Electron gain enthalpy of chlorine is more negative as compared to fluorine.

Explanation

All halogens are strong oxidising agents, however, reactivity decreases down the group, thus fluorine is the strongest oxidising agent and iodine the weakest. This is mainly due to the fact that electron gain enthalpy becomes less and less negative as we move down the group. However, as we know electron gain enthalpy of fluorine is unexpectedly less negative as compared to chlorine. Despite this fact fluorine `(F_2)` is a stronger oxidising agent than chlorine `(Cl_2)` as is evident from the fact that fluorine can oxidise other halide ions even in solid state.
`F_2(g) + 2NaCl (s) to 2NaF(s) + Cl_2 (g)`
(In aqueous solutions, `F_2` can oxidise water to `O_2` and `O_3`)
It is due to the following two factors.
(i) Enthalpy of dissociation of F-F bond is less than `CH- Cl` bond.
(ii) Enthalpy of hydration of `F^-` ion is more than `Cl^-` ion.
`1/2 X_2 (g) overset(1//2 Delta_("diss")H^(Theta))to X(g)`
`X(g) + e^(-) overset(Delta_(eg) H^(Theta))to X^(-) (g)`
`X^(-) (g) + aq overset(Delta_("hyd.")H^(Theta))to X^- (aq)`
Here step (i) is endothermic while steps:(ii) and (iii) are exothermic.