The process of dissociation is much more unstable than in aqueous solutions. The ionic charges are somewhat stabilized in water, and you can also consider this dynamic in relevance to transition states.
Not quite sure, but you'll probably need to consider the bond enthalpies of the acid compounds as well as other physical parameters (temperature) and on entropical factors. You should read a standard text on this issue, I don't know of any off the top.
ChemBuddy | 
Chem Reddit | 
Topic: Gas Phase Acidities (Read 3603 times)