CHAPTER16 - Chapter 16 CHEMICAL AND PHASE EQUILIBRIUM n...

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 equilibrium if no changes occur
within the system when it is isolated from its surroundings.
An isolated system is in mechanical equilibrium if no changes
occur in pressure, in thermal equilibrium if no changes occur
in temperature, in phase equilibrium if no transformations
occur from one phase to another, and in chemical equilib-
rium if 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.

16–1
■
CRITERION FOR CHEMICAL EQUILIBRIUM
Consider a reaction chamber that contains a mixture of CO, O
2
, and CO
2
at
a specified temperature and pressure. Let us try to predict what will happen
in this chamber (Fig. 16–1). Probably the first thing that comes to mind is a
chemical reaction between CO and O
2
to form more CO
2
:
This reaction is certainly a possibility, but it is not the only possibility. It is
also possible that some CO
2
in the combustion chamber dissociated into CO