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APSU CHEM 3610 - Phase diagrams of mixtures

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Pressure/composition phase diagramThere are 2 maintypes of composition diagrams –pressure and temperature. This is an example of how the total vapor pressure changes for an ideal solution.Pressure/composition diagramThe vaporphase is enriched in the more volatile component. The relationship is not exactly linear and is found on page 177 and 178. The line labeled a-b is called a “tie line”. It connects the liquid and gaseous phases that are in equilibrium.Temperature/composition Diagram –Ideal solutionThe temperature/compositiondiagram is the type we have found in lab. The tie line connects the composition of the vapor and liquid phases at a given temp.Distillation of an “Ideal Solution”The temp/composition diagram shows how you can separate 2 liquids with different boiling points. The number of simple distillations needed to separate 2 liquids is called the number of theoretical plates for the fractional distillations. Again these solutions show “ideal solution” behavior.Azeotropes–Nonidealsolution behaviorIn this situation we have a high boiling azeotrope. This composition boils at a higher temp than the pure liquids. The interactions reduce the VP and stabilizes the liquid. At the azeotropept. the vapor and liquid have the same composition and separation cannot be effected without adding a 3rdcomponent.Azeotropes–Nonidealsolution behaviorThis diagram shows a low boiling azeotrope. This composition boils at a lower temp than the pure liquids. The interactions increase the VP and destabilizes the liquid. At the azeotrope pt. the vapor and liquid have the same composition and separation cannot be effected without adding a 3rdcomponent.Partially Miscible Liquids –upper critical limitMany liquids show the behavior diagrammed above. This phase diagram for hexane/nitrobenzene shows that above the upper critical limit there is one phase. Below this point there can be 2 phases. The tie line connecting these 2 phase gives us (1) the compositions of the phases, and (2) the relative amounts of the phases.Partially Miscible Liquids –upper critical limitThe lever rule allows the calculation of the relative amounts of each phase. Example: At 290K find the compositions and relative amounts of hexane and nitrobenzene if the MF of nitrobenzene in the entire mixture is 0.42 (dotted line)? (page 182)These systems have positive enthalpy and entropy changes of mixing so that ΔG <0 at high temps.Partially Miscible Liquids –lower critical limitAnother possibility is to have a lower critical limit. Below this temp there is 1 phase; above there may be 2 phases. This type of behavior is shown when a weak complex occurs at a low temp with ΔH < 0 and ΔS <0. Thus at low T, ΔG < 0.Partially miscible liquids –upper and lower limits presentThis type of behavior occurs when the liquids form a weak complex which is broken and the entropy change of mixing then becomes positive.Distillation of partially miscible liquids which form a low boiling azeotropeNext we lay the liquid/gas diagram on top of the liquid miscibility diagram to see how one might distill a mixture of partially miscible liquids with an upper critical limit.What if boiling comes before complete miscibility?Example 5.6 –page 185Liquid / Solid Phase Diagrams –Ideal Behavior If the solids form a series of liquid soluble solutions then the diagram looks like this.Liquid –Solid Phase Diagrams –NonidealbehaviorThis diagram shows that many solid mixes are not soluble and form a eutectic composition where the mix melts at one temp. At other compositions the melting occurs over a range of temperatures. This is similar to our lab work with naphthalene and para-dichlorobenzene.Compound formation of solidsCompound C forms from solid A and solid


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APSU CHEM 3610 - Phase diagrams of mixtures

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