Consider the reactions: 2 S₂O₃²⁻ (aq) + I₂(s) → S₄O₆²⁻ (aq) + 2I⁻(aq) S₂O₃²⁻ (aq) + 2Br₂(l) + 5 H₂O(l) → 2SO₄²⁻ (aq) + 4Br⁻(aq) + 10H⁺(aq) Why does the same reductant, thiosulphate react differently with iodine and bromine?
The same reductant, thiosulphate, reacts differently with iodine and bromine because bromine is a stronger oxidising agent than iodine.
In the reaction with iodine, thiosulphate is oxidised to the tetrathionate ion (), as iodine is a weaker oxidant and cannot oxidise it further.
In contrast, when reacting with bromine, thiosulphate is oxidised to the sulphate ion () because bromine, being a stronger oxidant, oxidises the sulphur to a higher oxidation state. This aligns with the trend that the oxidising power of halogens decreases down the group, as demonstrated by the ability of bromine to displace iodide ions in solution.
Explanation
The textbook context establishes the relative oxidising strengths of halogens, noting that chlorine can displace bromide and iodide, and bromine can displace iodide. This indicates that bromine is a stronger oxidant than iodine. Consequently, bromine is capable of oxidising thiosulphate to a much higher oxidation state (sulphate, ), whereas the weaker oxidant iodine only oxidises it to tetrathionate ().