Final answer:
Heats of vaporization increase with the strength of intermolecular forces. Stronger IMFs require more energy for a substance to vaporize. The heat of vaporization is thus higher for substances capable of hydrogen bonding compared to those with just dispersive forces.
Step-by-step explanation:
The heats of vaporization are directly related to the strength of intermolecular forces (IMFs). In substances with strong IMFs, such as hydrogen bonds, more energy is required to vaporize a liquid because these forces must be overcome for molecules to escape into the gas phase. Therefore, a higher heat of vaporization corresponds with stronger IMFs. For instance, molecules with hydrogen bonding generally have higher heats of vaporization than those with only dispersive forces. Moreover, as temperature increases, the kinetic energy of the molecules increases, allowing them to more readily overcome the IMFs, which contributes to a higher vapor pressure of the liquid.
As an example, alcohol molecules which exhibit hydrogen bonding tend to have lower vapor pressures because of the difficulty in overcoming these strong IMFs. The relative vapor pressures of different alcohols can be explained by the varying extents of IMFs such as hydrogen bonding and dispersion forces. As the size of the alcohol increases, dispersion forces become more significant, leading to lower vapor pressures in a sequence from methanol to butanol: Pmethanol > Pethanol > Ppropanol > Pbutanol.