The Equilibrium Condition Applied to Phase Equilibria

We now have the foundation for applying thermodynamics to chemical processes. We have defined the potential that moves mass in a chemical process and have developed the criteria for spontaneity and for equilibrium in terms of this chemical potential. We have defined fugacity and activity in terms of the chemical potential and have derived the equations for determining the effect of pressure and temperature on the fugacity and activity. Finally, we have introduced the concept of a standard state, have described the usual choices of standard states for pure substances (solids, liquids, or gases) and for components in solution, and have seen how these choices of standard states reduce the activity to pressure in gaseous systems in the limits of low pressure, to concentration (mole fraction or molality) in solutions in the limit of low concentration of solute, and to a value near unity for pure solids or pure liquids at pressures near ambient. 

Chemical processes are central to the study of chemistry. The thermodynamic principles and relationships we have developed provide powerful tools for describing these processes, especially in predicting the spontaneity of the process and the equilibrium conditions that apply. 

As we proceed, we should keep in mind that the classification of a chemical process as a phase change or as a chemical reaction is somewhat arbitrary, and 

See Chapter 5 for a derivation of the equilibrium condition in terms of the chemical potential. 

for each component, its chemical potential is the same in all phases that are in equilibrium. We will see below that the relationships involving the pressure and temperature variations of the chemical potential that we have developed earlier will be helpful in explaining the effect of these variables on phase equilibria. 

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Chapter 8 - The Equilibrium Condition Applied to Phase Equilibria,... 

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