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Equilibrium in Thermodynamic Systems



This chapter introduces a general principle that is the
basis for determining the internal condition of the most
complex kind of thermodynamic system when it reaches its
equilibrium state. This general criterion for equilibrium has
the same level of importance in the development of
thermodynamics as do the laws themselves, as implied in
Figure 1.4. It is the foundation that provides the strategy for
the calculation equilibrium maps: phase diagrams, chemical
equilibria in predominance diagrams without and with
electrical effects, the role played by capillarity effects, the
chemistry of defects in crystals, as well as the primary
results of statistical thermodynamics. These applications of
this criterion occupy most of the rest of this text.

Some intuitive notions of equilibrium are hrst
presented; these ideas are then formalized to provide a
thermodynamic statement of the criterion that determines
when a system that is isolated from its surrounding during
its approach to equilibrium has attained its equilibrium


an isolated system, the entropy is a maximum at

5.5 This thermodynamic extremum principle (the entropy

has an extreme value, a maximum) is then formulated
mathematically. A set of equations, called the
conditions for equilibrium, that describe the
relationships that the internal properties of the
isolated system must have when it achieves its
equilibrium state are derived from this extremum
principle. It is then demonstrated that although these
conditions for equilibrium are derived for an isolated
system, they are valid for any system at equilibrium
whether or not it was isolated during its approach to
that hnal condition.
The strategy for obtaining these conditions for
equilibrium is illustrated in this chapter by applying it to the
simplest case: equilibrium in a unary, two-phase system.

5.8 This strategy is applied repeatedly in subsequent
chapters to derive the conditions for equilibrium in systems
of progressively increasing sophistication. These equations
are the basis for making practical calculations about the
hnal resting condition of complex systems.
5.9 The chapter ends with a collection of alternate
statements of the criterion for equilibrium, each of which is
useful in the description of systems with specihc external

The idea that a system changes toward some hnal
ultimate condition ...
Equilibrium in Thermodynamic Systems
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