Consider the evaporation of water. The standard change in the free energy is positive with ΔG∘rxn=+8.59kJmol. One might expect t
his process to occur in the opposite direction, with water vapor condensing into liquid water. How can the evaporation be spontaneously under normal conditions?
The partial pressure of water is much less than 1atm, so the free energy change for the process must be negative with ΔGrxn<0.
Explanation:
Water vapor will condense into liquid water when the water vapor is in its standard state, with a pressure of 1atm. Under normal circumstances, the partial pressure of water vapor is much less that 1atm. It is in a nonstandard state. The free energy change of this reaction has to be negative in order to make the reaction spontaneous.
