hvap

Hvap, in thermodynamics, denotes the molar enthalpy of vaporization. It is the amount of energy required to convert one mole of a liquid into its vapor at a given constant pressure. Hvap is a latent heat and is inherently positive, reflecting the endothermic nature of vaporization. The standard molar enthalpy of vaporization, ΔHvap°, is the Hvap value measured at 1 atmosphere (the standard state) and at the liquid’s boiling point, or more generally at a specified reference temperature.

Hvap is temperature dependent: in a given liquid, the value generally decreases as temperature rises and tends toward zero as the critical point is approached. At the normal boiling point of water (100 °C at 1 atm), ΔHvap° is about 40.6 kJ/mol. For many organic liquids, Hvap values typically range from about 20 to 60 kJ/mol, depending on molecular structure and intermolecular forces.

Hvap can be determined experimentally by calorimetry or estimated from vapor pressure data using the Clausius–Clapeyron relation. Under the common approximation that Hvap is relatively constant over a modest temperature range, ln(P) is linearly related to 1/T with slope −ΔHvap/R, where P is vapor pressure and R is the gas constant. This relationship enables estimation of Hvap from vapor pressure measurements or its use in modeling phase equilibria.

In practical use, Hvap is essential for understanding evaporation, distillation, and boiling processes in chemistry and chemical engineering, and it serves as a fundamental parameter in thermodynamic databases and process simulations.