Sum of the internal energy E and the product of the pressure P and volume V of a thermodynamic system (see thermodynamics ).
So, enthalpy H = E + P V . Its value is determined by the temperature, pressure, and composition of the system at any given time. According to the law of conservation of energy (see conservation law ), the change in internal energy is equal to the heat transferred to the system minus the work done by the system. If the only work done is a change of volume at constant pressure, the enthalpy change is exactly equal to the heat transferred to the system.