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Consider the reactions: 

2S2O32- (aq) + I2(s) S4O62-(aq) + 2I-(aq)        

S2O32-  (aq)  +  2Br2(I) + 5 HO(l)  →  2SO42-  (aq) + 4Br (aq) + 10H+(aq) 

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

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The average oxidation number (O.N.) of S in S2O32- 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 , S2O32- reacts differently with iodine and bromine. 

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