Lecture 16 - Lecture 16: Phase Transitions We already...

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Lecture 16: Phase Transitions We already discussed that the stability of one phase relative to another is a tradeoff between lowering enthalpy by increasing intermolecular interactions and increasing entropy by increasing freedom and disorder. In this lecture, we discuss specific cases of phase transitions. Liquid-vapor equilibria: There is always some finite, specific pressure of a vapor above/around the surface of a liquid in a closed container. (Equilibria always refer to closed systems.) The vapor pressure is defined by the enthalpy of keeping the molecules together in the liquid in competition with the entropy of releasing them into the gas phase. The entropy of releasing molecules into the gas phase is essentially the same for all gases, because they all have the same PV = nRT relationship at low pressures. So, the vapor pressure is really determined for any one liquid by the enthalpy term. An expression for the vapor pressure is: P = Ae -ΔH/RT where the A term comes from the entropy that is the same for all gases. ΔH is highest for liquids like water and alcohols. Also, P goes up as T goes up. When P equals atmospheric pressure, then the liquid boils . This is because of the mechanical aspects of the system; boiling can occur at any vapor pressure (and therefore any temperature) if the atmosphere matches the pressure. Water boils at a lower temperature at higher altitude. It is possible for a solid to have a vapor pressure also.
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This note was uploaded on 09/29/2009 for the course ME 530.230 taught by Professor Katz during the Spring '09 term at Johns Hopkins.

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Lecture 16 - Lecture 16: Phase Transitions We already...

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