117
Thermo Chapter 7
Questions on Concepts
Q7.1)
Explain why the oscillations in the twophase coexistence region using the
RedlichKwong and van der Waals equations of state (see Figure 7.4) do not correspond
to reality.
The oscillations predict that as
V
increases,
P
will increase.
No real gas exhibits this
behavior.
Q7.2)
Explain the significance of the Boyle temperature.
The Boyle temperature provides a way to classify the way in which
z
varies with
P
at low
values of
P
for different gases.
If
T
>
T
B
,
z
increases with increasing
P
; if
T
<
T
B
,
z
decreases with increasing
P
.
Q7.3)
The value of the Boyle temperature increases with the strength of the attractive
interactions between molecules. Arrange the Boyle temperatures of the gases Ar, CH
4
,
and C
6
H
6
in increasing order.
Ar < CH
4
< C
6
H
6
Q7.4)
Will the fugacity coefficient of a gas above the Boyle temperature be less than
one at low pressures?
No. The integral
0
1
P
z
dP
P
−
′
′
∫
is always greater than zero for this case.
Therefore,
γ
> 1 for
all
P
.
Q7.5)
Using the concept of the intermolecular potential, explain why two gases in
corresponding states can be expected to have the same value for
z
.
Two different gases will have different values for the depth of the intermolecular
potential and for the distance at which the potential becomes positive.
By normalizing
P
,
T
, and
V
to their critical values, the differences in the intermolecular potential are to a
significant extent also normalized.
Q7.6)
By looking at the
a
and
b
values for the van der Waals equation of state, decide
whether 1 mole of O
2
or H
2
O has the higher pressure at the same value of
T
and
V
.
A is significantly larger for H
2
O.
Therefore, the attractive forces between H
2
O molecules
are greater than between O
2
molecules.
Consequently, O
2
will have a higher pressure.
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Q7.7)
Consider the comparison made between accurate results and those based on
calculations using the van der Waals and RedlichKwong equations of state in Figures 7.1
and 7.5. Is it clear that one of these equations of state is better than the other under all
conditions?
The RedlichKwong gives more accurate results for almost all of the values of pressure
shown in these figures.
However, it is not better under all conditions.
Q7.8)
Why is the standard state of fugacity,
f
º, equal to the standard state of pressure,
P
º?
If this were not the case the fugacity would not become equal to the pressure in the limit
of low pressures.
Q7.9)
For a given set of conditions, the fugacity of a gas is greater than the pressure.
What does this tell you about the interaction between the molecules of the gas?
If the fugacity is greater than the pressure, the repulsive pat of the potential dominates the
interaction between the molecules.
Q7.10)
A system containing argon gas is at pressure
P
1
and temperature
T
1
. How would
you go about estimating the fugacity coefficient of the gas?
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 Spring '03
 Nefzi
 Physical chemistry, pH, Van der Waals

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