question_answer 24)

Consider the reactions: \[2{{S}_{2}}O_{3}^{2-}(aq)+{{I}_{2}}(s)\to {{S}_{4}}O_{6}^{2-}(aq)+2{{I}^{-}}(aq)\] \[{{S}_{2}}O_{3}^{2-}(aq)+2B{{r}_{2}}(l)\to 2SO_{4}^{2-}(aq)\] \[+4B{{r}^{-}}(aq)+10{{H}^{+}}(aq)\] Why does same reductant, thiosulphate react differently with iodine and bromine?  

Answer:

\[B{{r}_{2}}\]is stronger oxidising agent than \[{{I}_{2}}\] , it oxidises\[{{S}_{2}}O_{3}^{2-}\] to \[SO_{4}^{2-}\]i.e., from +2 state to +6 state of sulphur. However, \[{{I}_{2}}\] being weaker oxidising agent, oxidizes \[{{S}_{2}}O_{3}^{2-}\]to\[{{S}_{4}}O_{6}^{2-}\] ion i.e., from +2 to +2.5 state of sulphur.