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.
Liquidvapor 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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 Spring '09
 KATZ
 Vapor pressure

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