Subject: Boiling Points and Hydrogen Bonds

A classic exercise in chemistry is to plot the trends in boiling points 
for series of compounds against their molar masses. Its a dramatic way 
to point out the strength of hydrogen bonds, when you use sequences 
like H2O-H2S-H2Se-H2Te or HF-HCl-HBr-HI. However, I'm a bit confused when 
it comes to explaining the trends in the non-hydrogen bonded members of 
these series. I understand that in a nonpolar molecule the major attractive 
forces are London forces. And bigger atoms with more electrons and more 
volume are more polarizable, so the London forces on HI can get larger 
than for HBr. Fine and dandy. But what about HCl? The difference in 
electronegativity between hydrogen and chlorine would suggest a fairly 
polar character for the H-Cl bond. And I thought that dipole foces between 
polar molecules were an order of magnitude stronger than the instantaneous 
dipoles of London forces. So hydrogen chloride should have a higher boiling 
point than HBr. But it doesn't. Is percent ionic character that much less 
important than atomic size in determining the strength of these intermolecular 
attractions? Or am I missing a piece of the puzzle?

Re: Boiling Points and Hydrogen Bonds