Consider the reactions: Why does the same reductant,

Question

Consider the reactions: 

2S₂O₃^2- _(aq) + I_2(s) S₄O₆^2-_(aq) + 2I^-_(aq)        

S₂O₃^2-  _(aq)  +  2Br_2(I) + 5 HO_(l)  →  2SO₄^2-  _(aq) + 4Br^– _(aq) + 10H⁺_(aq) 

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

Answer 1

The average oxidation number (O.N.) of S in S₂O₃^2- is +2. Being a stronger oxidising agent than I₂, Br₂ oxidises , in which the O.N. of S is +6. However, I₂ is a weak oxidising agent. 

Therefore, it oxidises , in which the average O.N. of S is only +2.5. As a result , S₂O3^2- reacts differently with iodine and bromine.