Chem 112 Exam AID Course Pack

The heat absorbed or released is given by the

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Unformatted text preview: diagrams). In the case where the solid phases are immiscible but the liquid phases are, the phase diagram will resemble a horizontal line with two parabolic arcs over top (presented in your lecture notes with the example of NaCl and NaBr). - in the part below the horizontal line, both substances are solid; between the horizontal line and each parabolic arc, both are found in the liquid phase but only one in the solid phase - tie lines on this phase diagram are applied in the same way, except that there are different places you can put it (so a tie line between a parabolic arc and the side of the graph is associated with the equilibrium between the solid phase on that side and the mixture) The point between the two arcs, where they meet the horizontal line, is the eutectic point. Ethan Newton & Barry Zhang for SOS Winter 2012 24 - - - as temperature changes, the mixture will always cross the horizontal line at this point if we try to freeze the mixture (at any mole fraction), one of the substances will start to change phase first; this moves the mole fraction of what remains closer to the fraction of the eutectic point in fact, the path will be along the parabolic arc, until it reaches the eutectic point (and then freezing completes, and the original mole fractions are restored (though now everything is solid) similarly, melting the mixture will result in the first liquid that forms to have the mole fraction associated with the eutectic point the path will then follow the parabolic arc until the entire mixture is liquid and the original mole fractions are restored if your starting mole fraction is that of the eutectic point, the substances will freeze/melt together This also applies to solutions of solids in water. The main ideas are the same; it’s just that the melting points of the pure substances will be very far apart. You can also find the maximum possible solute concentration (i.e. the concentration of the saturated solution) at any given temperature by seeing how far to the right you can go before you reach the region where solid solute starts to occur. (Increasing the temperature will let you dissolve more). An azeotrope is a mixture for which the boiling point is either at a minimum or maximum. - changing the composition in either direction will decrease it (for a maximum- boiling azeotrope) or increase it (for a minimum- boiling azeotrope) - if the mixture is boiled, it gives a gas with the same composition as the liquid - this also means that distillation cannot be used to purify a minimum- boiling azeotrope (as distillation ma...
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