Thermodynamics

Chapter 16CHEMICAL AND PHASE EQUILIBRIUMIn Chapter 15 we analyzed combustion processes underthe assumption that combustion is complete when there issufficient time and oxygen. Often this is not the case,however. A chemical reaction may reach a state of equilibriumbefore reaching completion even when there is sufficienttime and oxygen.A system is said to be in equilibrium if no changes occurwithin the system when it is isolated from its surroundings.An isolated system is in mechanical equilibrium if no changesoccur in pressure, in thermal equilibrium if no changes occurin temperature, in phase equilibrium if no transformationsoccur from one phase to another, and in chemical equilibriumif no changes occur in the chemical composition of thesystem. The conditions of mechanical and thermal equilibriumare straightforward, but the conditions of chemical andphase equilibrium can be rather involved.The equilibrium criterion for reacting systems is based onthe second law of thermodynamics; more specifically, theincrease of entropy principle. For adiabatic systems, chemicalequilibrium is established when the entropy of the reactingsystem reaches a maximum. Most reacting systems encounteredin practice are not adiabatic, however. Therefore, weneed to develop an equilibrium criterion applicable to anyreacting system.In this chapter, we develop a general criterion for chemicalequilibrium and apply it to reacting ideal-gas mixtures. Wethen extend the analysis to simultaneous reactions. Finally,we discuss phase equilibrium for nonreacting systems.ObjectivesThe objectives of Chapter 16 are to:• Develop the equilibrium criterion for reacting systems basedon the second law of thermodynamics.• Develop a general criterion for chemical equilibriumapplicable to any reacting system based on minimizing theGibbs function for the system.• Define and evaluate the chemical equilibrium constant.• Apply the general criterion for chemical equilibrium analysisto reacting ideal-gas mixtures.• Apply the general criterion for chemical equilibrium analysisto simultaneous reactions.• Relate the chemical equilibrium constant to the enthalpy ofreaction.• Establish the phase equilibrium for nonreacting systems interms of the specific Gibbs function of the phases of a puresubstance.• Apply the Gibbs phase rule to determine the number ofindependent variables associated with a multicomponent,multiphase system.• Apply Henry’s law and Raoult’s law for gases dissolved inliquids.| 793