HCl: 36.5
H2S: 34.1
F2: 38.0
Ar: 39.9
Are differences in molar mass sufficient to explain the differences in boiling points?
H2S: 34.1
F2: 38.0
Ar: 39.9
Are differences in molar mass sufficient to explain the differences in boiling points?
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Differences in molar mass aren't always sufficient to explain the differences in boiling points because only dispersion forces increase (and hence boiling points increase) as the molar mass increases, assuming only dispersion forces are present. This explains the trend for F2 and Ar, which are nonpolar, and only have dispersion forces as intermolecular forces. And since dispersion forces are extremely weak, substances that have only this kind of intermolecular force have very low boiling points.
However, dipole-dipole forces and hydrogen bonding can also occur in addition to dispersion forces. Dipole-dipole forces exist in H2S and HCl, and when molar masses are about the same, dipole-dipole forces tend to be more decisive in determining the strength of intermolecular forces, relative to dispersion forces. As the molecule's size increases, however, dipole-dipole forces become less effective, and dispersion forces then become the decisive factor. Hence, the boiling point of H2S is higher than that of HCl.
However, dipole-dipole forces and hydrogen bonding can also occur in addition to dispersion forces. Dipole-dipole forces exist in H2S and HCl, and when molar masses are about the same, dipole-dipole forces tend to be more decisive in determining the strength of intermolecular forces, relative to dispersion forces. As the molecule's size increases, however, dipole-dipole forces become less effective, and dispersion forces then become the decisive factor. Hence, the boiling point of H2S is higher than that of HCl.