How can I calculate delta G of vaporization?
All phase changes at constant temperature and pressure are in an equilibrium such that
The Gibbs' free energy for a closed system is a function of the temperature and pressure:
#G = G(T,P)#
Even for an open system, the Gibbs' free energy is zero for any process in chemical equilibrium where the temperature and pressure are both constant.
Any liquid that evaporates is in chemical equilibrium with itself, and is already at constant temperature (i.e. its boiling point!). At a constant altitude, we can safely assume that a constant pressure is in equilibrium with the atmospheric pressure (the condition for vaporization). Hence,