Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal
energy, which is the energy required to create a system, and the amount of energy required to make
room for it by displacing its environment and establishing its volume and pressure.
The enthalpy of a homogeneous system is defined as[5][6]:
where
H is the enthalpy of the system
U is the internal energy of the system
p is the pressure of the system
V is the volume of the system.
The enthalpy H(S,p) of homogeneous systems can be derived as a characteristic function of the
entropy S and the pressure p as follows: we start from the first law of thermodynamics for closed
systems for an infinitesimal process
Here, δQ is a small amount of heat added to the system and δW a small amount of work performed
by the system.
In a homogeneous system only reversible processes can take place so the second law of
thermodynamics gives δQ = TdS with T the absolute temperature of the system. Furthermore, if only
pV work is done, δW = pdV. As a result
Adding d(pV) to both sides of this expression gives
or
So