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Enthalpy 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).