Question

Thiosulphate reacts differently with iodine and bromine in the reaction given below:  
$$ 2S_2O_3^{2-} + I_2 \rightarrow S_4O_6^{2-} + 2I^- $$ $$ S_2O_3^{2-} + 5Br_2 + 5H_2O \rightarrow 2SO_4^{2-} + 4Br^- + 10H^+ $$ 
Which of the following statements justifies the above dual behaviour of thiosulphate?

• A. Bromine undergoes oxidation and iodine undergoes reduction by iodine in these reactions
• B. Thiosulphate undergoes oxidation by bromine and reduction by iodine in these reactions
• C. Bromine is a stronger oxidant than iodine
• D. Bromine is a weaker oxidant than iodine

Answer 1

The Correct Option is C

The given reactions show the behavior of thiosulphate with iodine and bromine:

• With iodine: \(2S_2O_3^{2-} + I_2 \rightarrow S_4O_6^{2-} + 2I^-\)
• With bromine: \(S_2O_3^{2-} + 5Br_2 + 5H_2O \rightarrow 2SO_4^{2-} + 4Br^- + 10H^+\)

Let's analyze the reactions:

Thiosulphate with Iodine: In the first reaction, iodine \((I_2)\) is reduced to iodide \((I^-)\), while thiosulphate \((S_2O_3^{2-})\) is oxidized to tetrathionate \((S_4O_6^{2-})\). This indicates that iodine acts as an oxidizing agent.

Thiosulphate with Bromine: In the second reaction, bromine \((Br_2)\) is reduced to bromide \((Br^-)\). Here, the thiosulphate \((S_2O_3^{2-})\) is completely oxidized to sulfate \((SO_4^{2-})\). This shows that bromine acts as a stronger oxidizing agent compared to iodine.

Conclusion:

The difference in behavior is due to the relative strengths of iodine and bromine as oxidizing agents. Bromine is a stronger oxidant than iodine, which is why it can oxidize thiosulphate to a greater extent, all the way to sulfate.

Based on this analysis, the correct statement that justifies the dual behavior of thiosulphate is:

Bromine is a stronger oxidant than iodine.

Answer 2

In the reaction with I$_2$, the oxidation state of sulphur changes from $+2$ to $+2.5$.
In the reaction with Br$_2$, the oxidation state of sulphur changes from $+2$ to $+6$.
Thus, both I$_2$ and Br$_2$ are oxidants, but Br$_2$ is stronger as it increases the oxidation state of sulphur further.
Final Answer:
Bromine is a stronger oxidant than iodine.