1 Standard States and Chemical Potentials

The chemical potential ? _i of a species i is the partial molar derivative of the Gibbs energy G, enthalpy H, Helmholtz energy A, or internal energy U of substance i ^[1]

In Equation (1), n _i is the amount of component i, T is the temperature, P is the pressure, S is the entropy, and V is the volume. Matter flows spontaneously from a region of high chemical potential to a region of low chemical potential just like electric current flows from a region of high electric potential to a region of low electric potential and mass flows from a position of high gravitational potential to a position of low gravitational potential. The chemical potential can therefore be used to determine whether or not a system is in equilibrium. When the system is in equilibrium, the chemical potential of each substance will be the same in all phases of the system.

In an ideal solution, the chemical potential ? _i ^ideal of species i is calculated from the standard chemical potential ? ⁰ _i, the gas constant R, the Kelvin temperature T, and the mole fraction x _i as ^[2]

The chemical potential of component i will be equal to the standard chemical potential when the mole fraction of component i equals 1. The standard state is thus pure component i.