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

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Chapter 16 CHEMICAL AND PHASE EQUILIBRIUM | 793 I 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.
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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
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