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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
Richard Lee Ponting - Educational Consultant
BACKGROUND - THE IDEAL MODEL
The Behavior of Ideal Gases is Governed by two Principles :
The Ideal Gas Law => PV = nRT P -> Absolute Pressure ; V -> Volume
T -> Absolute (Kelvin Temperature) *
*Kelvin Temperature is Centrigrade Temperature plus about 273 deg. The introduction of the Kelvin scale for temperature (T) makes possible this simplest form of the Ideal Gas Law
Note, according to the Ideal Gas Law, either P or V goes to zero as Temp. (K) goes to zero !
Kinetic Theory => KE (molec.) = 3/2 k T Model molecules as point non-interacting moving masses.
Kinetic Energy directly related to Temperature. "k" is the Boltzman Constant !
Note Kinetic Theory predicts that Kinetic Energy (hence molecular speed) are zero when Temp (Kelvin) = 0
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 becomes 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.
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