Thiosulphate reacts differently with iodine and bromine in the reactions given below: 2S2O32- + I2 S4O62- + 2I- S2O32-…

Step 1 — Analyse the oxidation of S in each reaction:

With $\mathrm{I_2}$:

• S in $\mathrm{S_2O_3^{2-}}$: average OS = $+2$
• S in $\mathrm{S_4O_6^{2-}}$ (tetrathionate): average OS = $+2.5$
• Change: $+2 \rightarrow +2.5$ → mild oxidation (only partial oxidation of S)

With $\mathrm{Br_2}$:

• S in $\mathrm{S_2O_3^{2-}}$: average OS = $+2$
• S in $\mathrm{SO_4^{2-}}$: OS = $+6$
• Change: $+2 \rightarrow +6$ → complete oxidation to sulphate

Step 2 — Interpret the difference:

$\mathrm{Br_2}$ is a stronger oxidising agent than $\mathrm{I_2}$. It can oxidise thiosulphate completely to sulphate ($+6$), while the weaker oxidant $\mathrm{I_2}$ can only partially oxidise it to tetrathionate ($+2.5$).

Step 3 — Evaluate options:

• (1) Bromine is stronger oxidant → explains why it oxidises S more completely ✓
• (2) Incorrect — thiosulphate undergoes oxidation in BOTH reactions
• (3) Incorrect — Br₂ is stronger, not weaker
• (4) Incorrect — bromine is reduced (not oxidised) in both reactions

Answer: Option (1) — Bromine is a stronger oxidant than iodine