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IntroductiontoPhaseChange
Modeling the Behavior of Real Substances
Inserting Reality into the Ideal Gas Laws
A Series of Exercises produced in Molecular Workbench
Bennett College for Women
BACKGROUND - THE IDEAL MODEL
The Ideal Gas Law predicts that, as we remove heat (energy) from a substance, its temperature will decrease. For a gas, there is the additional requirement that the pressure(P) or the volume(V) remain constant - then the other will vary linearly in response to the removal of heat.
Kinetic Theory, repesenting the molecules as point non-interacting masses, suggests that if heat is continually removed, the molecules will slow down until eventually all motion will stop. This is the ideal extrapolated point called Absolute Zero, which is zero degrees on the Kelvin scale. The introduction of the Kelvin scale for temperature (T) makes possible the simplest form of the Ideal Gas Law:
PV = nRT . In this model, when the (Kelvin) temperature goes to zero either P or V on the left side must go to zero.
THE BEHAVIOR OF ALL REAL GASES
While the Ideal Gas Law very accurately reflects the behavior of gases under a wide variety of conditions, the model and laws above do not reflect the behavior of any real substance. In particular, assume we start with a real gas and begin to remove heat under conditions of constant pressure. The system will decrease slowly in volume, and indeed its linear progression will move towards zero volume at zero temperature (Blue line below) . But real substances never reach absolute zero. Instead, the gas steadily become cooler until it reaches the temperature at which a phase change occurs. Such a change for water vapor (gaseous H20) occurs at 373 K (or 100 ºC) and the volume suddenly reduces dramatically as the vapor changes to liquid water (Red line below).
The next section provides some experience with a phase changes and a model which can be used to describe them. Next
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