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Chem 481L
Fall 2009
Lab Module #10
Properties of Real Gases
Due Date:
Noon, Tuesday, November 17
Submit via Blackboard
Goal:
To explore the physical properties of real gases in the context of nonnegligible
intermolecular forces.
Research questions:
Can real substances be quantitatively modeled by Virtual Substance in the high
density context where intermolecular forces are important?
How does VS compare with predictions made
by different equations of state and with experimental data?
Introduction:
Real substances can be described by models that take into account the different types of
intermolecular interactions – repulsions as well as attractions – and describe these interactions
in different ways.
For example, we used the Ideal Gas Law, which assumes no intermolecular forces, to describe
noble gases:
[1]
This week we consider alternative equations of state that use various ways to account for
intermolecular forces.
HardSphere Model:
Another (simplified) model is to describe a gas as a collection of
impenetrable "billard" balls of diameter σ. Each atom would then take up a volume of roughly σ
3
,
and a collection of N atoms would occupy a volume of Nσ
3
. The volume available to atoms in
the box would be reduced by this "excluded" volume. Making this adjustment to the ideal gas
law results in:
[2]
where b is the excluded volume for a particle and nb is the excluded volume for n moles of
particles. Unlike the ideal gas model, in this "hard sphere" model there are
repulsive
intermolecular forces acting between the particles.
In classical physics, the force between two atoms can be related to the slope (i.e. derivative) of
an
intermolecular potential
, U(R), where R is the separation between particles. The
intermolecular potentials corresponding to the
hard sphere
model is as follows:
for R > σ
[3]
for R
≤
σ
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 Spring '07
 PIELAK
 Mole, pH, Properties of Real Gases, Chem 481L Lab, 481L Lab Module

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