Halide Ions as Reducing Agents

Fluorine is the most powerful oxidising agent and Iodine is the least powerful oxidising agent.By contrast, Iodide ions have the greatest reducing power and fluoride ions have the least reducing power.
The reducing power of a halide can be demonstrated by their ability to reduce concentrated sulfuric acid. Iodide ions have the greatest reducing power and are able to reduce concentrated sulfuric acid from +6 to +4 (SO₂), 0 (S) and -2 (H₂S). Chloride ions are unable to reduce concentrated sulfuric acid.
Concentrated sulfuric acid dissociates in two stages.
H₂SO₄ → H⁺ + HSO₄^- (strong acid)
HSO₄^- ⇌ H⁺ + SO₄^2- (weak acid)Concentrated sulfuric acid can act as an oxidising agent. The three possible reduction products are• sulfur dioxide: H₂SO₄ + 2H⁺ + 2e^- → 2H₂O + SO₂• sulfur: H₂SO₄ + 6H⁺ + 6e^- → 4 H₂O + S•hydrogen sulfide: H₂SO₄ + 8H⁺ + 8e^- → 4H₂O + H₂S