1. Free Energy and Pressure
A system at constant temperature and pressure proceeds spontaneously in the direction that lowers its free energy.
Free energy of a reaction system changes as the reaction proceeds, because free energy is dependent on the pressure of a gas or on the concentration of species in solution.

2. Ideal gas: (recall G = H – TS)
Enthalpy (H) is not pressure dependent
Entropy (S) is pressure dependent because of dependence on volume
Slarge volume > Ssmall volume
Slow pressure > Shigh pressure

3. Since entropy and therefore free energy are pressure dependent, it can be shown that
G = Go + RT ln(P)
where Go is free energy of the gas at a pressure of 1 atm, G is the free energy of the gas at a pressure of P atm, R is the universal gas constant, and T is the Kelvin temperature.

4. The Meaning of ΔG for a Chemical Reaction
The value of ΔG for a given reaction system tells us whether the products or reactants are favored under a given set of conditions, but it does not mean that system will proceed to pure products (if ΔG is negative) or remain at pure reactants (if ΔG is positive).
The system will instead go spontaneously go the equilibrium position, or the lowest possible free energy available to it.