Phase_Equilibria_1

# Phase_Equilibria_1 - Phase Equilibria Phase refers to a...

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1 Phase Equilibria Phase refers to a form of matter that is uniform w/r/t chemical composition and the state of aggregation on both microscopic and macroscopic length scales The thermodynamics of transition - From now on we will be making use of the concept of the Gibbs ( free) energy per mole (chemical potential, µ ) For a system of one component, / m GG n == For a system in equilibrium, the chemical potential of each constituent must be the same everywhere in the system.- condition for equilibrium (2) (1) [ (1)] mm m m Gn G n G n G G ∆= = A substance has a spontaneous tendency to change into a phase with the lower molar Gibbs energy If there are several phases present (several phases in equilibrium with one another), the chemical potential of each substance must have the same value in every phase in which that substance appears. Variation of Gibbs energy with pressure and temperature We know that dG VdP SdT =− and if we keep dT = 0 then dG VdP = and dG SdT / m n so that, VdP SdT dG + = becomes dP V dT S d m m + = and , m P S T = m T V P = For the 3 phases of single substance: solid m P solid S T , = liquid m P liquid S T , = gas m P as S T , lg =

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2 At any temperature, solid m liquid m gas m S S S , , , > >> The thermodynamic conditions for equilibrium between phases at constant pressure are: solid and liquid coexist at the intersections of the curves, at which temperatures, µ solid = liquid at T f liquid = gas at T b Below T f the solid has the lowest µ Between T f and T b the liquid has the lowest . Above T b the gas has the lowest The phase with the lowest value of is the stable phase What happens when a solid is heated under constant pressure is entirely a
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## This note was uploaded on 04/07/2012 for the course CHEM 340 taught by Professor Staff during the Spring '08 term at Ill. Chicago.

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Phase_Equilibria_1 - Phase Equilibria Phase refers to a...

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