The born equation

The solvation of a gaseous ion is always highly exothermic. The born equation estimates the Gibbs energy of this process, using a model which treats the solvent as a continuous dielectric medium:

z = nuclear charge
e = electron charge
r  = effective radius, which includes part of the radii of solvent molecules

The equation can take another form using a very squigly E to represent the electrostatic parameter of the ion, which equals z²/r:

From this we can tell that small, highly charged ions are more stabilized in solvents. It also shows that solvents with large relative permitivity have a more negative G. Water has a very high relative permitivity compared to nonpolar solvents.

The born equation does not factor in hydrogen bonding. For example, NH₄⁺ is less acidic than the equation predicts, while HF is more acid than predicted - since F^- can form stronger hydrogen bonds than HF.