Ncert Exercises & Solutions

8.14 Consider the reactions :

2 S2O32– (aq) + I2(s) → S4 O6 2–(aq) + 2I–(aq)

S2O3 2–(aq) + 2Br2(l) + 5 H2O(l) → 2SO42–(aq) + 4Br–(aq) + 10H+(aq)

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

The average oxidising number (O.N.) of S in $S_{2} O_{3}^{2 -}$ is +2.

Being a stronger oxidizing agent than $l_{2}, {Br}_{2}$ oxidises, in which the O.N. of S is +6. However, $l_{2}$ is a weak oxidising agent.

Therefore, it oxidises, in which the average O.N. $S_{2} O_{3}^{2 -}$ or $S_{4} O_{6}^{2 -}$ of S in only +2.5.

As a result, $S_{2} O_{3}^{2 -}$ reacts differently with iodine and bromine.