SOME THERMODYNAMIC
PROPERTIES OF METHYL
FLUORIDE
I. M. A. FONSECA
L. Q. LOBO
Departamento de Engenharia Química
Universidade de Coimbra
3000 Coimbra
PORTUGAL
118
Methyl fluoride is one of the rare examples
found in nature of a substance whose molecules possess a large dipole moment but
almost no quadrupole (t = 1.86 x 10 -18 esu;
Q = 0.04 x 10 -26 esu) [1], a fact that attaches considerable theoretical (and otherwise)
interest to this compound. It is therefore
somewhat surprising that the information
available in the literature on experimentally
measured thermodynamic properties of CH 3 F
is sparse and not always of the precision
required for most purposes.
In the course of a recent thermodynamic
study of liquid mixtures of (methyl fluoride +
xenon) [2] we carried out measurements of
the vapour pressure p and the orthobaric
liquid molar volume V of pure CH 3 F at, respectively, four and two temperatures. Moreover, in the subsequent theoretical treatment
of the raw data for the mixtures, reliable
values for both p and Vi over the entire liquid
range of the pure components were needed.
This led us to try and find adequate equations
for the temperature dependence of p and V.,
valid from the triple-point to the critical-point
of methyl fluoride, by combining our own experimental results with previously published
(selected) ones. In the present note we report
on the equations obtained in this way, and
also on some of the derived thermodynamic
properties of CH E F.
The methyl fluoride used was a Matheson
product, of purity better than 99.0 moles per
cent, which was redistilled in the laboratory
low-temperature fraccionation column (similar to that described by Davies et al [3]).
The middle fraction used in our experiments gave a triple-point pressure
p t = (0.379 ± 0.003)kPa, which is the average of six determinations made with a widebore precision mercury manometer. This pressure does not seem to have been measured
directly before.
The technique used to measure the vapour
pressure of the pure sample at the remaining
temperatures has already been described, as
Reu. Port. Quím., 31, 118 (1989)
THERMODYNAMIC PROPERTIES OF CH 3 F
well as that for the determination of the liquid
molar volumes [2]. The sample was maintained at a constant and reproducible low-temperature by surrounding it with an appropriate
pure substance melting at its own triple-point.
The vapour pressure measurements were
made, using a quartz-spiral gauge (Texas Instruments, model 145-01, capsule type 811), at
three temperatures: 161.39, 182.33, and
195.48 K, the triple-point temperatures of,
respectively, xenon, nitrous oxide, and ammonia [4].
To represent the dependence of vapour
pressure p on temperature T, we have chosen
the equation proposed by Wagner [5]. This
equation is
ln (p /p c ) = (Aot + A iz 15 + A2t3 + A3t6 )/T R ,
(1)
where t = (1-T R ) , T R = T/T e , and T and p c
are respectively the critical temperature and
critical pressure. In applying this equation to
the vapour pressure of CH 3 F, we have used,
in addition to our own results in table 1: (1),
the measurements of Michels and Wassenaar
between 164.28 and 288.42 K [6]; (2), the
values given by Oi et al. covering the range
132.48 to 213.12 K [7]; and (3), the critical
constants found by Bominaar et al.
(p c = 5.870 MPa, T c = 317.4 K) [8] in a
recent study of liquid methyl fluoride at high
pressures. The latter are in good agreement
with the values •recomended by Ambrose [9].
Each point has been given unit weight. Where
it was necessary to convert temperatures
given in earlier scales into their IPTS-68 equivalents, we have used the methods summarized by Lobo and Staveley [10]. Since we have
not measured the triple-point temperature of
methyl fluoride, the coordinates of this point
were not included in the Wagner fitting.
The best fit was obtained with the following
values of the four parameters in equation (1):
Ao = -6.80940; A l = 0.93782; A 2 = -1.4002;
and A3 = -2.229. The standard deviation of
lnp is 2.1 x 10 -3 . Table 1 includes the
Rev. Port. Quim., 31, 118 (1989)
Table 1
Experimental vapour pressures p and molar volumes Vm of
liquid methyl fluoride. by is the differenc p-p (equation 1),
and W. is V. - Vm (equation 2).
T/K
p/kPa
bp/kPa
130.46
0.379
161.39
10.520
0.118
182.33
48.163
-0.045
195.48
105.178
0.315
V /cm 3 mol - '
bV m /cm 3 mol '
35.769
-0.028
38.608
-0.046
values of 6, = p - p (equation 1), where p
(equation 1) is the vapour pressure of CH 3 F
calculated from equation (1) with the parameters given above. Table 2 gives values of p
(equation 1) at 10K intervals and at the triple-point temperature T t , and the normal boiling
temperature Tb .. The values reported for both
Tt and Tb are derived from equation (1). While
T t = 130.46K found by us is significantly
lower than the 131.4K estimated by Grosse et
al. [11] from their Rankine vapour pressure
equation, our value of Tb = 194.855K compares well with the 194.65K found in the literature [11].
Table 2
Calculated vapour pressures p, liquid molar volumes V.,
and enthalpy of vaporization 41Hm of methyl fluoride
T/K
p/kPa
V„ /cm 3 mol - '
4g H /Jmol 'K '
130.46 ^
140
0.379
1.258
3.677
34.363
34.504
34.994
19280
18888
9.245
35.691
20.574
41.455
36.490
37.326
38.178
150
160
170
180
190
194.855 b
200
210
220
230
240
76.961
101.326
133.458
38.599
39.060
40.032
218.556
340.987
510.484
41.191
42.677
737.644
44.670
18498
18119
17741
17354
16947
16740
16513
16046
15544
15000
14413
Triple-point temperature;
b
Normal boiling temperature.
119
I. M. A. FONSECA and L. Q. LOBO
The molar volume measurements were carried out at 161.39 and 195.48 K by the well
known pyknometric technique. The vessel had
been calibrated at the two temperatures by
using pure liquid ethane and the accurate
density values obtained by Haynes and Hiza
[12]. The experimental results are given in
Table 1. Our two experimental points were
combined with the five values by Grosse et al.
[11] and the critical molar volume V, of 109
cm 3 mol - ' [8] in obtaining the best set of parameters in the equation
terson [13] also made measurements of the
molar volume of liquid CH 3 F between 293.19
and 317.13 K but their values were found to
be too low (and scattered) to be inclued in the
fitting.
Since there seems to be no measured values of
the molar enthalpy of vaporization Og H. of
methyl fluoride it was decided to derive it
from the Clapeyron equation,
T(Vg- V„)
dT
(3)
(V,,,/cm 3 mol -1 ) =
4
=
(
Vm, ref/cm 3 mol - ') + E vi [(T - Trer) /K]' , (2)
i =1
which was fitted to the experimental data.
The following values were obtained for the
parameters: Vm, ref = 34.363; v 1 = -7.3690 x
x 10 -3 ; v2 = 2.6316 x 10 -3 ; v 3 = -3.415 x 10 -5 ;
v 4 = 1.7 0 x 10 -7 . For T rel we used the value
of the triple-point temperature of CH 3 F derived from equation (1).
The standard deviation of this fitting is ±0.026
cm 3 mol - '. A deviation plot is shown in figure
1. In this figure 8V>r = Vm - V (equation 2),
where Vm (equation 2) is the molar volume
calculated from equation (2). Cawood an Pat-
❑
N
m
E
>
Tc
Ti r
0
E
>
o
•
+ 0.1
100
150
••
200
250
T/K
300
and to include in Table 2 the values so obtained. This procedure needs, of course, that
'the molar volumes of the orthobaric vapour
Vg are known. They were calculated from
the virial equation of state truncated after the
second term. As for the second virial coefficients B at the several temperatures required
in the calculations, they were obtained from
the expression
2
=
i=0
b i TR ,
(4)
with parameters b i resulting from the fitting
of equation (4) to the experimental B
values compiled by Dymond and Smith [14]:
b o = -6.0 x 10 -2 ; b 1 = 8.8 x 10 - '; b 2 = -2.46. To
judge the quality of the ,6,f H. values listed in
Table 2 is difficult due to the lack of experimental results at any temperature. To compare our calculated value of the enthalpy of
vaporization at the normal boiling temperature (16740 Jmol - ') with those derived from
the vapour pressure equations presented by
others — 17556 Jmo1 -1 [11] — is perhaps the
furthest step we can take in the present
circumstances.
Figure 1
Deviation plot for the molar volume V of liquid CH3 F. The
quantity plotted is 100[Vm Vm (equation 2)]/V where V is
the experimental value. •, reference [11],
O, reference [8], a this work
120
(Received, 16th December 1987)
Rev. Port. Quím., 31, 118 (1989)
THERMODYNAMIC PROPERTIES OF CH 3 F
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Rev. Port. Quím., 31, 118 (1989)
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121