The halogens ($X_2$) have strong electron accepting tendency and have positive standard oxidation potential values. They are therefore, powerful oxidising agents. The decreasing order of oxidising powers of halogens is : Fluorine is the strongest oxidising agent (oxidant) because it can liberate the other halogens from their respective compounds. For example, $2\mathrm{KCl}+\mathrm{F}_2$ → $2\mathrm{KF}+\mathrm{Cl}_2$ $2\mathrm{KBr}+\mathrm{F}_2$ → $2\mathrm{KF}+\mathrm{Br}_2$ $2\mathrm{KI}+\mathrm{F}_2$ → $2\mathrm{KF} + \mathrm{I}_2$ Among halogen acids (HX), the HI is the strongest reducing agent or reductant because it has minimum bond dissociation energy. enthalpy $(\text{kJ mol}^{-1})$ The iodination of methane is of reversible nature because HI produced in the reaction being a reducing agent converts Iodomethane back to methane. $\mathrm{CH}_4+\mathrm{I}_2$ → $\mathrm{CH}_3\mathrm{I}+\mathrm{HI}$ $\mathrm{CH}_3\mathrm{I}+\mathrm{HI}$ → $\mathrm{CH}_4+\mathrm{I}_2$ ––––––––––––––––––– Net reaction : $\mathrm{CH}_4+\mathrm{I}_2$ ⇌ $\mathrm{CH}_3\mathrm{I}+\mathrm{HI}$