The first law of thermodynamics is expressed where the infinitesimal heat transfer into a gas is expressed as the infinitesimal change in internal energy of the gas plus the infinitesimal amount of work done by an infinitesimal expansion of the gas. The definition for enthalpy is suggested by this form of the first law of of thermodynamics. The first law of thermodynamics is then expressed incorporating the enthalpy, which turns out to be a state variable.
