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JAMES L. BILLSAND F. ALBERTCOTTON
of Ge in 20% HF to give a solution of Ge in 10% HF, but the relevant data for a check are apparently unavailable. Our final result, -132.2 f. 1.2 kcal./mole, for the AHrO of GeOz(c, I) is in excellent agreement with that
of Yokokawa, et (-132.3 f 1.0) and with the value recalculated from the data of Jolly and Latimerg (- 132.6 f 2.0). It therefore appears that the value in the N.B.S. tables6 based on combustion measurements (- 128.3) is in error by about 4.0 kcal./mole.
The Enthalpy of Formation of Tetraethylgermane and the Germanium-Carbon Bond Energy'
by James L. Bills2 and F. Albert Cotton3 Department of Chemistry, Massachusetts Institute of Technology. Cambridge 39?Massachusetts (Received October 10,1963)
The heat of formation of tetraethylgermane has been determined by combustion calorimetry. A rotating-bomb calorimeter of the type devised by Hubbard, Katz, and Waddington was used with 10% aqueous HF as the bomb liquid. The method of comparative measurementi was employed to minimize errors. The chief results are: AHf'2~~3.16= -50.0 f 1.9 kcal./mole for Ge(C2H6)4(1); mean thermochemical bond energy of Ge-C bond = 58 f 2 kcal./mole.
Introduction
-1515.6 f 1.5 kcal./mole. A comparison of this result with ours will be given in the Discussion section.
At the time this work was begun there was no reliable value for the germanium-carbon bond energy. I n the (1) Acknowledgment is made t o the donors of the Petroleum Reinterim, two different values have been reported. search Fund, administered by the American Chemical Society, and The entire literature on the thermochemistry of organoto the National Science Foundation for support of this work. germanium compounds is very small and will be briefly (2) National Science ,Foundation Cooperative Predoctoral Fellow. reviewed here. (3) Alfred P. Sloan Foundation Fellow. The first compound studied was tetra(@-styry1)- (4) K. H. Birr and D. Kraeft, 2 . anorg. allgem. Chem., 311, 235 (1961). gerniane14for which the standard energy of combustion, (5) K . H. Birr, ibid., 315, 175 (1962). AE,", was reported to be -4293.1 f 2.9 kcal./mole. (6) In evaluating AHrO from AE,', auxiliary AHfo values have been No effort was made to derive a Ge-C bond energy. added directly to AEco. Using A ( P V ) = A n R T , should be -3281.0 kcal./mole and this, together with the correct AHf' for Later, one of these authors burned tetraphenylgermane,6 GeOn(c, I)Y gives AHr' for Ge(CeHs)r(c) as 208.3 kcal./mole. Taking obtaining AE," = -3277.4 kcal./mole, from which a the heat of sublimation5 of Ge(CsHs).r as 20.9 kcal./mole,' A H f o of Ge(g) as 89 kcal./mole, and the heat of formation of the phenyl Ge-C bond energy of 32.2 kcal./mole was derived. radical as 71 kcal./mole, the Ge-C bond energy comes out as 36 However, because of various errors in the c a l ~ u l a t i o n s , ~ ~kcal./mole. ~ I t may also be noted that Birr's own data give 66.5 kcal./mole (not 51.3) for the Si-C bond energy, and that the numbers this result should be 36 kca1,jmole. in column 4 of Table I V are actually - AHr". Very recently, some Russian workers* have measured (7) J. L. Bills and F. A. Cotton, J . Phys. Chem., 68, 802 (1964). the heats of combustion of tetraethylgermane (TEG) (8) I. B. Rabinovich, V. I. Tel'noi, N. V. Kariakin, and G. A. Razuvaev, Dokl. A k a d . h'auk SSSR, 149, 324 (1963). and hexaethyldigermane, obtaining for the former AH,O
The Jozrrnal of Physical Chemistry
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Experimental3 Apparaltus. Except for a few small changes, the calorimeter and platinum-lined bomb are identical with those of the U. 6 . Bureau of Mines, Bartlesville, Oklahoma, and designated by them as BMR2 and Pt-5.'0 Jacket temperature was regulated by a Model S Sargent Thermonitor, powered by a Sola constant voltage transformer, which provided regulation to within about 0.001'. An Automatic Timing and Controls electronic dial timer was used to limit the ignition current, drawn from a, 6.3-v. transformer, to a fixed period of time Quartz crucibles were used, rather than platinum ones, which were found to alloy with the germanium. Crucibles of 7-ml. capacity (Thermal American Fused Quartz Co.) were treated with HF to reduce their mass to 3-4 g. Recording Thermometry The practices of the Bureau of Mines workers were modified t o provide thermistor temperature sensing and millivoltmeter recording. A 2000-ohm disk thermistor (Veco 32 D 12) TVSZS preaged a t 110" for a few weeks to improve its stability and then coated with epoxy resin t o maintain it in a constant immediate environrnent.'l The use of this large thermistor required some modification of the lead arrangements, since it is too large to be inserted through the thermometer hole of the calorimeter can. The use of smaller thermistors was undesirable because they respond too quickly t o a small volume of the water, thus giving a wavy recording trace. The thermistor formed one arm of a Wheatstone bridge. The adjacent arm of the other branch was a 1-10,000-ohm decade box of 0.05% accuracy. The other two arms were 2000-ohm wound Manganin wire resistors. The bridge was powered by a Sorensen Model &:If 3.0-0.66 transistorized d c . power supply of nominal voltage 3.0 with 0.05% regulation, line and load. The bridge output was fed to a Leeds and Northrup Speedomax G millivolt recorder equipped with a special range switch providing for ranges of 0 t o 1, 2, 5 , 10, 20, 50, and 100 mv. The fore- and after-periods of each run were recorded on the 0-1mv. range but the main reaction periods covered several spans of the 0-10-mv. range. The algebraic details of converting the mv. us time traces to temperature rises will not be given here.9 Calibration and Test of Calorimeter. The energy equivalent of the bomb and calorimeter was determined using benzoic acid supplied by the National Bureau of Standards (sample 39h). Measurements were made over temperature intervals corresponding to those in the cornbustion and comparison experirnents. The average values in cal./deg. and their standard deviations were 3519.2 f 0.9 for At = 1.3" and 3518.3
*
1 8 for At = 2.2" (four measurements each). The heat of combustion of succinic acid was measured (four runs) against that of benzoic acid in a separate set of experiments, giving a value (with standard deviation) of 3020.1 f 0.5 cal./g. which may be compared with the value of 3019.8 i. 0.4 cal./g. from the Bureau of Mines.12 Sample Enclosure. Several attempts t,o burn TEGr sealed in thin-walled Pyrex bulbs were unsuccessful due to incomplete combustion and the impossibility of separating the residues from fused glass. We therefore had recourse to polyester film, which burns completely and thus also serves to promote combustion, Mylar Type A, 100 gage, was used and the procedureH for making bags, filling and sealing them, and correcting for the heat of combustion of the film was similar to that recommended in the literature,l 3 Unmercerizecl cotton thread (heat of combustion taken12 as 4050 cal./g.) was used as fuse material, Materials. T'etraethylgermane was purchased from Peninsular ChemResearch Inc. It was redistilled under a reduced pressure of nitrogen through a 4-ft, Todd fractionating column a t a high reflux ratio. A, middle fraction was used for the combustions, The germanium(1V) oxide was from the same batch as the completely soluble hexagonal material used earlier. Oxygen gas was purified by passage over hot copper oxide and then through Ascarite and Drierite. The 10% hydrofluoric acid solution used in the bomb was prepared by dilution of Baker Analyzed reagent HF. A 25-ml. volume was weighed in a stoppered polypropylene vessel. Procedure. The method of comparative measure-. ments14 was used. The comparison experiments were designed to produce the same amount of ca,rbon. dioxide as the corresponding combustion experiments. Thus corrections for dissolved carbon dioxide are nearly identical and residual errors about cancel. Equivalent amounts of benzoic acid (N.B.S. sample 39h) and GeOz(c, I) were substituted for tetraethyl-. germane. The temperature rises were about 1.3" in the comparison experiments as compared to 2.2" in (9) A much more detailed account will be found in the Ph.D. thesiEi of J. L. Bills, Massachusetts Institute of Technology, 1963. (10) W.D. Good, D. W. Scott, and G . Waddington, J . Phgs. Chem., 6 0 , 1080 (1956). (11) S. A. Friedberg in "Temperature, Its Measurement and Control in Science and Industry," Vol. 11, H. C. Wolfe, Ed., Reinhold Publishing Corp., New York, N. Y., 1955, pp. 364, 365. (12) W. D. Good, e l ul., J . Phys. Chem., 6 3 , 1133 (1959). (13) H. A. Skinner, "Experimental Thermochemistry," Vol. 11, Interscience Publishers, Inc., New York, N. Y., 1956, p. 19. (14) See ref. 13, Chapter 4.
Volume 68, Number .G April, IO64
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the combustion experiments ; as noted earlier, calibyations were made for each of these approximate temperature rises, In the comparison experiments, the GeO, wax weighed into a quartz crucible. A platinum insert-type crucible c o ~ e rserved t o cover the GeOz and to hold the polyester film and benzoic acid, With the appropriate materials in place, the bomb was flushed with about 3 1. sf purified oxygen and filled to a pressure of 30 atm. Rotation of the bomb was begun when the temperature had risen about 60% of the expected amount and continued to the end of the experiment, (Subsequent calculations revealed the mid-time to lie closer to 58% far these experirneiits,) Complete combustion of tetraethylgermane was not achieved; the residues of o ~ b o n ranging. , froin 0.2 t o 0.5 mg. were assumed to have the game heat of combustion, -7.8 caL/mg., as pure carbon, and corrections made were taken t o have an uncert'ainty of &0.10 mg. of carbon. A corresponding contribution was included in the standard deviation assigned t o the combustion reaction. Entirely negligible amounts (
