Heats of fusion Vaporization Sublimation

The absorption or release of heat without any accompanying temperature change is characteristic of a change in the state of aggregation of a substance. The quantity of heat absorbed in the transformation of solid to liquid is the heat of fusion. The quantity of heat absorbed in the transformation of liquid to vapor is the heat of vaporization. The direct transformation of a solid to vapor is called sublimation. The quantity of heat absorbed is the heat of sublimation, which is equal to the sum of the heats of fusion and vaporization. 

Liquids composed of molecules that have comparatively strong forces acting between them have high heats of vaporization, while those composed of weakly interacting molecules have low heats of vaporization. The van der Waals a is a measure of the strength of the attractive forces we expect the heats of vaporization of substances to fall in the same order as the values of a. This is in fact correct it can be shown that for a van der Waals fluid the heat of vaporization per mole, is equal to a/b. 

---