Why is the normal boiling point of #HF# (#19.5# #""^@C#), so high compared to that of #HCl# (#-85.0# #""^@C#)?

1 Answer
Mar 2, 2016

Answer:

For the same reason that water is a liquid at room temperature and pressure, whereas #H_2S#, #H_2Se#, and #H_2Te# are gases.

Explanation:

So the simple reason underlying these phenomena is hydrogen bonding. Fluorine is a smaller, more polarizing atom than chlorine. Charge separation in the #H-F# molecule should be greater, and the bond strength of #H-X# should thus be greater when #X=F#. Of course, this principle manifests in the acid strength of #HCl# versus #HF#. #HF# is a poor acid in water.

Because there is greater polarity in the #H-F# bond, the opportunity for hydrogen bonding in the bulk solvent is greater. The best metric for this INTERMOLECULAR force is the boiling point. #HF# has a normal boiling point of #19.5# #""^@C#, whereas that of #HCl# is #-85.0# #""^@C#. Also compare the boiling points of water versus #H_2S#, and #H_2Se#.