791
THERMODYNAMIC AND PHYSICAL PROPERTIES OF BERYLLIUM COMPOUNDS
The Thermodynamic and Physical Properties of Beryllium Compounds. V. Heat of Formation and Entropy of Beryllium(1) Chloride@)
by Michael A. Greenbaum, M. Louis Arin, Madeline Wong, and Milton Farber Rocket Power, Inc., Research Laboratories, Pasadena, California
(Received October 8,1963)
+
The equilibrium BeClz(g) Be(1) = 2BeCl(g) was studied over the temperature range 1573-1723OK. by the molecular flow effusion technique in order to obtain the heat of form,stion and entropy for BeCl(g). Over this temperature range AH, was found to he 89.1 f 7.6 kcal./mole and US, was 39.4 f 4.6 cal./deg./mole. From recent experimental data and available thermal functions, the second law AHt,zes for BeCl(g) was +3.7 f 3.8 kcal./mole, which compared favorably with the corresponding third law for BeCl(g) was found value of +2.0 f 0.8 kcal./mole. The experimental value for Sozg8 to be 53.0 f 2.3 cal./deg./mole.
Introduction The heat of formation of BeCl(g) has not been determined experimentally. As was the case with BeF(g),2 estimates of the heat of formation have been based on the dissociation energy of the BeCl molecule, for which sevleral values have been reported. The first spectroscopically determined dissociation energy value (Doo) for the BeCl(g) species was reported by Fredrickson and Hogan3 in 1934. Ely employing new physical conststnts and conversion factors, Herzberg, in 1950, recalculated a new Doovalue from the original data of Fredrickson and Hogana and obtained a value of 4.2 e.v. Gaydon6 applied further corrections dependent upon the ionic character of BeCl and obtained a value of 3.0 e.v. for Do”. In 1960, Novikov and Tunitskii6 obtained a new set of spectral data for BeCl(g) which yielded a value of 5.9 f 0.5 e.v. for the dissociation energy of BeCl. This wide variation in the dissociation energy of BeCl results in a correspondingly large variation in the heat of formation of this molecule. Using the heat of sublimation of beryllium of 78 kcal./mole’ and the dissociation energy of chlorine of 57.8 kcal./mole,* the values obtained for the heat of formation of BeCl(g) are: +9.5 kcal./mole (Herzberg), 4-37.1 kcal./mole (Gaydon), and -29.7 kcal./mole (Novikov and Tunitskii) . Since all previously available values for the heat of
formation of I3eCl(g) were based on widely varying spectroscopic data, an experimental thermodynamic study of the reaction was carried out. This reaction BeClz(g>
+ Be@)-+-2BeCl(g)
(1)
was studied over the temperature range 1573-1723 O K . using the molecular flow effusion technique.
Experimental
Introduction. The apparatus and general experimental procedures for the determination of thermodynamic properties of BeF as reported previously2 have been employed in the present investigation. The (1) This research was supported by the Air Research and Development Command of the United States Air Force. (2) M. A. Greenbmm, R. E. Yates, M. L. Arin, M. Arshadi, J . Weiher, and M. Farber, J . Phys. Chem., 67, 703 (1963). (3) W. R. Fredrickson and M. E. Hogan, Phys. Rev.,46, 454 (1934). (4) G. Herzherg, “Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules,” 2nd Ed., D. Van Nostrand Co., New York, N. Y., 1950. ( 5 ) A. G. Gaydon, “Dissociation Energies and Spectra of Diatomic Molecules,” Chapman and Hall, Ltd., London, 1953. (6) M. M. Novikov and L. N. Tunitskii, Opt. i Spektroskopiya, 8 , 396 (1960). (7) G. T. Armstrong, H. W. Wooley, W. H. Evans, and L. A. Krieger, National Bureau of Standards (U. S.), Report No. 6928, U. S. Govt. Printing Office, Washington, D. C., July 1, 1960. (8) D. R. Stull and G. C. Sinke, “Thermodynamic Properties of the Elements,” American Chemical Society, Washington, D. C., 1956. (9) M. Farber, J . Chem. Phys., 3 6 , 1101 (1962).
Volume 68, Number 4
April, 1964
792
M. A.
following sections discuss in some detail the modifications in this apparatus and procedure necessary for the study of the reaction of BeC12and beryllium. Apparatus. The molecular flow effusion method developed by Farberg was employed to determine the requisite experimental data for the reaction of gaseous beryllium chloride with liquid beryllium a t temperatures between 1573 and 1723°K. This procedure consists of vaporizing solid BeC12 and allowing it to pass over the sample of Be which is heated to the desired temperature, and allowing the resulting vapor species to escape through an effusion orifice into a high vacuum. The container for the BeC12consisted of an aluminum oxide tube, 50 mm, in length and 20 mm. in diameter, closed a t one end with a high purity, high density graphite plug. The other end was fitted with a graphite adapter. The alumina tube was wrapped with heating wire contained in an asbestos matrix. The sample of BeC12 was contained in a nickel boat placed inside the alumina cell. The graphite adapter had an opening 6 mm. in diameter. Connecting the BeC12cell to the Be cell was a high purity Be0 tube 150 mm. long with an inside diameter of 6 mm. This Be0 tube was connected to the Be cell by another graphite adapter. The Be0 cell was made of two tight-fitting Be0 tubes. The back part had a 1-mm. orifice to allow entry of the gaseous BeCI2, while the front section had a 0.7-mm. effusion orifice. N o reaction was found to occur between the A1203and the BeC12. This was established by passing BeC12 vapor through an empty A1203 tube over the temperature range of interest and weighing the tube after this treatment. No change in weight of the hlz03tube was found to occur under these conditions. The remaining parts of the cell were constructed of BeO. To permit uniform distribution of heat over the Be0 cell, as well as to prevent contamination of materials on the outside of the cell (and thus result in an apparent weight gain), the Be0 cell was encased in a graphite shield. The opening in the shield above the cell orifice permitted unhindered effusion. Weighing of the Be0 cell before and after reaction was done with the graphite shield removed. Measurement of the Be temperature was accomplished by means of a Leeds and Northrup optical pyrometer which was calibrated before and after the experimental determinations. Correlation of the temperature inside the Be0 cell with that read by the pyrometer on the outside of the cell was accomplished by using a calibrated platinum-rhodium thermocouple placed inside of the cell and taking readings with thermocouple and pyrometer simultaneously. This was done in the empty cell before and after experimental The Journal of Physicd Chemistry
GREENBAUM,
M. L. ARIN, nf. WONG,
AND
M.FARBER
determinations were made as well as during some experimental runs. KOsignificant difference in temperatures was observed in any case. Experimental Procedure. The extreme hygroscopicity of BeC12(s)necessitated very special handling precautions. Even when these precautions were taken early samples of BeClz were found to be totally unsatisfactory. The first samples of BeCl? obtained were reported to be in excess of 99% purity. However, they were provided in the form of a very fine, fluffy material. After many unsuccessful attempts to obtain reproducible data with this material some of it was resublimed in vacuo. Under these conditions large amounts of material were left behind. In addition, the sublimed materials had only a slightly larger crystalline size and thus did not significantly reduce the surface area of the material. Suitable material of 99.5+% purity in the form of large crystals was finally obtained from Beryllium Corporation of America. This material yielded reasonably reproducible results when handled in a drybox or drybag under dry nitrogen. All of the BeCI2cells were filled under these conditions and on completion of a reaction the entire three-part cell assembly was immediately placed in a drybag under nitrogen. For weighing purposes, the BeC12 remaining in the cell was transferred to a weighing bottle (nickel boat and all) and then weighed on a semimicro balance. Initial weighings were made in the same manner. The bottles of BeC12 were kept under dry nitrogen in a drybox at all times. The beryllium employed has been described previously. The Be sample was heated to the desired temperature in the 1573-1723" K. range. After reaching temperatures in the Be cell the temperature of the BeCh cell was generally around 470-500°K. The temperature of the cell was then so raised that a reasonable pressure of BeClz could be obtained. Based on a recent vapor pressure study of BeC12(s)*0this was generally around 525-555°K. The length of the runs was between l and 3 hr. and corrections were made for the weight loss of BeC12 during the heat-up and cooldown cycle a t each temperature studied. Temperatures of both BeC12 and Be containers were constantly monitored and controlled, the former manually and the latter automatically. The variation in temperature was usually less than =k7"1
