Oxidation of thiosulphate by iodine gives tetrathionate ion

The oxidation of thiosulphate (S₂O₃²⁻) by iodine (I₂) results in the formation of tetrathionate ion (S₄O₆²⁻). This reaction is commonly known as the iodine-thiosulphate reaction and is often used in analytical chemistry to determine the concentration of iodine or thiosulphate in a solution.

Reaction:
The reaction between thiosulphate and iodine can be represented by the following equation:

2S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2I⁻(aq)

Explanation:
1. Thiosulphate ion (S₂O₃²⁻) is a reducing agent that can be oxidized by iodine (I₂) to form tetrathionate ion (S₄O₆²⁻). In this reaction, thiosulphate is oxidized from a +2 oxidation state to a +6 oxidation state.

2. Iodine acts as the oxidizing agent and gets reduced from an oxidation state of 0 to -1 in the form of iodide ion (I⁻).

3. The reaction proceeds through a series of intermediate steps. Initially, the iodine reacts with thiosulphate to form tetrathionate ion and iodide ion. This reaction involves the transfer of electrons from thiosulphate to iodine, resulting in the oxidation of thiosulphate.

4. The tetrathionate ion (S₄O₆²⁻) is formed as the main product of the reaction. It contains four sulphur atoms bonded together with alternating single and double bonds, along with six oxygen atoms. The overall charge of the tetrathionate ion is 2-.

5. The reaction between thiosulphate and iodine is often used in iodometric titrations, where the concentration of one of the reactants (either thiosulphate or iodine) is determined by the other reactant.

6. The tetrathionate ion formed in this reaction can be further oxidized to sulphate ion (SO₄²⁻) under certain conditions. However, in the given question, the correct answer is the tetrathionate ion (S₄O₆²⁻).

Summary:
The oxidation of thiosulphate by iodine results in the formation of tetrathionate ion. Thiosulphate is oxidized to tetrathionate, while iodine is reduced to iodide ion. This reaction is commonly used in analytical chemistry for titrations and determining the concentration of either thiosulphate or iodine.