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Redox Reactions — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryRedox Reactions3 Marks
Question
Consider the reactions:$2\text{S}_2\text{O}^{2-}_{3}(\text{aq})+\text{I}_2(\text{s})\rightarrow\text{S}_4\text{O}^{2-}_{6}(\text{aq})+2\text{I}^{-}(\text{aq})$
$\text{S}_2\text{O}^{2-}_{3}(\text{aq})+2\text{Br}_2(\text{l})+5\text{H}_2\text{O(l)}\rightarrow\\2\text{SO}^{2-}_{4}(\text{aq})+4\text{Br}^{-}(\text{aq})+10\text{H}^+(\text{aq})$
Why does the same reductant, thiosulphate react differently with iodine and bromine?
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Answer

The average oxidation number (O.N.) of S in $\mathrm{S}_2 \mathrm{O}_3^{2-}$ is +2 . Being a stronger oxidising agent than $\mathrm{I}_2, \mathrm{Br}_2$ oxidises $\mathrm{S}_2 \mathrm{O}_3^{2-}$ to $\mathrm{SO}_4^{2-}$, in which the O.N. of S is +6 . However, $\mathrm{I}_2$ is a weak oxidising agent. Therefore, it oxidises $\mathrm{S}_2 \mathrm{O}_3^{2-}$ to $\mathrm{S}_4 \mathrm{O}_6^{2-}$, in which the average O.N. of S is only +2.5 . As a result, $\mathrm{S}_2 \mathrm{O}_3^{2-}$ reacts differently with iodine and bromine.

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