The criterion for equilibrium at constant temperature and pressure is the change in the integral molar free energy of the solution, i.e.
for an infinitesimal process and
(3)
for a finite process
Where is change in integral molar free energy
At constant temperature and pressure when , a process occurs spontaneously. For an isothermal chemical reaction say , where J is activity quotient and is the standard free energy change. At equilibrium
i.e.
for an infinitesimal process and 
for a finite process
, a process occurs spontaneously. For an isothermal chemical reaction say 
, where J is activity quotient and 
is the standard free energy change. At equilibrium 
, where K is equilibrium constant.
