117
Thermo Chapter 7
Questions on Concepts
Q7.1)
Explain why the oscillations in the two-phase coexistence region using the
Redlich-Kwong 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.

This
** preview**
has intentionally

**sections.**

*blurred***to view the full version.**

*Sign up*