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The heat of vaporization is the energy required to convert a liquid to vapor at the boiling point, and is influenced by intermolecular bonding forces and expansion work. A conceptual model assumes a flexible liquid surface and surface drag. The heat of vaporization is important in industrial distillation and steam heating systems, and can be calculated using the Clausius-Clapeyron equation.
The heat of vaporization, ΔHvap, sometimes called the enthalpy of vaporization, is the amount of energy required to convert a liquid to vapor at the boiling point. This energy is independent of any component resulting from an increase in temperature. The heat of vaporization is often measured at atmospheric pressure and the ordinary boiling point, although this is not always the case. Since the boiling point of any liquid varies with the surrounding pressure, and the heat of vaporization also depends on that pressure, the heat of vaporization of a liquid must be temperature-dependent. Two-dimensional (2-D) graphs represent a simple, nearly parabolic relationship for the most common liquids.
There are many influences that must be considered if the process of boiling, or vaporization, is to be fully understood. Among these are intermolecular bonding forces such as van der Waal forces – which include at least the London dispersion forces – and much stronger hydrogen bonding forces, if applicable. The work required to expand the gas must be included. Also, for the most part, the potential energy of the liquid has been converted into kinetic energy in the gas. It is incorrect to assume that all of this kinetic energy exists in the form of translational energy; some of it becomes rotational energy and vibrational energy.
At a more basic level, a conceptual model first described in 2006 in the journal Fluid Phase Equilibria is promising. In that model, the empirical data for 45 elements matched well when two assumptions were made: The surface of a liquid is flexible, and a particle uses all of its latent energy to free itself from particles blocking its escape: surface drag. In this study, the maximum surface area that a particle in the surrounding liquid can hold was used in the calculations. Small deviations between calculations and reality have been explained in terms of approximations, such as the hardball sphere approximation for atoms.
The heat of vaporization is of considerable importance for industrial distillation apparatus. It is also important in situations where vapor pressure needs to be considered, such as in the design and operation of steam heating systems. A mathematical expression of particular interest in this regard is the Clausius-Clapeyron equation. This equation combines the heat of vaporization with the pressures and temperatures of the system. Using the equation, from a particular temperature and vapor pressure, a second vapor pressure at another temperature can be determined.
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