Heat Capacity, Enthalpy, Entropy, and the Third Law of Thermodynamics
This chapter provides the knowledge of the heat capacities and the entropies of substances and the heats of formation of compounds, which allow the enthalpy and entropy changes to be evaluated for any process—that is, for phase changes and chemical reactions. The heat capacity of solids can be modeled by the statistical thermodynamic models of Einstein and Debye. The entropy of all substances which are in complete internal equilibrium is zero at 0 K. This is the Nernst–Planck–Simon statement of the Third Law of Thermodynamics. Both the enthalpy and entropy are dependent on pressure and temperature. However, the pressure dependence of the enthalpy and entropy of condensed phases is normally small enough to be ignored, especially when the pressure of interest is in the range 0-1 atm. The variation of enthalpies of transformation with temperature and pressure can be calculated by the application of the First Law of Thermodynamics to the changes in enthalpy with temperature and pressure.