The oxidizing power of an element depends on three factors.
1. Bond dissociation energy
2. Electron gain enthalpy
3. Hydration enthalpy
Fluorine is considered strongest oxidizing agent compared to all known elements due to its small size (being the first member of 17 group and ionic radii decrease downwards), low bond enthalpy of F-F (due to high electron-electron repulsion). High negative value of electron gain enthalpy (highest electronegativity ) and very high hydration energy of F- ions (hydration energy is the energy released when a compound is dissolved in water . In case of fluorine it is more due to the strong interaction between F- ions and water, as it is electro-negative). The comparison of both the elements makes this clear that F2 is a strong oxidizing power as compared to Cl2.
The electron gain enthalpy of chlorine (-349 KJ mol-1) is more negative than that of fluorine (-333 KJ mol-1). However, the bond dissociation energy of fluorine (158.8 KJ mol-1) is much lesser than that of chlorine (242.6 KJ mol-1). Also, because of its small size, the hydration energy of fluorine (515 KJ mol-1) is much higher than that of chlorine(381 KJ mol-1). Therefore, the latter two factors more than compensate for the less negative electron gain enthalpy of fluorine. Thus, fluorine is a much stronger oxidizing agent than chlorine.
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