionate `(S_(4)O_(6)^(-2))` ion.
`2S_(2)O_(3(aq))^(-2)+I_(2(s))to S_(4)O_(6(aq))^(-2)+2I_((aq))^(-)`
The above reaction has high rate of reaction.
The reaction between Thiosulphate and Bromine involves the formation of sulphate ion.
`S_(2)O_(3(aq))^(-2)+2Br_(2(l))+5H_(2)O_((l))to 2 SO_(4(aq))^(-2)+4Br_((aq))^(-)+10H_((aq))^(+)`
`to Br_(2)` is some what better oxidising agent than `I_(2)`
Hence the difference observed in the above reactions.