Çengel Boles Chemical and Phase...
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WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermo Thermo dynamics dynamics Çengel engel Boles Boles Third Edition Third Edition 15 CHAPTER CHAPTER Chemical and Phase Equilibrium
Transcript of Çengel Boles Chemical and Phase...
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998
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15CHAPTERCHAPTER
Chemicaland Phase
Equilibrium
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(fig. 15-2)
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Equilibrium Criteria for a Chemical Reaction That Takes Place AdiabaticallyEquilibrium Criteria for a Chemical Reaction That Takes Place Adiabatically
15-1
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Criteria for Chemical Equilibrium at a Specified Temperature and PressureCriteria for Chemical Equilibrium at a Specified Temperature and Pressure
15-2
(Fig. 15-4)
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Three Equivalent KP Reactions for Reacting ideal-gas mixturesThree Equivalent KP Reactions for Reacting ideal-gas mixtures
15-3
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The Larger the KP Reaction,the More Complete the Reaction
The Larger the KP Reaction,the More Complete the Reaction
15-4
(Fig. 15-9)
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Exothermic Reactions are Less Complete at Higher TemperaturesExothermic Reactions are Less Complete at Higher Temperatures
(Fig. 15-16)
15-5
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Phase EquilibriumPhase Equilibrium15-6
The multicomponent multiphase system is in phase equilibrium when the specific Gibbs function of each component is the same in all phases
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Equilibrium Diagram for a Two-Phase Mixture of Oxygen and Nitrogen at 0.1 MPaEquilibrium Diagram for a Two-Phase Mixture of Oxygen and Nitrogen at 0.1 MPa
15-7
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Mole Fraction of Species on the Two Sides of a liquid-Gas InterfaceMole Fraction of Species on the Two Sides of a liquid-Gas Interface
(Fig. 15-22)
15-8
Unlike temperature, the mole fraction of species on the two sides of a liquid-gas (or solid-gas or solid-liquid) interface are usually not the same
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Dissolved Gases in a Liquid can be Driven Off by Heating the LiquidDissolved Gases in a Liquid can be Driven Off by Heating the Liquid
(Fig. 15-23)
15-9
Gas: ALiquid: B
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Chapter SummaryChapter Summary• An isolated system is said to be in chemical
equilibrium if no changes occur in the chemical composition of the system. The criterion for chemical equilibrium is based on the second law of thermodynamics, and for a system at a specified temperature and pressure it can be expressed as
For the reaction
where the v's are the stoichiometric coefficients.
15-10
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Chapter SummaryChapter Summary
• The equilibrium criterion can be expressed in terms of the Gibbs functions as
which is valid for any chemical reaction regardless of the phases involved.
15-11
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• For reacting systems that consist of ideal gases only, the equilibrium constant KP can be expressed as
where the standard-state Gibbs function change G*(T) and the equilibrium constant KPare defined as
and
Here, Pi's are the partial pressures of the components.
Chapter SummaryChapter Summary15-12
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• The KP of ideal-gas mixtures can also be expressed in terms of the mole numbers of the components as
where v = vC + vD - vA - vB, P is the total pressure, and Ntotal is the total number of moles present in the reaction chamber, including any inert gases. The equation above is written for a reaction involving two reactants and two products, but it can be extended to reactions involving any number of reactants and products.
Chapter SummaryChapter Summary15-13
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Chapter SummaryChapter Summary
• The equilibrium constant KP of ideal-gas mixtures depends on temperature only. It is independent of the pressure of the equilibrium mixture, and it is not affected by the presence of inert gases. The larger the KP, the more complete the reaction. Very small values of KP indicate that a reaction will not proceed to any appreciable degree. A reaction with KP > 1000 is usually assumed to proceed to completion, and a reaction with KP < 0.001 is assumed not to occur at all. The mixture pressure affects the equilibrium composition, although it does not affect the equilibrium constant KP.
15-14
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Chapter SummaryChapter Summary
• The variation of KP with temperature is expressed in terms of other thermochemical properties through the van't Hoff equation
where hR(T) is the enthalpy of reaction at temperature T.
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Chapter SummaryChapter Summary
• For small temperature intervals, the van't Hoffequation can be integrated to yield
This equation shows that combustion processes will be less complete at higher temperatures since KP decreases with temperature for exothermic reactions.
15-16
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Chapter SummaryChapter Summary
• Two phases are said to be in phase equilibrium when there is no transfor-mation from one phase to the other. Two phases of a pure substance are in equilibrium when each phase has the same value of specific Gibbs function. That is,
gf = gg
15-17
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Chapter SummaryChapter Summary• In general, the number of independent variables
associated with a multi-component, multiphase system is given by the Gibbs phase rule, expressed as
IV = C - PH + 2
where IV = the number of independent variables, C= the number of components, and PH = the number of phases present in equilibrium
15-18
• A multicomponent, multiphase system at a specified temperature and pressure will be in phase equilibrium when the specific Gibbsfunction of each component is the same in all phases.
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Chapter SummaryChapter Summary
• For a gas i that is weakly soluble in a liquid (such as air in water), the mole fraction of the gas in the liquid yi liquid side is related to the partial pressure of the gas Pi, gas side by Henry's Law expressed as
where H is Henry's constant.
15-20
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Chapter SummaryChapter Summary• When a gas is highly soluble in a liquid (such as
ammonia in water), the mole fractions of the species of a two-phase mixture in the liquid and gas phases are given approximately by Raoult'slaw expressed as
where Ptotal is the total pressure of the mixture, Pi,sat(T) is the saturation pressure of species i at the mixture temperature, and yi, liquid side andyi, gas side are the mole fractions of species i in the liquid and vapor phases, respectively
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