enthalpy of a system
is its actual energy
(termed the internal energy)
plus the product of its volume and the external pressure. Though sometimes
content,'' the enthalpy in fact includes energy
not contained in the system.
Enthalpy proves convenient for describing processes in gases and liquids in
laboratory environments, if one does not wish to account explicitly for energy stored in the atmosphere by work done when a system
expands. It is of little use, however, in describing processes in nanomechanical systems,
where work can
take many forms: internal energy
is then more convenient. Enthalpy is to energy
what the Gibbs free energy
is to the Helmholtz free energy.