Chemical potential
Partial molar free energy, usually symbolized by µ; the increase in energy of a system (due to an infinitesimal addition of an element or compound, without affecting thermal and mechanical energies) divided by the amount of the element or compound added.
Formula
https://upload.wikimedia.org/math/5/c/b/5cbebf6b87f31e551884256cb43d8fcd.png where H is the enthalpy, U the internal energy, F the free energy and ni the amount of substance.