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Karnataka Board PUCPUC Science Class 11

Consider the reactions: 2SX2OX3X(2−)(aq)+lX2(S)⟶SX4OX6X(2−)(aq)+2lX−(aq) SX2OX3X(2−)(aq)+2BrX2(l)+5HX2O(l)⟶2SOX4X2−(aq)+4BrX−(aq)+10HX+(aq) Why does the same reductant, thiosulphate react - Chemistry

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Question

Consider the reactions:

\[\ce{2S_2O_3^{(2-)}(aq) + l_2(S) -> S_4O_6^{(2-)}(aq) + 2l-(aq)}\]

\[\ce{S_2O_3^{(2-)}(aq) + 2Br_2(l) + 5H_2O(l) -> 2SO_4^{2-} (aq) + 4Br-(aq) + 10H+ (aq)}\]

Why does the same reductant, thiosulphate react differently with iodine and bromine?

Numerical
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Solution

The average oxidation number (O.N.) of `"S"_2"O"_3^(2-)` in is +2. Being a stronger oxidising agent than I2, Br2 oxidises `"S"_2"O"_3^(2-)` to `"SO"_4^(2-)`, in which the O.N. of S is +6. However, I2 is a weak oxidising agent. Therefore, it oxidises `"S"_2"O"_3^(2-)` to `"S"_4"O"_6^(2-)`, in which the average O.N. of S is only +2.5. As a result, `"S"_2"O"_3^(2-)` reacts differently with iodine and bromine.

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Oxidation Number - Types of Redox Reactions
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