Qualitatively we can envision that if the extra

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Unformatted text preview: d 2) the melting point of A. We can do this qualitatively using a diagram of chemical potential versus temperature, examining how changing the chemical potential for the liquid phase of A (by mixing A with B) affects the melting and boiling temperatures. pure solid A µ pure liquid A S A in mixed liquid L V Tm(A) pure vapor A Tm*(A) Tb*(A) Tb(A) T The arrows in the above figure show what happens when you mix a solute B with a liquid A, such that the solute B is of very low vapor pressure (i.e. B remains mostly in the liquid phase, which would be the case if we consider B to have a much higher boiling point that A) and such that B does not solidify or co- crystallize with A (B remains in solution as A crystallizes). Marand’s Notes: Chapter 5 - The Properties of Simple Mixtures 177 So, what happens as you mix B with A? The chemical potential of A in the liquid phase decreases (because aA is less than unity). The chemical potential of A in either the solid or the vapor phases does not change (since in these cases A remains pure). Therefore the melting temperature decreases and the boiling point increases. Practical applications: 1) Did you know that adding salt to the water you cook your pasta in, actually allows the water temperature to be at slightly higher temperature than 100°C and therefore to cook your pasta faster!!! 2) Did you know that salting a road in the winter allows H2O to remain in the liquid state down to a lower temperature, which prevents (in some case) roads from icing...I am sure you did. But now, you also know why. Also think of “solder”, an alloy of two high melting metals, which itself melts at much lower temperature than either metal. Now, we are going to calculate experimentally how much melting temperature depression and boiling point elevation to expect, based on some given mole fraction of solute B added to A. Marand’s Notes: Chapter 5 - The Properties of Simple Mixtures 178 Melting Point Depression: When A is pure under pressure P, it melts at TA* given by the Clapeyron equation. Obviously, at this temperature and pressure, the chemical potential of A in the solid phase is equal to that of A in the liquid phase. We write: µA*L(TA*,...
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This note was uploaded on 01/26/2014 for the course CHEM 3615 taught by Professor Aresker during the Spring '07 term at Virginia Tech.

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