(a)HF(20oC) and HCl (-85oC)
HF is the higher boiling substance because hydrogen bonding is stronger than dipole-dipole forces (but note that hydrogen bonding is a unique case of dipole-dipole forces.)
(b)CHCl3 (61oC) and CHBr3 (150oC)
Both molecules are polar (work out the Lewis structures and the VSEPR geometries, and see why this is so.) Both have dipole-dipole forces, but the CHBr3 is higher boiling because it has the higher molecular weight, which leads to higher polarizability and stronger dispersion forces.
(c)Br2 (59oC) and ICl (97oC)
The molecular weights of these substances are very similar (160 amu vs 162 amu); however, ICl has the higher boiling point because it is a polar molecule, whereas Br2 is not. For molecules with similar structures and molecular weights, dipole-dipole forces are stronger than dispersion forces.
Doug Chapman firstname.lastname@example.org 6/30/08