Chemical Potential

Chemical potential occupies the same role for matter transfer that temperature occupies for heat: a substance moves spontaneously from a region of higher chemical potential to one of lower, and equality across phases defines equilibrium. That single criterion underlies phase diagrams, partition between immiscible solvents, and membrane transport alike. For a real mixture the potential is written as a standard-state value plus RT times the natural logarithm of activity, activity replacing concentration to absorb non-ideal interactions between species. The condition for chemical equilibrium in a reaction is that the stoichiometrically weighted sum of the chemical potentials of products equals that of reactants, a formulation that leads directly to the equilibrium constant. Electrochemistry extends the quantity to the electrochemical potential by adding a term for charge in an electric field.