Re: Boiling Points and Hydrogen Bonds

You have actually worked out the puzzle fairly well. Some more data will
throw light on the situation.

(1) Is HCl a lot more polar than HBr? On some electronegativity scales,
there is very little difference between Cl & Br. But if we look at actual
dipole moments, we find HCl = 1.084 D ; HBr = 0.78 D. Since the dipole
moment enters as a squared term in the dipole-dipole attraction, we can
conclude that dipole-dipole forces are roughly double in HCl to what they
are in HBr. So your supposition there is correct.

(2) What about London forces? Well, London forces are only one of several
weak interactions between the atoms of non-polar molecules. Taken as a sum,
these forces are much larger between large atoms than between small atoms.
A really good comparison would be to look at the series HCl, HBr, HI 
alongside the neighbouring noble gases Ar, Kr, and Xe. These gases are
isoelectronic with the hydrogen halides, but no dipole effects are involved.
An isoelectronic comparison can also be made with non-polar group IVB
hydrides.

Boiling Points:   HCl 188 K    HBr 206 K    HI 238 K
Boiling Points:   Ar   87 K    Kr  121 K    Xe 166 K
Boiling Points:  SiH4 161 K   GeH4 185 K  SnH4 221 K

Clearly, the dominant effect is that of atomic size. The polarity of the
hydrogen halides can be seen in the fact that the boiling points in this
series are significantly higher than in the other two series, and in the
slightly smaller range -- a difference of 50 K between HCl and HI compares 
with 60 K in the silane series, and 80 K in the noble gases, because, as you
rightly suppose, dipole effects are working in the other direction.

It is often said that dipole forces are stronger than the forces between
non-polar molecules. This is only a half-truth. They are ***longer range***
than the other forces. A dipole-dipole potential has a radial dependence
proportional to 1/r^3, whereas the non-polar interactions have potentials
proportional to 1/r^5 and 1/r^6. But at short range -- atomic contact
distances are those relevant to condensation phenomena -- the magnitudes are
fairly similar, particularly when heavy atoms are present. Even then, the
dipole forces appear to be stronger, because they are ***additional to***
the non-polar forces, which are still present in polar molecules.

I hope this long and involved answer has helped.

John.