Final answer:
The enthalpy of vaporization is higher than the enthalpy of fusion because more energy is required to completely separate molecules in the gas phase, compared to the energy required to only partially overcome intermolecular forces in the liquid phase.
Step-by-step explanation:
The enthalpy of vaporization of a substance is always higher than the enthalpy of fusion. This is because the process of vaporization, which is the conversion from a liquid to a gas, requires more energy to completely separate molecules compared to the process of fusion, which is the conversion from a solid to a liquid.
When a substance vaporizes, the intermolecular forces must be completely overcome to allow the particles to move freely in the gas phase. This requires a significant amount of energy, resulting in a higher enthalpy of vaporization. On the other hand, during fusion, the particles only need to partially overcome the intermolecular forces to change from a solid to a liquid, resulting in a lower enthalpy of fusion.
For example, the enthalpy of vaporization of water is 40.8 kJ/mol, while the enthalpy of fusion is only 6.02 kJ/mol. This difference in enthalpy values can be attributed to the higher energy required to convert liquid water to water vapor compared to the energy required to convert ice to liquid water.