the amount of energy possessed by a thermodynamic system (see
Thermodynamics) for transfer between itself and its environment. For example,
in a chemical reaction, the change in enthalpy of the system is the heat of
the reaction. In a phase change, as from a liquid to a gas, the change in
enthalpy of the system is the latent heat of vaporization. In a simple
temperature change, the change in enthalpy with each degree is the heat
capacity of the system at constant pressure. The German physicist Rudolf J.E.
Clausius originated the term in 1850. Mathematically, enthalpy H is
identified as U + PV, where U is internal energy, P is
pressure, and V is volume. H is measured in joules or British
termal units (BTUs).