Problem 11.24

Rationalize the difference in boiling point between the members of the following pairs of substances:

(a)HF(20^oC) and HCl (-85^oC)

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 (61^oC) and CHBr₃ (150^oC)

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 (59^oC) and ICl (97^oC)

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.