2332
J. BLAUER,XI. A. GREEXBAYM, A N D M. FARBER
The Heat and Entropy of Formation of Boron (I) Fluoride( g)
by Jay Blauer, M. A. Greenbaum, and Milton Farber Research and Debelopment Laboratories, Maremont Corporation, Pasadena, California (Received April 6 , 1064)
+
The equilibrium 2’3B(c) 3B173(g) = BF(g) was studied in the temperature range 1307-1505°K. by means of transpiration at pressures below 300 y. The enthalpy and entropy changrs of the reaction in this temperature raiqe were found to be 59.7 i 2.6 kcal. / mole and 25.1 f 1.9 cal./deg., mole, respectively. The data have been interpreted and extrapolated to yield t,he second law heat of formation, AHE2980K. = -29.0 f 2.6 kcal.,’ mole, and entropy, S2g80~.= 47.6 f 1.9 cal. deg., mole, of gaseous BF. Partial pressure studies indicate that the quantity of BYL(g)present under the experinmital conditions is negligible.
Introduction S o experimental nieasurcnieiits of the heat of formation or entropy of gaseous BF have been reported to date. Estiiiiated values of these functions have been reported.’ These values are given as: AHiZg8 = -45.47 kcal.,’mole and Sozg8= 47.89 cal.,’deg. ’mole. As a continuing part of our studies of the thermodynamic properties of the light metal compounds it was decided to undertake an experimental program to determine these therniodynaiiiic values.
Experimental Sniall pirces of crystalline boron (obtained from Kern Cheiiiical Co.) of >99.5yG purity were placed in a rhenium combustion boat. The boron and cornbustion boat had been weighed previously. The combustion boat was placed in a zirconia furnace tube with a 2.5-cin. i.d. The combustion tube was flushed continuously with a stream of argon gas during the loading operation. The furnace used in the present study was a Leco tube furnace with automatic temperature controls. The absolute tciiiperature was established by nieaiis of a calibrated platinum-plati1iu11i-10~~ rhodium thermocouple. Once this was set the automatic controller niaintained thc teniperaturc to ,io. After the system had been evacuated, boron trifluoride (froni JIatheson Co.) having a purity iii excess of 99.5% was adiiiittcd through a very fine metering valve. The gas was prcheated by passage over a bed of alumina chips. S o reaction was found to occur bctwcen the BV3(g) and A120achips over the tenipera-
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T h e Journal of Physical Chemistry
ture range specified. The total pressure of the system never exceeded 300 p . Pressures were deterniiried by means of a thermocouple gauge which was placed a t the end of the furnace tube and which had been calibrated for the pressure at the center of the tube by rneaiis of a AleLeod gauge. The transpired gascs were collected in a cold trap cooled in liquid nitrogen. At the end of each run the conibustion boat was removed from the furnace and allowed to cool in an atmosphere of argon gas. The boron trifluoride in the cold trap was allowed to expand at room teniperature and its pressure determined manometrically. The sample of boron chips was weighed separately due to the fact that the rhenium boat reacted to a slight extent with the boron trifluoride. However, the amount of this side reaction was quite sinall (less than 5y0)and had no effect on the experimental results when the boron saiiiple was weighed separately from the boat.
Treatment of Data The theory for the nioleeular flow reaction method at very low pressures has been adequately described in a previous paper.* If oiily Rk’(g) is formed under the experimental conditions considered, it is possible to shorn on the basis of the theory of niolecular flow2that the following equation describes the conditions holding a t equilibrium (1) JANAF Thermochemical Tables, USAF Contract AF 33(616)6149, Advanced Research Projects Agency, Washington 25, D. C. (2) M. Farber and J. Blauer, Trans. Faraday SOC.,5 8 , 2090 (1962).
2333
HEATA N D EKTRQPY OF FORYIATIOS O F BORON(I) FLUORIDE(g)
-~
The symbols appearing in eq. 1 have the following definitions: K , is the equilibrium constant, MBF is the molecular weight of BF, MBF, is the molecular weight of BF3, rBF is the number of nioles of BF escaping from the reaction zone per hour, I’BF, is the number of moles of unreacted BFB escaping from the reaction zone per hour, and PBF,is the equilibrium pressure of BIT3. The calibration of the apparatus indicated that the following equation applies
PBIP, = O.o0367r~~,
=
I‘BF
0.02368~-
r BF,’”
log
x,*- JW
where KPois the value of K , a t infinite surface area, J is a constant, and w is the weight of the sample. It was further found that J is not temperature dependent. The over-all eflect of both temperature and surface area upon the extent of the reaction was expressed by nieans of the following equation log K ,
AS AH J - - -- R RT w
= -
- 4.6
i
\
-1
;
0
1
3 4 I/@,E. -1.
2
5
6
1 7
Figure 1. Effect of surface area upon the extent of reaction: 0, data taken at 1476°K.; 0, data taken at 1375°K.
(3)
by changing the surface area, or equivalently the weight, of the boron sample. It was found that the effect of surface area could best be described by means of the following equation =
-I\
-4.2
A
It was found that the reaction is markedly influenced
log K ,
- 3.4
-5.0
n
K,
T
-3.8
(2)
where PBF,is expressed in units of atmospheres. combination of eq. 1 and 2 yields eq. 3, namely
-3.0
(5)
Results and Discussion The experinieiital data are tabulated in Table I. The data taken at 1375OK. and at 1476OK. are shown plotted in Fig. 1 in the form described by eq. 4. The values of J and log K,O for these two teinperatures were evaluated by the method of least squares and the results are tabulated in Table 11. A variance ratio test3 indicates that the variances of ,I are statistically identical at the two temperatures ; consequently, a Students’ t-test can be applied to test the identity of the slopes themselves. The t-test indicates that within cxperjniental error the slopes are identical and independent of temperature. The values of the equilibrium constants for each run at 1373’K. have been corrected for surface effects by means of eq. 4 and tabulated in Table I11 as a function
Table I : Experimental Data for the BF3(g)-B(s) Reaction Run
TBF,
rBFa,
Km
1/
