ABSTRACT
Once the second law of thermodynamics was introduced, dU and dH could also be redefined. Two kinds of “free energy” (non-PV work) are then defined, the sign of which will also determine whether the process is spontaneous or non-spontaneous. By using Gibbs free energy, G (or Helmholtz energy, A), it is no longer necessary to consider dSsurr for a described change in the system to define the direction of natural change. For pure substances, the molar free energy can be defined as the chemical potential. The chemical potential defines the criteria for equilibrium to exist between two phases or states. A phase is when a substance has a uniform chemical composition and physical structure for a range of P and T values. A phase diagram shows the thermodynamically stable phase at any P and T values and when two phases (such as solid and liquid) are in equilibrium with each other.
