Phase Equilibrium in a One-Component System
The intensive thermodynamic variables which control the equilibrium of a system are the temperature, the pressure, and the chemical potentials of the various species that compose the system. This chapter provides the knowledge of the dependencies, on temperature and pressure, of the changes in molar enthalpy and molar entropy caused by phase changes in a system allows the determination of the corresponding change in the molar Gibbs free energy of the system. Since a closed one-component system has only two independent variables, the dependence of G can be examined most simply by choosing the state variables T and P as the independent variables. The intersection of the surfaces for the solid and liquid phases gives the variation of the equilibrium melting temperature with pressure. The chapter presents a schematic representation of the variation of the molar Gibbs free energies with temperature of the body-centered cubic (BCC), face-centered cubic (FCC), liquid, and vapor phases of iron.