Heat capacity and phase transitions

Figure 4.7 Heat capacity and phase transitions in phosphine. The same entropy at point (a) is obtained by going the stable (lower) route or by going the metastable (upper) route. Point (b) is the normal boiling temperature.

The standard molar quantities appearing in Eqs. 12.10.1 and 12.10.2 can be evaluated through a variety of experimental techniques. Reaction calorimetry can be used to evaluate AfH° for a reaction (Sec. 11.5). Calorimetric measurements of heat capacity and phase-transition enthalpies can be used to obtain the value of Sf for a solid or liquid (Sec. 6.2.1). For a gas, spectroscopic measurements can be used to evaluate S° (Sec. 6.2.2). Evaluation of a thermodynanuc equilibrium constant and its temperature derivative, for any of the kinds of equilibria discussed in this chapter (vapor pressure, solubility, chemical reaction, etc.), can provide values of ArG° and AfH° through the relations AfG° = —RTln K and ArH° = -Rd aK/d /T). 

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