Nov., 1959
THERMODYNAMIC PROPERTIES OF SOLID BIBMUTH CHLORIDES
ficient of viscosity and of the dielectric constant in the electrical double layer are not greatly different from what they are in bulk solution.
1813
Acknowledgment.-Financial support for this research was provided by the National Science Foundation for which we wish t o express our thanks.
SOME THERMODYNAMIC PROPERTIES OF SOLID BISMUTH CHLORIDES’ BY A. J. DARNELL AND S. J . YOSIM Atomics International, A Division of North American Aviation, Inc., Canoga Park, California Received February 10, 1969
The thermodynamic stability of solid BiCl with respect to its disproportionation products, Bi( s) and BiC13(g),has been determined from 127 to 242‘. This was accomplished by measuring the pressure of BiCla gas over the solid subhalide and over pure BiC13 by the Knudsen technique. The pressures of BiCla from the sublimation and disproportionation reactions are, respectively, log P B ~ =c (-6200 ~ ~ f 30)/T 9.95 f 0.07 and log PBicla = (-6360 f 60)/T 9.29 f 0.14. These results show that the subchloride is barely stable with respect to its solid disproportionation products. At 298’K. the ma, AFO and AS0 of formation of BiCl(s) are, respectively, -30.4 kcal./mole, -24.0 kcal./mole and -18.0 e.u. New values for AFOtam, ASoformand So of BiCL(s) were calculated and are, respectively, -73.6 kcal./mole, -57.1 and 36.4 e.u.
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Introduction The bismuth chlorides consist of bismuth trichloride, BiC13, and the subhalide, BiC1.2 The dichloride, BiCl2,*appears to be unstable by disproportionation4 while the existence of the tetrachlorides has not been confirmed. Bismuth trichloride is a colorless salt, melting a t 232’ and boiling at 447”. Bismuth subchloride is not as well known. It was first isolated by Eggink6 who determined the compound to have a chlorine to bismuth ratio of unity. At 320°, the solid subchloride disproportionates to farm two immiscible solutions, a black salt-rich phase with an ’ bismuth and 53 over-all composition of 47 mole % mole % BiC13,and a metal-rich phase consisting of 99 mole yobismuth and 1mole yoBiChS Some of the physical and chemical properties of BiCl have been described by Corbett.’ Recently there has been some question concerning the stability of.BiC1. Brewer4 suggested that if BiCl exists in the solid form, it is just barely stable toward disproportionation and, therefore, has about the same free energy of formation per equivalent as the trichloride. However, Sokolova,8who studied BiCl by X-ray techniques a t room temperature concluded that it was unstable. This was based on the fact that after the compound was formed, predominant lines, which were attributed to BiC1, became quite weak in 20 minutes. Corbett’ determined that BiCl was inert in dry air, and attributed Sokolova’s results t o excessive impurities. Therefore, it was of interest to determine the thermody(1) This paper was supported b y the Atomic Energy Commission. and has been presented in part before the Division of Physical Chemistry at the National Meeting of the ACS in April, 1958. (2) Solid bismuth subchloride is referred to, in this paper, a8 BiCI, although i t is possible that, like HgZC12, the cations of this subhalide are dimerized, or form higher polymers. (3) R. Schneider, Ann. Physik., 96, 130 (1855); R . Weber, ibid., 107, 596 (1859); P. Muir, J . Chem. SOC.,29, 144 (1876). (4) L. Brewer, “The Chemistry and Metallurgy of Miscellaneous Materiala-Thermodynamics,” L. L. Quill, Ed., N N E S IV-19B, McGraw-Hill Book Co., New York, N . Y., 1950. (5) B. G . Eggink, 2. p h y s i k . Chem., 64, 449 (1908). (6) 8. J. Yosim, A. J. Darnell, W. G. Gehman and 8 . W. Mayer, THIS JOURNAL, 63, 230 (1959). (7) J. D . Corbett, J . A m . Chem. SOC.,80, 4757 (1958). (8) M. A. Sokolovrs, G. G. Urazov and V . G . Kuznetsov. Akad. Nauk, X.S.S.R. Inst. Gen. Inorg. Chem., 1, 102 (1954).
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namic stability of BiCl with respect to its solid disproportionation products, Bi and BiCl3. An earlier experiment in this Laboratory showed that the gas phase over solid BiCl was essentially BiCla gas. Thus, by studying the sublimation pressure of BiC13 and the pressure of BiCla over solid BiC1, the stability of BiCl with respect to its solid disproportionation products could be determined. Therefore, a series of vaporization experiments was carried out to determine the thermodynamic stability of BiC1. I n addition to the stability of BiC1, the heat and entropy of sublimation of BiCla and the heat of fusion of BiC13 were obtained. The absolute entropy, the entropy and free energy of formation of solid BiC1S9 are based, in part, on the entropy of fusion of BiCla.lo Since, as will be shown later, this value may be in error, new values for So, AXoformand AF’form of BiCla were calculated from the sublimation data. Experimental Materials .-Reagent grade bismuth was melted under an inert atmosphere and filtered through Pyrex glass wool to remove bismuth oxide. Reagent grade bismuth trichloride was dried under a current of HC1 gas, distilled under HC1 and then under argon. The first and last eighths of the distillate were discarded. The salt had a melting.point of 232.2’. A bismuth and chlorine analysis of the salt showed a 66.2 f 0.1 weight yo bismuth as compared to 66.27% theoretical. Bismuth subchloride, free of BiCla, could not be synthesized simply by direct combination of BiC13with excess bismuth, as was also noted by Corbett.7 Therefore, the excess BiClt was removed by sublimation. The BiCl used in this investigation was prepared from a mixture originally consisting of 80 mole % bismuth and 20 mole % BiC4, heated in a seal:d Pyrex tube with continuous mixing for two days at 305 This process converted approximately 85y0 of the BiC4 into BiC1. Most of the unreacted bismuth “lumps” were mechanically removed. Since one of the products of the reaction studied was solid bismuth, it was not necessary to remove completely the unrescted bismuth. Pressure Measurements .-The vapor pressure of solid BiCIs at the temperatures of interest is in the range where the effusion technique is applicable. The experimental apparatus was similar to that used by Farber and Darnel1,ll in their
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(9) F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine and 1. Jaffe, “Selected Values of Chemical Thermodynamic Properties,” National Bureau of Standarb, Circular 500, 1952. (IO) K. K. Kelley, IS.8. Bureau of Mines, Bulletin 393, Washington, D. C . (1936). (11) M . Frtrber and A. J. Darnell, THIS JOURNAL, 69, 156 (1955).
A. J. DARNELL AND S. J. YOSIM
1814
study of the lower halides of titanium. In that and in this work, a small portable dry box was attached to the vacuum manifold. Thus, the material studied was never exposed to the atmosphere. The effusion cells in this study were made of Pyrex glass 1.5 om. in diameter by 4.5 cm. in length. The lids were also of Pyrex except in the case where a very small orifice was desired, in which case a platinum lid, sealed to the cell with AgCl, was used. Orifice areas were measured with a microscope and were also calibrated by vaporization of mercury. Temperatures were measured with a chromelalumel thermocouple inserted in a thermocouple well in the cell. The uantity of effused material was determined from the weightloss of the cell contents during each run. Pressures of BiCls were calculated by the Knudsen equationla which irtcluded a Clausingla factor to correct for the non-ideality of the orifice. Since BiC13 gas is known to be essentially m ~ n o m e r i c ,315 ~ ~ was used as the moleoular weight. Initially, due to the presence of excess BiCls, the vapor ressure of the mixture was equal to that obtained for pure bic13. When about 15% of the material in the cell had effused, the pressure became constant at a value approximately one tenth that of pure BiC13. X-Ray diffraction patterns of the material remaining in the Knudsen cell showed a set of lines which agreed with the pattern of BiC1 obtained by Corbett? but disagreed with the pattern reported by Sokolova,* et al. The assumption that BiC13 was the predominant gaseous species was verified by determining a CI/Bi ratio of the effusate and by comparing the weight of the effusate with the weight loss of the sample. The results indicated that the gas consisted of more than 99% BiC13. Within the experimental error, the pressures obtained for the disproportionation process were independent of orifice area indicating that equilibrium pressures were attained in the cell.
Results The results for the pressures of BiC4 for the sublimation and disproportionation processes are shown in Tables I and 11. TABLE I SUBLIMATION PRESSURE OF BiCl3 BiCls(s) -+ BiCls(g) Temp., K.
Wt. loss,
Time int., see.
g.
Orifice
Pressure, atm.
371 4 . 1 0 X lo6 0.0247 a 1.90 X 394 5.76 X lo4 .0280 1.58 X 395 1.56 X 105 .0776 a 1.62 X 415 7.25 X loa .2172 9.98 x .1548 a 1.09 X 416 4 . 7 2 X lo4 416 7.50 X lo4 .2333 a 1.04 X .0589 4.85 X 435 4 . 1 4 X lo8 .0572 a 5.00 X 435 3.90 X lo3 .3110 a 2.32 X 456 4 . 6 8 X loa 457 4 . 0 8 X lo3 .2717 2.33 X 408 4.38 X lo3 .6446 a 5.21 X a Orifice area, 0.0152 cma. Clausing fact80r0.51. (1
lo-’ 1010-8
10-8 10-6 10-6 10-6
lo-’ lo-‘
A least-squares analysis of the sublimation pressure (PoBiCla) data and of the disproportionation pressure ( P B ~ cdata ~ J yielded, respectively log P’BiCli = -620;*
30
-6360
60
+ 9.95 f 0.07 (371-468’K.)
log Psicis =
(1)
+ 9.29 =k 0.14 (400-515°K.)
(2)
The heat and entropy of sublimation for solid BiC13 was calculated from equation 1 to be 28.4 f 0.1 kcal./mole and 45.2 f 0.3 e.u., respectively. (12) M. Knudsen. Ann. Phyeik, 29, 179 (1909). (13) P . Clausing, ibid.. 12, 961 (1932). (14) D. Cubicciotti, F. Keneshea, Jr., and C. Kelley, THISJOURNAL, 62, 463 (1958).
Vol. 63
TABLE 11 PRESSURES OF BiCla FROM DISPROPORTIONATION OF BiCl(s) 3BiCl(s) -+ 2Bi(s) Temp., 0 K.
Wt. loss,
Time int., sec.
g.
+ BiCla(g) Orifice
Pressure, atm.
400 2.36 X 106 0.0178 0 2.47 X 412 4 . 5 4 X lo4 .0136 9.93 X ,0359 a 1.42 X 419 8.46 X lo4 419 8 . 0 1 X lo4 .0310 a 1.30 X .0775 2.16 X 427 5 . 9 0 X lo4 5 . 5 8 X lo4 ,0464 2.82 X 429 ,1299 * 2.89 X 431 7.45 X lo4 437 6 . 1 1 X lo4 .2117 5.78 X .0919 a 5.24 X 437 6.00 X lo4 .1543 a 1.11 X 446 4.82 X lo4 .0299 9.56 X 446 1.08 X lo4 449 5.40 X lo4 ,0700 1.76 X .3968 a 2.62 X 457 5 . 2 8 X 10’ .0358 a 2.75 X 458 4.56 x loa .0366 2.67 X 459 4 . 8 0 x 103 461 8.70 X lo3 ,0756 a 3.05 X 7.15 X 6.06 x lo3 .1281 473 .0453 5 6.72 X 474 2 . 4 0 X 108 .1933 2.59 X 493 1.06 x lo4 ,2347 9.64 X 515 3 . 5 4 X lo3 Orifice area = 0.0152 crn.l. Clausing factor Orifice area = 0.0227 cm.2. Clausing factor 0.700. fice area = 0.00208 cm.2. Clausing factor 0.952.
10-7 10-6 10-8
10-6 10-8 10-6 10-6 10-6 10-8 10-6 10-6 10-6 10-6 10-6 10-6 10-6 lo-‘ lo-‘ 0.513. Ori-
In order to obtain the heat of fusion of BiC13, the heats of sublimation and vaporization were calculated at the melting point. Applying a ACP estimate15of - 10.4 to the heat of sublimation obtained in this work (28.4 kcal. at the mid-temperature, 420°K.) yielded a value of 27.3 kcal. a t the melting point. Applying a ACp of -12.816 to the heat of vaporization (20.414kcal. at 533°K.) resulted in a value of 20.8 kcal. Thus, the resulting heat of fusion of BiCl3 is 6.5 kcal./mole. The absolute entropy of solid BiC13 at 298°K. was calculated from the absoluie entropy of the gas (So = 85.5 e.u.)ls and the entropy of sublimation of BiCla at 298°K. (AXsubl. = 49.1). The latter was obtained by applying a ACp correction of - 10.4 to the entropy obtained from equation 1. Combining the value of the absolute entropy of solid BiCl3 ( S o s i c 1 8 = 36.4 e.u.) with the absolute entropy of solid bismuth and chlorine gas” yields an entropy of formation of BiC13 of -57.1 e.u. The free energy of formation of solid BiCl3 was calculated from Thornsen’s’* value for the heat of formation of solid BiCl3 (AHOform = 90.6 kcal.) t o be -73.6 kcal./mole. Assuming no solid solution between Bi and BiC1, the stability of BiCl(s) with respect to Bi(s) and BiC13(s) can be determined from these considerations
1J
BiCla(s) BiC13(g) AFolubl. = -RT In PoBlC18 (3) 3BiCl(s) J_ 2Bi(s) BiC13(g) AFo,,,. = -RT In Psicia (4)
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(15) 0. Ihbaschewski and E. Evans, “Metallurgical Thermo chemistry,” John Wiley and Sons, Inc., New York, N . Y . , 1956. (16) K . K. Kelley, U. 8. Bureau of Mines, Bulletin No. 434, 1940. (17) D. Stull and G. Sinke, “Thermodynamic Properties of the Elements,” American Chemical Society, Washington, D. C., 1956. (18) J. Thomsen, “Thermochemische Untersuchungen,” Barth, Laipzig, 1882-1886.
I
L
THERMODYNAMICS PROPERTIES OF SOLID BISMUTHCHLORIDES
Nov., 1959 Subtracting (3) from (4) 3BiCl(s) -+ 2Bi(s)
+ BiC13(s)
Therefore, the standard free energy change per chlorine for (5), i.e., the standard free energy of disproportionation (AF'disprop) of BiCl(s) to 2/3Bi(s) and 1/3BiC13(s),is
Substituting (1) and (2) in (6) AFodi,prop= (0.245 f 0.102) f (0.00101 f 0.00024)T kcd./BiCl
(7)
The enthalpy of the reaction of 1 mole of solid BiCl to yield solid Bi and solid BiC13 is obtained by the usual dAF/T/b 1/T and is 0.245 f 0.102 kcal. while the entropy of the above reaction is obtained from the equation AS" =
- AFo =
-1.01 f 0.24 e.u.
(8)
The thermodynamic values for the disproportionation reaction a t the mid-temperature, -450"K., are A F ~ ~ ~= O 0.700 O K f 0.144 kcal./BiCl AHOnsoo~ = 0.245 f 0.102 kcal./BiCl A S " 4 b p ~= -1.01 =IC0.24 e.u./BiCl
(9) (10) (11)
-
If the small heat capacity corrections (ACp 0.1 cal./mole") are neglected, the values a t 298°K. obtained are AF%* = 0.546 f 0.125 kcalJBiC1
(12)
the other values being unchanged. Using the above data for the formation of BiC4, the formation of 1 mole BiCl from the elements a t 298"K., was calculated, and the results are compared with those of BiC13 in Table 111. TABLEI11 THEFORMATION OF SOLID BISMUTH HALIDES l/sBiCla BiCl
AF'torm, kcal.
A p f o r r n , kcal.
-24.5 -25.0
-30.2 -30.4
e.u.
-19.0 -18.0
Discussion of Results The pure BiC13 sublimation pressure reported in this work is considerably lower than that of Maier,'9
1815
who measured the pressure of both liquid and solid BiCls by a static method. However, his values for the vapor pressure of liquid BiC4 in the lower temperature region (below 340') are considerably higher than those of other worker^,'^.^^ and thus, it is reasonable to suspect that his sublimation pressures are too high. The vapor pressure obt.ained by extrapolation of equation 1 to the melting point atm. at 232") is in good agreement with (4.68 X the pressure obtained from the extrapolation of the data of Cubicciotti, et al., for the liquid (4.80 X 10-3 atm. a t 232"). The equation of Kelley21gives 3.93 X lodaatm. at this temperature. Since the heat of fusion of BiC13 was the difference of two large numbers, this value is admittedly inexact. Nevertheless, this number is considerably greater than the value of 2.6 kcal./mole accepted in the literat~re.~."However, the latter value was selected'O on the basis of phase diagrams of systems consisting of BiC13 and solutes, such as bismuth, ferric chloride and cuprous chloride. The heat of fusion of BiC13 was calculated with certain assumptions as to the identity of these species. Preliminary cryoscopic experiments with different solutes, carried out in this Laboratory, indicate that the heat of fusion of BiCla lies between these extreme values.22 It appears that BiCl, as Brewer4suggested, is just barely stable toward disproportionation and has indeed about the same free energy of formation per chlorine as solid BiC13. While the thermodynamic values for the disproportionation of BiCl have large uncertainties relative to the small values, these uncertainties are quite small relative to the values for the formation of the subhalide from the elements. Thus, the accuracy for the formation values for BiCl is limited to the accuracy of the formation values of BiCl3. A repeat of Thornsen's calorimetric determination or a determination of the AFoformof BiC13 by an independent method would be desirable. Acknowledgment.-The authors wish to acknowledge the assistance of Mr. J. A. Rubin in carrying out the experimental work. (19) C. G. Maier, U. 8. Bureau of Mines, Tech. Paper No. 360, 1925. (20) E. V. Evnevich and V. A. Sukhodskii, J . Rusa. Phus. Chem. Soc., 61, 1503 (1929). (21) K. K. Kelley, U. 8. Bureau of Mines Bulletin No. 383, 1935. (22) L. E. Topol and 9. W. Mayer, Abstracts of Papers, ACS Meeting, Chicago, Ill., Sept. 1958.
