Introduction Gibbs energy change

The change in the Gibbs energy of the crystal by an amount AGy, due to the introduction of n v vacancies distributed over N possible atom sites is given by... 

The introduction of Schottky defects causes the Gibbs energy of the crystal to change by an amount AGs ... 

The calculation of the number of Frenkel defects in a crystal proceeds along lines parallel to those above. The introduction of Frenkel defects causes the Gibbs energy of the crystal to change by an amount AGp ... 

Thus the chemical potential corresponds to the change in Gibbs energy of a homogeneous multicomponent system on the introduction of an infinitesimal amount of a component into the mixture at constant p, T and constant amounts of the other eomponents. 

We have shown how models for volumetric equations of state can be used with stability criteria to predict vapor-liquid phase separations. However, not all phase equilibria are conveniently described by volumetric equations of state for example, liquid-liquid, solid-solid, and solid-fluid equilibria are usually correlated using models for the excess Gibbs energy g. When solid phases are present, one motivation for not using a PvT equation is to avoid the introduction of spurious fluid-solid critical points, as discussed in 8.2.5. A second motivation is that properties of liquids and solids are little affected by moderate changes in pressure, so PvT equations can be unnecessarily complicated when applied to condensed phases. In contrast, g -models often do not contain pressure or density instead, they attempt to account only for the effects of temperature and composition. Such models are thereby limited to descriptions of phase separations that are driven by diffusional instabilities, and the stability behavior must be of class I (see 8.4.2). In this section we show how a g -model can describe liquid-liquid and solid-solid equilibria. 

In this edition we retain the core organization of the previous edition with two notable exceptions. First, we have moved the chapter entitled Spontaneous Change Entropy and Gibbs Energy forward in the text. It is now Chapter 13. By moving the introduction of entropy and Gibbs energy forward in the text, we are able to use these concepts in subsequent chapters, cond, we have moved the chapter on chemical kinetics to Chapter 20. Consequently, the discussion of chemical kinetics now appears after the chapters that rely on equilibrium and thermodynamic concepts. 

Early chemists thought that the beat of reaction, —AH. should be a measure of the "chemical affinity" of a reaction. With the introduction of the concepl of netropy (q.v.) and ihe application of the second law of thermodynamics lo chemical equilibria, it is easily shown that the true measure of chemical affinity and Ihe driving force for a reaction occurring at constant temperature and pressure is -AG. where AG represents the change in thermodynamic slate function, G. called Gibbs free energy or free enthalpy, and defined as the enthalpy, H, minus the entropy. S. times the temperature, T (G = H — TS). For a chemical reaction at constant pressure and temperature ... 

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