Consider the reactions:
`2S_2O_3^(2-) (aq) + I_2 (s) rarr S_4O_6^(2-) (aq) +2I^(-) (aq)`
` S_2O_3^(2-)(aq) +2 Br_2 (I) harr 5H_2O (I) rarr 2SO_4^(2-) (aq)+ 4Br^(-) (aq) +10H^(+) (aq)`
Why does the same reductant, thiosulphate react differently with iodine and bromine ?

Explanation

S_2 O_3^(2-). is getting oxidised to .S_4 O_6^(2-). whereas .I_2. is acting as oxidising agent. .Br_2. is stronger oxidising agent than .I_2. therefore it oxidises .quad S_2 O_3^(2-). to .SO_4^2. -ions.