Explain chemical reactivity of halogens.

Explanation

Bond dissociation enthalpy : The bond dissociation enthalpy of halogen increases with the increase in size of halogen atom. However, fluorine has lower bond dissociation enthalpy because of its small size and large electron-electron repulsions between lone pairs of fluorine.
Bond dissociation energy sequence :
`Cl_(2) gt Br_(2) gt F_(2) gt I_(2)`
The electron pairs in fluorine are much more closer than in chlorine.
(ii) Oxidizing nature : Halogens are strong oxidizing agents. This is due to fact that halogens readily accepts the electrons. Fluorine is strongest oxidizing agent. It oxidizes other halide in ions in solution or even in the solid phase. For example :
`F_(2) + 2X^(-) to 2F^(-) + X_(2) (X = Cl, Br, I)`
`Cl_(2) + 2X^(-) to 2Cl^(-) + X_(2) (X = Br, l)`
`Br_(2) + 2I^(-) to 2Br^(-) + I_(2)`
The oxidizing strength of halogens depend on enthalpy of fusion, enthalpy of vapourisation, enthalpy of dissociation, electron gain enthalpy and hydration enthalpy.
`1/2X_(2)(g) overset(1/2Delta"diss"H^(-)) to X(g) overset(Delta_(eg)H^(-))to X_(g)^(-) overset(Delta"hyd"H^(-))to, X_(aq)^(-)`
The relative oxidizing power of halogens can be illustrated by their reactions with water.
`2F_(2)(g) + 2H_(2)O(l) to 4H_(aq)^(+) + 4F_(aq)^(-) + O_(2)(g)`
`X_(2)(g) + H_(2)O(l) to HX(aq) + HOX (aq) (X=Cl, Br)`
Fluorine oxidizes water to oxygen where as chlorine and bromine forms corresponding hydrohalic acid. The reaction of iodine with water is non spontaneous and infact `I^(-)` is oxidized by oxygen in acidic medium, just the reverse of the reaction observed with fluorine.
`4l_(aq)^(-) + 4H_(aq)^(+) + O_(2)(g) to 2l_(2)(s) + 2H_(2)O(l)`
Fluorine has less negative electron enthalpy than chlorine though it is a strongest oxidizing agent because of low bond dissociation enthalpy and high hydration enthalpy.