Electron Gain Enthalpy & Acidic Strength Trends

Electron Gain Enthalpy (Exothermic Order):

$$\text{Cl} > \text{F} > \text{Br} > \text{I}$$

Acid Strength of Hydrogen Halides (HX):

$$\text{HI} > \text{HBr} > \text{HCl} > \text{HF}$$

Reducing Power of Halide Ions ($$X^-$$):

$$\text{I}^- > \text{Br}^- > \text{Cl}^- > \text{F}^-$$

Bond Dissociation Enthalpy ($$X_2$$):

$$\text{Cl}_2 > \text{Br}_2 > \text{F}_2 > \text{I}_2$$

Concept: $$\text{F}_2$$ has an unexpectedly low bond dissociation energy compared to $$\text{Cl}_2$$ and $$\text{Br}_2$$ due to intense inter-electronic repulsions between non-bonding lone pairs on the compact 2p fluorine atoms.