Chapter 16

CHEMICAL AND PHASE EQUILIBRIUM

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n Chapter 15 we analyzed combustion processes under

the assumption that combustion is complete when there is

sufficient time and oxygen. Often this is not the case,

however. A chemical reaction may reach a state of equilib-

rium before reaching completion even when there is sufficient

time and oxygen.

A system is said to be in equilibriumif no changes occur

within the system when it is isolated from its surroundings.

An isolated system is in mechanical equilibriumif no changes

occur in pressure, in thermal equilibriumif no changes occur

in temperature, in phase equilibriumif no transformations

occur from one phase to another, and in chemical equilib-

riumif no changes occur in the chemical composition of the

system. The conditions of mechanical and thermal equilib-

rium are straightforward, but the conditions of chemical and

phase equilibrium can be rather involved.

The equilibrium criterion for reacting systems is based on

the second law of thermodynamics; more specifically, the

increase of entropy principle. For adiabatic systems, chemical

equilibrium is established when the entropy of the reacting

system reaches a maximum. Most reacting systems encoun-

tered in practice are not adiabatic, however. Therefore, we

need to develop an equilibrium criterion applicable to any

reacting system.

In this chapter, we develop a general criterion for chemical

equilibrium and apply it to reacting ideal-gas mixtures. We

then extend the analysis to simultaneous reactions. Finally,

we discuss phase equilibrium for nonreacting systems.

Objectives

The objectives of Chapter 16 are to:

• Develop the equilibrium criterion for reacting systems based
  on the second law of thermodynamics.


• Develop a general criterion for chemical equilibrium
  applicable to any reacting system based on minimizing the
  Gibbs function for the system.


• Define and evaluate the chemical equilibrium constant.


• Apply the general criterion for chemical equilibrium analysis
  to reacting ideal-gas mixtures.


• Apply the general criterion for chemical equilibrium analysis
  to simultaneous reactions.


• Relate the chemical equilibrium constant to the enthalpy of
  reaction.


• Establish the phase equilibrium for nonreacting systems in
  terms of the specific Gibbs function of the phases of a pure
  substance.


• Apply the Gibbs phase rule to determine the number of
  independent variables associated with a multicomponent,
  multiphase system.


• Apply Henry’s law and Raoult’s law for gases dissolved in
  liquids.

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