Whenever one of the halogens is involved in oxidising a species in solution, the halogen is reduced to a halide ion associated with water molecules. The following reactions illustrate this process F2(g)+2ι′→2F−(aq) ⇒Cl2(g)+2e−→2Cl−(aq) Br2(l)+2e−→2Br−(aq) I2(g)+2e−→2I−(aq) Smaller the size of ion, higher is the energy released when the ion is hydrated. Since, the size of ions increases in the order F−<Cl−<Br−<I−. The order of hydration energy is F−(aq)>Cl−(aq)>Br−(aq)>I−(aq) Because of very high hydration enthalpy of the fluoride ion, F2 gets very easily reduced followed by Cl2, then Br2 and I2. Therefore, the correct order of oxidising power of halogens is F2>Cl2>Br2>I2.
