Question

Explain the enthalpy of vaporization (numerical value) at the critical point, and discuss implications of this dependence of enthalpy of vaporization on temperature.

Answer 1

The enthalpy of vaporization, (symbol ∆H_vap) also known as the heat of vaporization or heat of evaporation, is the energy (enthalpy) that must be added to the substance, typically a liquid, to transform a quantity of that substance into a gas. The enthalpy of vaporization is a function of the pressure at which that transformation takes place.

The heat of vaporization is temperature-dependent, though a constant heat of vaporization can be assumed for small temperature ranges and for reduced temperature . The heat of vaporization diminishes with increasing temperature and it vanishes completely at a certain point called the critical temperature (). Above the critical temperature, the liquid and vapor phases are indistinguishable, and the substance is called a supercritical fluid.