Thiosulphate reacts differently with iodine and bromine in the reactions given below: 2S2O32- + I2 S4O62- + 2I- S2O32- + 5Br2 + 5H2O 2SO42- + 4Br- + 10H+ Which of the following statements justifies the above dual behaviour of thiosulphate?
- Thiosulphate undergoes oxidation by bromine and reduction by iodine in these reactions
- Bromine is a weaker oxidant than iodine
- Bromine is a stronger oxidant than iodine
- Bromine undergoes oxidation and iodine undergoes reduction in these reactions
Answer
(3) Bromine is a stronger oxidant than iodine
Solution
Oxidation number of S:
with I2: S2O32- (+2) → S4O62- (+2.5)
with Br2: S2O32- (+2) → SO42- (+6)
Bromine takes S to a much higher oxidation state, so bromine is the stronger oxidant.
E (Br2/Br-) = 1.09 V > E (I2/I-) = 0.54 V
(1): thiosulphate is oxidised in both reactions, so (1) is wrong
(2): it states the reverse of the fact, so (2) is wrong
(4): both Br2 and I2 are reduced (to Br- and I-), so (4) is wrong
Answer: (3)