Question

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

Answer 1

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`.