Enthalpy

A thermodynamic state function given by the following expression:

 H = U + PV

where H indicates enthalpy, U indicates internal energy, and P and V indicate pressure and volume, respectively.

Enthalpy is a useful function for two reasons. First, the expression U + PV occurs often in thermodynamics, and it is convenient to define this quantity. Second, if a process begins and ends at the same pressure, the amount of energy exchanged with the environment as heat is given by the change in the system's enthalpy.