15 Phase Diagrams - Phase Transformations Sections 8.4-8.6,...

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Phase Transformations Sections 8.4-8.6, 8th Ed.; Sections 6.1-6.3, 7th Ed. Temperature-Composition Diagrams Distillation of Mixtures Azeotropes Immiscible Liquids Liquid-Liquid Phase Diagrams Phase Separations Critical Solution Temperatures Distillation of Partially Miscible Liquids Liquid-Solid Phase Diagrams Eutectics Reacting Systems Incongruent Melting Last updated: Dec. 7, 2009, minor changes, added slides 18, 19 and 22, mod. 23
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Temperature Composition Diagrams Temperature compositions diagrams show the boundaries of compositions of phases at equilibrium at different temperatures at a given pressure (typically 1 atm) - liquid phase in lower part of diagram Region between lines: F & = 1, compositions of phases are fixed at given temperature Region outside lines, F & = 2, temperature and phase composition are variable a 1 : pure phase liquid is heated and boils at T 2 , with composition a 2 = a 1 , vapour has minuscule component a 2 & (location of the tie line gives us the b.p. T 2 ) AB system, A more volatile (lower b.p.) a 3 : first bit of condensation drawn off from a distillation (collect a 2 N ), richer in the more volatile component, with composition a 3 & a 4 : Vapour is drawn off, liquid condenses to this composition, very rich in volatile component, and almost pure A is obtained Boiling and condensation cycle is known as fractional distillation
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Fractional Distillation Fractional distillation is similar to simple distillation except that a fractionating column is placed between the boiling flask and the condenser. The fractionating column is usually filled with glass or plastic beads, which improve the separation between the liquids being distilled. # The glass beads in the fractionating column provide "theoretical plates" (i.e., additional surface area) on which the vapour can condense, re-evaporate, and condense again, essentially distilling the compound over and over. # Hence, more volatile liquids approach the top of the column, and the less volatile liquids stay in the bottom. # The more theoretical plates that are used, the higher the surface area, and the longer the distillation will take, and more energy is required to complete the distillation. Image from: h ttp :/ / w w w .chemh elper. com/ dis tilla tion .html
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High-Boiling Azeotropes Sometimes favourable interactions between molecules reduce vapour pressure of the mixture below the ideal value: i.e., A-B interactions stabilize the liquid phase - here G E = -ve, more favourable for mixing Examples: propanone/ trichloromethane and nitric acid/water mixtures Composition a heated to boiling ( a 2 & > a 2 ) • Vapour (rich in A) is removed, and liquid left is richer in B, composition a 3 , vapour with a 3 & • Vapour removed, composition shifts to a 4 , vapour composition at a 4 & • Composition of remaing liquid shifts to b as more A is drawn off, and b.p. of liquid, vapour becomes richer in B • Finally, at composition b the vapour of A has the same composition as the liquid • Evaporation occurs without change in composition, and the mixture is an azeotrope ( boiling without changing ), & distillation cannot separate the components
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15 Phase Diagrams - Phase Transformations Sections 8.4-8.6,...

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