August, 1962
1563
NOTES
(CF3-CCI3) = 46.8 ked. Combining this latter quantity with values of AH8 and A116O, we obtain for the reaction i-
~
Fp(g) -5 graphite -+ 2F(graphite) AHy0 = - 100.2 kcal. hiole FZ
(7)
The uncertainty in AH,O is estimated to be aboul =t6 kcal., and results largely from the use of empirical bond energies in the computation. In view of this, heat capacity corrections to AHI0and AH2O were considered insignificant. The availability of experimental heats of formation of CF3--CC13and CF2=CC12 should diminish the uncertainty. In the reaction of CF2C1-CFC12 with graphite, the only observable product in addition to CF2= CFCl was HCl(g). This is attributed to a reaction with hydrogen in graphite, which can diffuse into the reaction zone. The pressure of HF(g) in the reaction of CF3-CC13with graphite, however, was small compared to that of HCl(g) (as noted by the Fig. 1.--Pressure dependence dztta for the reaction of CF'?-CC18(g)with graphite. Electron energy = 50 v., re- HF+/€ICl* ratio). Thus it appears that most of the reacted fluorine remains in the graphite. It :wtion temperature 91 5°K. should be noted that the total amount of CFJCCl,(g) that has reacted during the course of the experiment was less than 10-3 mole, although a t least a mole of solid carbon was available for reaction. Since so little fluorine is finally present in the graphite, we cannot demonstrate whether a new phase such as C4F has been formed. For our purposes, it is perhaps only meaningful to consider that the reaction involves the bonding of fluorine atoms to active sites in the graphite, or to active -0.2 surface sites. Comparison of these data with 0.61-0.4 calorimetric heats may provide information that will allow us to distinguish between a surface re-0.6 N+ action and one involving internal sites. --OB It should be noted that reaction 1 must have a positive entropy change of the order of 20-25 e.u. -4.0 -g 0.0a t 1000°K. This probably is due in part to -0.2break-up of the graphite structure on the bonding of fluorine and chlorine atoms. The ease of removal -0.4of an F atom in reaction 1 must reflect the ease of -0.6removal of chlorine on the adjacent carbon atom. If we assume that the rate determining step is the -0.8transfer of a chlorine atom to graphite, an upper * , -''oO.B . L limit of 30 kcal. for the activation energy is ob1.0 1.2 1.4 1.6 1.8 tained by combining AH6O with a dissociation energy fO000). of C1, of 58 kcal. and a bond strength of 78 kcal. in Fig. 2.-Temperature dependence data for the reactions chloroethanes. of Cli'a-CCl,(g) and CF2Cl-Cl?C12(g) on graphite. Ionizing Acknowledgrnent.-'We wish to thank Professor electron energies were 50 and 75 v., respectively. A small correction has been applied to the intensities of C2F2C12+ W. T. Miller for samples of CF3-CClB and CF2C1and C*F&l+ due to ion fragmentation of react,ant gam. CFClz used in this work. Squares and! circles represent independent sets of data.
o.8t
I
I
+
2Cl:graphite) -+Clz(g) graphite ( 5 ) A H 8 = $38.3 kcal./moIe Combination of data for reactions 1 and 4 gives for the reaction
+
+
CFd!CIJ(g) graphite Cl,(g) + CFZCl-CC13 (F C1)graphite AHeO = -22.4 kcal./mole
+ +
-
(5) J. R. Lacher, J. J. McKinley, C. Talden, K. Lea, and J. D Park, J. Am. Chem. SOC., 71, 1337 (1949). (6) C. R. Patrick, "Sdvances in Fluoline Chemistry," Vol. 2, Butterworthi, 1961.
THE VAPOR PRESSURE OF GERMS1\JIITAI TELLURIDE' BY
(6)
CHIKARA HIRAYAM-4
Westznghouse Research Laboratorzes, Pzttsburgh 86, Pennsglaanla Received March 28, 18622
Heats of formation computed from bond additivity The germanium-tellurium system and the therCQnsid@ratjOnB6 givg. AHfo (CFa-CICC&) A l l t o moel~~trio prapertisa ~f germanium telluride have
1564
NOTES
been reviewed recently by Miller.2 Only a single compound, GeTe, with a melting point of 725’, has been known to be present in this system.s However, a recent detailed study4 of this system has shown the compound to be congruent melting and of composition GeTel.oea. Germanium telluride undergoes a phase transformation6 at about 440’ from the low-temperature rhombohedral form to the cubic. The thermodynamic properties of GeTe(g) have been computed by Kelley16based on the spectroscopic data’ for this compound. The thermodynamic properties of GeTe(s) are still lacking. I n view of the interesting thermoelectric properties of the ‘(pure” and bismuth-doped GeTe2 a t temperatures above 400°, w c have undertaken to measure the vapor pressure of germanium telluride. The heat of sublimation thus enables one to calculate the thermodynamic properties of GeTe(s). This information adds further to the high temperature thermodynamic properties and volatility characteristics of intermetallic compounds of tellurium. Experimental The apparatus consisted of a 30-in. long vertical tube. This tube consisted of a 22 in. long, flanged Pyrex brand pipe of 1.5 in. i.d. (Corning Glass Works), to which was sealed a flat-bottomed 8 in. long mullite tube of 1J/8 in. i.d. A stainless steel flange, through which passed the iron-constantan thermocouple and a hook for suspending the sample, was scaled to the flanged top of the Pyrex tube. A gasket of Viton-A (E. I. duPont) was used to facilitate a vacuum seal. The tube was connected to a vacuum system just below the flanged top. Allowance was made also for introduction of argon to the system to break the vacuum. The mullite section was heated by raising a tube furnace over it. The temperature of the furnace was controlled to f2’, or better, and the bottom 4-in. section of the tube was at the same temperature. The Knudsen effusion cells were machined from graphite rods supplied by the National Carbon Company. The dimensions were l-’/.8 in. high, 8/4 in. diameter with wall thjckness of approximately ”16 in. The cap, on which was drilled the orifice, was approximately 1/4 in. thick. Three different orifice diameters were used, namely, 0.250, 0.125, and 0.0625 in. The Clausing correction* was used to calculate the effective orifice area. The cells were heated above 800” in vacuo of approximately 1 X 10+ mm. for several hours until constant weight was obtain d . To obtain the effusion rate, the Knudsen ce1.I was contained in a silica cup which was placed on a, platinum pan. The pan then was lowered into the mullite tube. The thermocouple was placed adjacent to the cell. The s stem was flushed several times with oxygen-free argon, and inally filled with the latter. The sample then was heated by raising the furnace over the tube and allowed to come to constant temperature. The time required was 20 to 30 min. The system subsequently was evacuated to about 3 X lo-‘ (1) This work was supported in part under a contract with the Bureau of Ships. (2) R. C. Miller, in “Thermoelectricity: Science and Engineering,” Ed. R. R. Heikes and R. W. Ure. Jr.. Interscience Publishers, New York, N. Y., 1981, pp. 434-439. (3) M. Hansen and K. Anderko, “Constitution of Binary Alloys,” 2nd Ed., MoGraw-Hill Book Co., New York, N. Y., 1958. (4) J. P. McHugh and W. A. Tiller, Trans. Met. SOC.A.Z.M.E.. 218, 187 (1980). (5) K. Schubert and H. Frioke, 2. Metal., 44, 459 (1953). ( 8 ) K. K. Kelley, “Contributions to the data on theoretical metallurgy,” U.S. Bur. Mines, Bull. No. 584, 1960. (7) G. Herzberg, “Molecular Spectra and Moleoular Structure. I. Spectra of Diatomic Moleoules,” 2nd Ed., D. Van Nostrand Co.. New York, N. Y., 1950. (8) S. Dushman, “Scientific Foundations of Vacuum Technique,” John Wiley and Sons , h a . , New York, N. Y., 1949, p. 99.
Vvl. 66
mm. in about 8 min. The pressure during any run was maintained between 5 X lo-eand 3 x 10-5 mm. Check runs indicated no detectable loss of material during the heat-up time in the argon atmosphere. Each run was terminated by breaking thevacuum with argon, and lowering the furnace to cool the tube with a forced draft. The effusion rate was determined from the weight loss over the heating period under vacuum. The germanium telluride was prepared* similarly to $hat described by Johnston and Sestrichlo from stoichiometric amounts of germanium and tellurium, and the material was crushed and ground to a powder. A wet analysis indicated 36.2% Ge and 63.75% Te (theoretical: Ge = 36.21%, Te = 63.79%). An X-ray powder diffraction pattern indicated no unreacted metals present, and the lattice parameter agreed with that reported”J; L e . , 4 = 5.98 A. Separate saFples of germanium telluride were volatilized a t 525 to 650 . The X-ray diffraction patterns of the condensate indicated the presence of GeTe only. Because we do not have mass Hpectroscopic data, the volatile species is assumed to be the monomer GeTe.
Results The results of the determinations are summarized in Table I, with the data recorded in the order of runs. The vapor pressure was calculated with the aid of the equation p = 0.0225~(m/At)(T/M)”~ (1) where p is in atmospheres, m the weight loss in grams. t the effusiontime in seconds, A the effective orifice area, T the absolute temperature, and M the molecular weight of GeTe. The pressure in mm. also is recorded. A least squares treatment of the data in Table I gave the equation -(10,255 f 451)1/T 8.255 i 0.598 log pat,. (2) with a linear regression coefficient of 0.992. The good linear relationship of the data obtained with cells of three different effective orifice areas shows that the accommodation coefficient is very close to unity. I n view of the small ratio of orifice area to surface area of the vaporizing solid, it can be concluded that the equilibrium vapor pressure of germanium telluride is identical to the measured dynamic pressure, p .
+
TABLE I VAPORPRESSURE OF GeTe Eff. orifice area,
T, OK.
Time, sa.
Wt. loss, mg.
om.’ X
725 702 751 767 787 680 816 796 837 823 683
10300 13520 8360 5150 3480 18900 2100 6060 3310 3340 10620
10.2
2.784 2.784 2.784 2.784 2.784 2.784 2.784 0 444 0,444 0.444 14.11
5.4
22.2 18.2 23.8 2.0 38.4 16.4 44.6 28.5 4.7
1Ot
P, mm. 1.16 X 4.60 X 3.17 X 4.26 X 8.34 X 1.19 X 2.27 X 2.08 X 1.08 X 6.69 X 9.94 X
10-3 IO-‘
10-8 10-8 10-8 10-2 10-2 IO-’ 10-6
p ? atm.
1.52 X 6 05 X 4.17 X 5 61 X 1.10 X 1.57 X 2.99 X 2.74 X 1.39 X 8.80 X 1.31 X
lo-’ 10-7 10” 10-6 IO-’
lo-‘ IO-‘ 10”
lo-’
The heat of sublimation over the temperature range studied has a constant value, AHs&. = 46.86 (9) The GeTe was kindly supplied by Mr. Y. Ichikawa, Westinghouse Electric Corporation, Youngwood, Pennsylvania. (10) W. D. Johnston and D. E, Sestrich, J . Znorg. N w l . Cham., I?, 229 (1981).
Augutit, 1962
NOTEB
f 2.06 kcal./mole. From the heat capacity of GeTe(g) given by Kelley,B and a reasonable estimate of 13.5 cnl./deg. mole for the average heat capacity of GeTe(s) between 298.16O and the median experimental temperature of 768OK., the standard heat of sublimation is 49.0 2 kcal./ mole. From the hea,ts of dissociation" for GeTe(g) and Te2(g), their spectroscopic datal7 and the heats of sublimation for germanium12 and tellurium,13 the standard heat of formation for GeTe(s) is calculated as
(1)
may be evaluated by measuring the increase in aolubility of the metal chromate a6 a function of added halide. Duke determined the solubility of the metal chromate by sampling and subsequent analysis. The development of a high temperature spectrophotometric technique suggested that an in situ analysis could be performed using the absorption peak of the chromate ion in the fused Nan'OaKN03 mixture. Investigation showed that chromate ion could be accurately determined spectrophotometrically in the presence and absence of halide ion and in the presence of excess lead nitrate. In the presence of excess lead ion the chromate ion concentration will be defined by the solubility product
(2)
Ksp = [Pb+2][Cr04-2] (1) If now halide ion is added, some of the lead chro-
*
GeTe(g) -+ GeTe(s) AH = -49.0 Ge (g)
+ Te (9)
-+
2 kcal./molc
GeTe (g)
AH = -94.6
-
f
=k
91 kcal./mole
1/2 ?'el(g> -+ Te(g) AH = 26.2 rt 2 kcal./mole Ge(s) Te(s) Ge(s)
(3)
Gek)
-+1/2
+ Te(s)
=
91.5
f
3 kcal./niole
Tedg) AH = 34.8 kcal./mole -+
mate solid will dissolve due to formation of PbXn2-% species. The following expression holds if only species of the form PbX,2-n exist [Pbltotal
Aky
1565
=
[Pb+']
+ [PbXf] + [PbXz] + . . [PbX,2-n]
(4) (3)
The successive formation constants for lead halide complexes, KI, 1'2, K3,.. . l I L are expressed as
GeTe(s) ANo = 8.9 i 9 kcal./mole
The entropy of sublimation of GeTe is 42.1 e.u., and using t,he data of Stull and Sinke14the entropy change for reactions 4 and 5 are 32.7 and 20.2 e.u., respective1;y. The standard ent:ropy of formation is therefore ASo = 0.6e.u. The standard free energy of formation is AFo = 8.7 ct 9 kcal./mole. There is a large uncertainty in AFO due to uncertainties in the dissociation energies, which probably results in the positjive free energy. ( 1 1 ) A. G. IGaydon, "Dissociation Energies and Spectra of Diatomic Molecules," Chapman and Hall, Ltd., London, 1953. (12) A. W. 13earcy and R. D. Freeman, J . Chsm. Phys., 23, 88 (1955). (13) I. V. Rornecva, A. 5. Pashinkin, A. V. Novoselova. and Yu. A. Prisdkov, Zh. Arrorg. Khim., 2 , 1720 (1957). (14) D. R . S.tull a n d G . C. Sinke. "Thermodynamic Properties of the Elements," Advances i n Chemistry Series, 1959.
(2)
K1 =
IPbX+] [Pb+2][X-]
(4)
(5) Substitiition of cq. 3-5 into eq. 2 results in the equation
T W ~ K ~ [ X T. .-.] ~( 6 ) A function K e p tnow is defined as Ksp'
=-L
(7)
[Pbltota~[Cr04-2]
Ry multiplying the numerator and denominator of equation 6 by [Cr04-2]and introducing a function FOR DETERbl'INATION OF FORb'CATION Po the following can be written. CONSTANTS OF LEAD HALIDE K ~ K P [ X -4 ]~ Fo K B D t / K s p = 1 K1rX-I COMPLEXES IN F I S E D TC,TC,Ii,3[X-]~ . . (8) SODIUM KITRATE-POTASSIUM NITRATE If a fuiiction PI is defined by BY J. D. VAS NOR VAN^ AND R. -4.OSTERYOUNG Department of Chemzstry, Rensselaar PolZrtschnic Instztute, Troy, NEW Fo - 1 F York I [X-] Recezved March 10,1968 Theoretical and F , is plotted us. [X-1 the intercept of the plot Duke and Iverson2 have shown that the succes- as [X-] goes to zero will be K1. If F2is now defined sive formation constants for chloro and bromo com- as plexes of lead and cadmium in molten nitrate media
h SPECTROPHQTOMETRI C I\lETIXOI)
+
(1) Ilrookhavm National Laboratory, Upton, Xew York. (2) F. R. Duke and M. L. Iverson, J . Phys. Chem., 6 2 , 417 (1958).
+
.
