3444
RALPHF. KRAUSE, JR.,AND THOMAS B. DOUGLAS
RAi
f
RAa
f
(1 - f)l'"
1 1
R - A- -~ (1 - f)l'r* R A ~ (1 - f)lIr1
(6)
(7) (8)
The calculated values of rl and r2 were used with the experimental values off to calculate values of R A and ~ R B from ~ combining eq 5-8. The derived values of R A and ~ R B were ~ used for RNCand RCN,respectively, in eq 3 of the test to calculate a theoretical average isotope effect, r, for comparison with the experimental values of r derived from measured values of RONand
RNC.
The Heats of Formation of AIClF, and AlC1,F from Subliming AIF, in the Presence of AlCl, Vapor' by Ralph F. Krause, Jr., and Thomas B. Douglas National Bureau of Standards, Washington, D. C. 20286
(Received March 12, 1988)
The volatility of AlF, which was measured between 1196 and 1256°K by an entrainment method, was enhanced by the presence of A1C13 vapor. When the AlCl3 vapor was generated by subliming a t 373 or 394"K, the amount of sublimed AlF3 was observed to be from 1.7 to 7.4 times that expected in an inert medium. The vapor mixture saturated with AlF, was considered to include the two mixed-halide monomers formed accordin which m = 1 or 2. After correcting for ing to the reaction (rn/3)AlC& [l - (rn/3)]AlF3 + AlCl,F3-,, the formation of AlZFa, AlZCls, and the 22 possible mixed-halide dimers, a least-squares fit to the entrainment data yielded A H O l z z s = 0.5 rt 1 kcal (estimated uncertainty) for both of the above all-monomer reactions.
+
Introduction The mixed-halide monomers of AlC1, and AlF3 have been commonly postulated, but no experimental work has heretofore indicated their existence, let alone given information as to their properties. Several aUthors2-6 have estimated zero (with uncertainties up to I 2 0 kcal) for values of the AH'S of the two gaseous reactions represented by (m/3)AIC13
+ [l - (m/3) ]A1F3
---f
AlCl,F,-,
(1)
in which m = 1 or 2. The sum of these two reactions is analogous to the gaseous reaction BCla
+ BF3 +BClF:! + BClzF
(2)
on which several spectroscopic measurements have been made by others. Higgins, Leisegang, Raw, and Rossouw' determined the equilibrium constant of reaction 2 (300-302°K) by measuring the intensity of infrared absorption due to BC&. Lindeman and Wilsons assigned vibration fundamentals to the four species of the reaction. Gunn and Sanborns calculated AH" for the reaction of 1.68 lccal from the preceding results and 1.1kcal (uncertainty of -0.5 to +0.8 kcal) from their own measurement of the variation of the equilibrium The Journal of Physical Chemistry
constant with temperature (288-318°K). More recently, Porter, Bidinosti, and Watterson'O measured the mass spectrometric intensity of the four species of reaction 2 from about 300 to 600°K; they obtained second-law values of AHl" = 0.49 f 0.03 kcal and AHzO = 0.70 f 0.04 kcal for the t\vo reactions of eq 1with B replacing Al. The present paper gives comparable val(1) This work was sponsored by two agencies of the U. 5. Department of Defense: the Advanced Research Projects Agency (Order No. 20) and the Air Force Office of Scientific Research, Propulsion Division (ISSA-65-8). (2) D. L. Hildenbrand, Publication No. C-623, Aeronutronic, Newport Beach, Calif., Sept 1959. (3) R. R. Koppang, C. M. Sherwood, and G. S. Bahn, "Some Provisional Tables of Species Thermodynamic Properties," Marquardt Corp., Van Nuys, Calif., Oct 1959. (4) C. B. Henderson and R. S. Scheffee, "Survey of Thermochemical Data," Atlantic Research Corp., Alexandria, Va., Jan 1960. (5) J. 9. Gordon, "Thermodynamic Data for Combustion Products," Thiokol Chemical Corp., Denville, N. J., Jan 1960. (6) D. R. Stull, et al., "JANAF Thermochemical Tables," Dow Chemical Co., Midland, Mich., Sept 1964. (7) T. H. S. Higgins, E. C. Leisegang, C. J. G. Raw, and A. J. Rassouw, J. Chem. Phya., 23, 1544 (1955). (8) L. P. Lindeman and M. K. Wilson, ibid., 24, 242 (1956). (9) S. R. Gunn and R. H. Sanborn, ibid., 33, 955 (1960). (10) R. F. Porter, D. R. Bidinosti, and K. F. Watterson, ibid., 36, 2104 (1962).
3445
THEHEATSOF FORMATION OF A1C1F2AND AIClzF ues for the AHO's of the corresponding reactions involving the aluminum halides. While the boron halides are believed to have no appreciable association,* the vapor dimerization of the aluminum halides 2A1X3 +AlzXs
+ [2 - (n/3)]AlF3 +Al&lnFs-n
c
r
Ar
-
Gasometer
n
(f-
Y
(3)
is known. Using mass spectrometric data of others for A12Fe, I'hause and Douglas" recently evaluated 4.6 f 1.4 mol % of dimer in the saturated vapor of the A1F3 system at 1225"M. M e a ~ u r e m e n t s l ~of- ~the ~ vapor density of A1C13 over the region 513-944°K have shown that its saturated vapor is predominantly dimeric below its melting point (465"K, >1 atm); however, extrapolation to 1200°K and 0.02 atm indicates an upper limit of only 0.04 mol % of dimer for the case of the present work. It was recognized that the A1C13-A1F3gaseous system may contain an even larger proportion of dimers than indicated by the above-mentioned dimerization on each halide alone. Although there is no information on the mixed-halide dimers, their contribution may be postulated by considering their formation &s demonstrated by the reaction (n/3)AlCL
AIC13
(4)
in which n = 0, 1, 2, . . ., 6, and by assuming that any dimer has the same general structure as that known for AlzC16. Small but detectable amounts of trimers and even higher polymers are likely,l5 but only monomers and dimers, each behaving as an ideal gas, will be assumed. I n fact the total number of physically distinguishable dimers that can be formulated, namely, 24, is so great that even a more arbitrary assumption will be involved, grouping the properties of many of them together before attempting an evaluation from our data.
Experimental Method As illustrated in Figure 1, this entrainment work consisted of nearly saturating a flow of argon gas in one cell with the vapor of AlC13, which was generated at 373 or 394"I
