April, 1963
$UBLIMATION PRESSURE OF
CaF2 AKD
shown species like (Q) to be very strongly acidic with a pK value of about 1. Consequently one would expect species (Q) to be rearranged because of th.e transfer of the proton to the nitrogen of the pyridine nucleus with the establishment of species shown in two extreme resonance forms as
DISSOCIATION
ENERGY OF Cap
877
I n an organic solvent with a low dielectric constant it is unlikely that the charge will be separated to any greater extent. The SPectruni given jn Fig. IB is concluded to be the absorption spectrum of species (R)* From th.e quantum yields it is seen that in EPA glass at liquid nitrogen temperature only a small percentage of the light quanta absorbed is effective in accomplishing this chemical isomerization. It is interesting to note that the quantum yield seems to be of the same order of magnitude in liquid ethanol, EPA and ether, as coinparable irradiation times were needed to obtain the same optical density in these solvents as in EPA glass.
QcHpc, - pHQca -
+H
N -0 /
N
OF ' 0
\0
(R)
THE SUBLIMATION PRESSURE OF cALcIuni(I1) FLUORIDE AND THE DISSOCIATION ENERGY OF CALCIUX(1) FLUORIDE' BY GARYD. BLUE,JOHN W. GREEN,REXATO G. BAUTISTA, AXD JOHN L. MARGRAVE Department of Chemistry, [Jniversity of Wisconsin, Madison, Wisconsin Received October 18, 1962
A vacuum microbalance and a mass spectrometer have bem used to determine sublimation pressures and heats of sublimation for CaFz(s) over the range 1242-1669°K. The data are represented by log Pat,,,= -(95.46 i. 0.49)/45.76 X 104/T 8.141 f 0.006. The errors quoted are standard deviations of the least-squares fit. The heat of sublimation a t 298°K. is 103.3 f 2.0 kcal./mole. From studies of the reaction of CaF2 with alumifor CaF(g) was determined to be 5.4 =t0.2 e.v. from two independent num over the range 1271-135loK., DOZSS equilibria. Close agreement with other investigators is found for the sublimation of CaF2(s)but the dissocia-
+
tion energy oi CaF(g) is much greater than the previously accepted value.
I. Introduction Reliable Knudsen or Langmuir vaporization and sublimation rat,es for alkaline earth halides have not been available, in spite of the fact that many of these coinpounds are easily procured as high-purity single crystals and that they find applications as refractories and reactants in high temperature systems. Brewer, Somayajulu, and Brackett2 have reviewed the thermodyiiamic properties of the gaseous metal dihalides and present vaporization data for CaFz based on the early work of Ruff and L e B ~ u c h e r . ~More recently Schulz and Searcy4 have investigated the vapor pressure of Capz in the temperature range 1400 to 1850OK. by the torsion-eff usion method. Finally, Pottie6 has made a single Knudseii effusion measurement of the vapor pressure of liquid Ca,F2at 1823OK. I n order to extend the temperature range of observat,ions and identify the vapor species, we have measured the sublimation rate and heat of sublimation of CaFz by both the Langmuir and Knudsen techniques employing a vacuum microbalance and a mass spectrometer. The dissociation energies of the alkaline earth monohalides are poorly known. The recent and extensive spectroscopic work on BeFj6BeCl,'" and thermochenii( 1 ) Abstracted, in part, from the theses presenied by G. D . Blue and J . W. ( : ~ e c nin partial fulfillment of the requirements for the P1i.D. degree at tilo Univcrsity o f Wisconsin, 1063. ( 2 ) L. Brewer, (:. It. Souiayajulu. and ld. Brackett, Lawrence Radiation Laboratory Report UCRL-9840, September, 1961; Chem. Rev., 63, 111 (1963). (8) 0. Ruff and L. LeBoucher, 2. anorg. allgem. Chem., 219, 376 (1934). (4) D. A. Schulz and A. W. Searcp, Lawrence Radiation Laboratory Report UCRL-10141. March, 1962; .I. P h y s . Chem., 67, 103 (1963). (j) R. W.Potiio, E. I. du Pont Company, private communication. (6) 1'. M. Tatevskii, L. N. Tunitskii, and M. >l. Novikov, O p t . Speulr.. (USSR), 6, 521 (1958). ( 7 ) (a) M. M. Novikov and L. N. Tunitskii, (bid., 8 , 396 (1960); (b)
cal work on BeF,7b suggest, that the stability of these molecules may be much greater than previously suspected. Gaydons and Herzbergg have tabulated recommended values for the dissociation energies of a large number of diatomic molecules and both suggest a value of 3.15 e.v. or less for the CaF gaseous molecule, based on an apparent predissociation observed by Hellwegela and Harvey.ll It, is not possible to obtain a reliable value for the dissociation energy of this molecule by means of a Birge-Sponer extrapolation because the data reported depend on head measurements of bands of a weakly degraded system. In the work reported here, the dissociation energy of the CaF molecule was coinpared with that of the A1F molecule by utilizing the mass spectrometer to study the equilibrium Ca(g) hlF(g) = CaF(g) Al(g) in a Knudsen cell. A second, independent value of D(CaF) was derived from the heat of the reaction Ca(g) CaF2(s) = 2CaF(g).
+
+
+
11. Experimental Methods The Lnngmuir studies were carried out with a microhalance built inside a vacuum system. The apparatus has been previously described in detail by Dreger and Margrave1z and by Pa111e.l~I t consists basically of an inductively-heated graphite furnace, an electromagnetic beam balance, and an electrical circuit for the balance which permits changes in weight to be measRocket Power, Inc., Fourth Quarterly Report. QIt-7414-4, Contract AI' 01(611)-7414, April, 1962, tlir(iugli June, 1962. (8) A. U. Osydon, "llissociation Rneygies," Cliapnlan and Hall, Ltd., London, lS50. (9) G. Heraberg, "Molecular Spectra and Molecular Structure-I. Spectra of Diatomic Molecules," D. Van h'ostrand Co., Ino., Kew York, N. Y., 1950. (10) I D(CaF) even though a bonding electron is r e n i o ~ e d . ~The tenipera(17) C. E. Moore, Natl. Bur. Std. Circ. 467, 1949.
Llpril,1963
SUBLIMATION PRESSURE OF CaFz AND DISSOCIATIQX EKERGY OF CaF
Ca F2
Fig. 1.-Vapor
pressure data for CaFz by various investigators.
ture dependences OF CaF+ ions formed by bombardment with 10.5 and 75 volt electrons were found to be greatly different, demonstrating that the low energy tail is not due to fragmentation of CaFz(g). Further evidence, discussed in section IV, supporting CaF(g) as the precursor of the CaF+ ions formed a t low energy was found in the ionization efficiency curve for CaF+ after aluminum was added to the calcium fluoride in the cell. The observed intensity of Ca+ and F+ cannot be due to decomposition of CaFz to atoms since Pca = 5.4 X 10-l1 atm. a t 1600'K. It was not possible to follow the temperature depencleiice of low energy F+ due to its high ionization potential and background, but the temperature dependence of F+ from 75 volt electrons was very nearly equal to that of CaF+ from 10.5 volt electrons, so it is conceivable that CaF(g) and F(g) may be formed in the same reaction which may possibly be with the tantalum cell. Two independent methods were employed to determine the heat of sublimation of CaFz from the experimentally observed ion currents of CaF+. The inteqsity of the 40Ca1gF+ peak, using 75 volt electrons, was followed as a function of temperature, taking into account the CaF+ due to CaF molecules. By making use of the ion current pressure relationship18P = kI+T (18) W. A. Chupka and >! G.I. Inghrem,
J. Chem. Phus., 21, 371 (1953).
x 50
0
15
20
ELECTRON
Fig. 2.-Ionization
VOLTS
25
30
35
ICQRRECTEQI.
efficiency curves for species observed over CaF2.
and the integrated form of the Clausius-Clapeyron equation, a value of 4HT which is independent of the proportionality constant 16 may be found from the slope of the curve obtained by plotting log (I+T)us. 1/T. The slope of this plot yielded a heat of sublimation of 4H014z4 = 95.46 i 0.49 kcal./mole where the uncertainty given is again the standard deviation of the least squares treat-
G. D. BLUE,J. \'v. GREEN,R. G. BAUTISTA, A ~ J. D L. I I A R G R A V E
880
ment. The true uncertainty may be several times this figure due to such factors as temperature gradients in the = 103.6 kcal.jniole, cell. A heat of sublimation, AHoZg8 was obtained from AHo1424by using Brewer's2 molecular constants to calculate the enthalpy change of the gas and taking H"HZ,- Hozvs= 24.3 kcal.jmole directly from Kaylor's heat coiiteiit measurements on CaFz(s).l 9 There was no break in the vapor pressure curve a t or near 1424'K. t o suggest a phase change. From a review of Naylor's work, it appears that the designations (Y and p were simply used to describe the two different equations which fit the H T - H02g,data above and below 1424OK. and that there is no real evidence for a phase change in CaFz(s) in this temperature range. hi alternative approach is to calculate AH0298at each temperature from the absolute pressure and the free energy function change for the reaction. In order to determine the instrument constant k , a weighed amount of silver was vaporized from the crucible and a pressure calibration made in the usual ~ a y . ' ~ When J~ it was determined that the pressures from the microbalance studies were only ca. 20% higher than those calculated using the silver calibration, the constant k was calculated by normalizing the mass spectrometric data to the inicrobalance data by means of the leastsquares equations. The pressures then were used for a third law calculation of the heat of sublimation and yielded an average value of AHo298= 103.3 kcal./mole. The results of the mass spectrometric runs are listed in Table I1 and plotted in Fig. 1. TABLE I1 MASSSPECTROUETRIC SUBLIMATION DATA I (CaF+),a OK.
1041~
-A
FoT
arbitrary units
(
-log Pat,
- Hu,w T
)
AH0m
42.25 103.2 42.44 103.6 42.65 103.3 42.85 103.4 43.05 103.3 43.25 103.0 43.40 102.8 43.17 103.4 42.97 103.5 42.80 103.5 103.5 42.55 42.38 103.5 42.27 103.5 ' 41.86 103.0 103.0 42.12 42.55 103.2 42.78 103.2 43.16 103.2 103.3 43.43 Av. = 103.3 5 0 . 2 kc a1./mole a Second set of measurements corrected for change in eniission and multiplier gain 1573 1521 1461 1403 1347 1290 1250 1312 1370 1419 1490 1535 1567 1669 1606 1491 1426 1317 1242
6.357 6.575 6.845 7.128 7.424 7.752 8.000 7.622 7.299 7.047 6.711 6.515 6.382 5.992 6.227 6.707 7.013 7.593 8.052
log Patrn= -
5.098 5.604 6.154 6.760 7.348 7.996 8.494 7.782 7.112 G.580 5.881 5.468 5.190 4.339 4.806 5,834 6.484 '7.685 8.685
4660 1503 442 114 30.7 7.20 2.36 11.6 51.9 171 810 2040 3800 25400 8950 909 211 14.5 1.53
95.4ti
f
0.49
45.76
X
lo4 -
T
+ 8.141
f
0,006
While the small standard deviation of h 0 . 2 kcal. mole for the third law heat reflects the high reproduci(10) B. F.Nayloi, J . A m . Chem. Soe., 67, IS0 (1045). (20) M. G. lnghrain, W..IChugka, . and R. F. Pwtei, J . Chem. P h y s . , 23, 2161 (1955).
T'ol. 67
bility of the measurements, the true uncertainty must reflect errors in entropy estimates, primarily of CaFz(g). The quite low bending frequency of 95 cm.-l is particularly open to question. The third law heat of sublimation therefore was taken to be 103.3 f 2.0 kcal. /mole. In using the second law method to obtain the enthalpy of reaction the major inherent errors are those in temperature measurement. The enthalpy correction for the gas is very insensitive to the choice of molecular parameters. I n practice, uncertainties of the order of h 3 kcal./mole are obtained in the temperature range covered by this investigation. The second law heat of sublimation then was taken to be 103.6 =t 3.0 kcal./ mole. IV. Stability of the CaF G.aseous Molecule I n order to achieve reducing conditions and enhance the amount of CaF(g) present, about 100 mg. of aluminum wire was added t o the CaFz sample in the Iinudsen cell. On heating to about 133OoK.,ion peaks produced by 7 5 volt electrons were observed corresponding t o Ca+, CaF+, ,41+, and AlF+ in the relative amounts 41 : 1: 100 :850. Ionization efficiency curves for these ions are shown in Fig, 3. Using IP(Ca) = 6.1 e.v. as a standard," one finds AP(CaF+) = 5.5 e.v., AP(hlF+) = 9.7 e.v. and AP(,41+) = 6.1 e.v. The appearance potential of CaF + reproduces within the uncertainty (*0.3 e.v.) of the measurements, the appearance potential of AlF+ agrees with work by Porter,21and the appearance potential of AI+ agrees with the spectroscopic value17of 5.98 e.v. and the electron impact value of 6.1 0.2 e.v. obtained by Porter, et aLZ2 The fact that a new process was then responsible for the production of CaF(g) was established by the observations that the CaF+ intensity was much greater than that due to CaFz(g) when observed a t the same temperature and that the temperature dependence of the CaFT ion formed by 10.5 volt electrons differed greatly from that observed in the earlier sublimation experiments. Fragmentation effects were minimized by working 5 v. above the appearance potential for Ca+, CaF+, and AlF+ and only 2 v. above the appearance potential of Al+ since there was a break in the ,41+ ioiiization efficiency curve about 3.1 e.v. above onset. The Al+ formed by higher energy electrons was almost entirely due to fragmentation of A1F.2i Temperature dependences of the indiyidual ions formed a t these voltages then established that Ca+ and Alf were not produced by fragmentation of CaF and A1F. An important observation was that the activity of aluminum in the system was definitely much less than unity since the pressure of Al(g) was much less than, and the heat of formation from the condensed phase much greater than, that expected from the reaction Al(1) = Al(g). X-Ray diffraction powder patterns of the sample showed no lines due t o A1 even after long exposure though copious amounts of BlF(g) were still being produced a t the completion of the expwinient. Thr cqiiilibrium constant for tho lioiriogciicous retiction (h(g) 4- Alk'(g) = 'U(g) -t- Cal;(g) wah detcrmined at a series of temperatures using low energy ionizing electrons, The equilibrium constant, Ki, for this reaction is pressure independent so that instrument geometry and sensitivity factors cancel and it was
*
(21) K. r Porter, zbzd., 23, SI51 (1960). (22) R F Porter, P. Schissel, a n d 1LI G. Inghram, a h d . 23, 339 tlRT,5).
SUBLIMATION PRESSURE OF Capz AND DISSOCIATIOX ESERGY OF CaF
.kprii, 1963
SS 1
only necessary to correct the ion current constant, Kl’ = (Al+)(CaF+)/(Ca+)(AlF+), for relative cross sections, differences in energy above threshold, and gains. The tabulated cross sections of Otvos and Stevenson,23 an experimental value for the gain ratio of AlF+ and CaF+ and an estinnate16of the relative gains of Al+ and C a + were used to calculate K , = 2.3&’. The values of log K1 then were used together with free energy fuiictions for A1 and AJF from the JAKAF Tables,24Ca from Stull and Sinke,5:6and CaF from Kelley26and Kelley and Kingz5(SO258 := 54.9 f 0.2 e.u.) to calculate third law heats of reaction a t 298OK. The average value is AH0298 = 32.7 f 10.2 kcal./mole. The data and results are presented in Table 111. TABLEI11 EQUILIBRIUM COSSTANT AXD HEATOF THE REACTION Cak) AWg) = A k ) CaF(g)
+
+
-A
(POT -
T,
-log
OK.
104/T
1289 1311 1335 1323 1292 1276 1349 1281 1334 1271 1351
7.758 7.628 7.491 7.559 7.740 7.837 7.413 7.806 7.496 7.868 7.402
K i‘
3.891 X 2.554 X 4.04% X 3.2582 X 2.716 X 2.472 X 4.821 X 2.03, X 3.79 X 2.06 X 4.69 X
Ki’
-log KI
10-6 4.41 4.05 10-6 4.59 4.23 lop5 4.39 4.03 10-5 4.48 4.12 4.58 4.22 10-5 4.61 4.25 10-6 4.32 3.96 10-6 4.69 4.33 4.42 4.06 10-b 4.69 4.33 loW64.33 3.97
Horn T) 5.93 5.93 5.93 5.93 5.93 5.93 5.93 5.93 5.93 5.93 5.93
AHozes
31.6 33.2 32.6 32.8 32.6 32.4 32.5 33.0 32.7 32.7 32.6
0
10
5
ELECTRON
Fig. 3.-Ionization
15 VOLTS
20
25
(CORRECTED).
efficiency curves for species observed over hl-
C3Fa.
Ca(g).AIF(g): Al(g). CaF(g)
Av.
= 32.7f0.2 kcal. /mole
The ion current constants, Kl’, may be used as ‘before to find an independent second law heat from the plot of I I I 1 I 1 1 log K1’ us. 1/T shown in Fig. 4. A least squares treat73 74 75 76 77 70 79 80 ment yields AH31310= 34.8 f 2.7 kcal./mole which 9 T when corrected gives AHoz,, = 34.6 2.7 kcal./m.ole. Fig. 4.--.Temperature dependence of the equilibrium constant for By combining .the more reliable third law heat of this Ca(g) AIF(g) = AUg) CaF(g). reaction with the known heat of d i s s o ~ i a t i o n(157.7 ~~~~~ kcal./mole) of AlF(g), one computes DOZ9,(CaF)= the heat is AHOzvs= 120.7 f 0.8 kcal./mole. The 125.0 kcal./mole (5.44 e.v.). By combining this result data and results are presented in Table IV. with the heat of sublimation of calcium25 and the dissociation energy of the heat of forniation of TABLEIV CaF(g) from the elements in their reference states at EQUILIBRIUV CONSTAST A N D HEATOF THE REACTIW 298’K. was computed to be AHOf = -63.9 kcal./mole. Ca(g) CaFz(s) = 2CaF(g) An independent value of the dissociation energy of -A CaF(g) was obtained by using the same ion intensity T. AHOBos OK. Kz’ log Kz’ -log Rz data to determine a third law heat for the heterogeneous reaction Ca(g) -t. CaF2(s) = 2CaF(g). The equilib51.47 120.1 1335 2.089 X l o 3 3.320 8.411 51.52 120.2 1323 1.356 X lo3 3.132 8.599 rium constant, K’1, for this reaction was obtained from 51.67 121.5 1292 2.928 X l o 2 2.467 9.264 the ion current constant, K2’ = (CaF+)2T/(Ca+),by 51.74 121.2 1276 1.937 X l o 2 2.287 9.444 using the instrument sensitivity constant obtained in 51.40 120.9 1349 2.368 X l o 3 3.374 8.357 the sublimation studies and correcting for cross sections 51.72 121.6 1281 1.911 X lo2 2.281 9.450 and gains as before. The calculation yielded Kz = 1334 2.709 X l o 3 3.433 8.298 51.47 119.3 1.86 X The values of log I
