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A. S. DWORKIN, H. R. BRONSTEIN, AND M. A. BREDIG
intercept the vertical coordinate are significant in that they represent the mole fraction of SsRwhich can convert to SaChupon molecular equilibration at given temperatures. As could be expected, these values increase with rising temperature. The mole fraction of SsCh which forms in sulfur upon equilibration at 130” is 0.038; at 140”,0.046; and at 150°, 0.057. From the slope of plots shown in Figure 1, the reaction rate constants were found to be 0.50 X 1.10 X and 3.34 X min-l at 130, 140, and 150”, respectively. The activation energy obtained from the slope of the straight line which provides the best fit for these three points is 32 kcal/mole. Since it is assumed that ring opening is the rate-determining step, this activation energy expresses the bond dissociation energy of octatomic sulfur rings. This value
is in good agreement with the enthalpy of opening an Ss ring, 32.8 kcal, as reported by Tobolsky and Eisenberg.6 Our value is also in reasonable agreement with previously reported data reviewed by Kende, et aL7 These data were obtained in kinetic studies of systems containing rubber and sulfur. Depending on experimental conditions, these data range from 30.5 to 37.8 kcal/mole, averaging about 35 kcal/mole. The presently reported value was obtained on the pure sulfur system and confirms previous estimates for the dissociation energy of a sulfur-sulfur bond in SsR.
Miscibility of Liquid Metals with Salts.
(6) A. V. Tobolsky and A. Eisenberg, J. Am. Chem. Soc., 81, 780 (1959). (7) I. Kende, T . L. Pickering, and A. V. Tobolsky, ibid., 87, 5582 (1965).
VIII.
Strontium-Strontium
Halide and Barium-Barium Halide Systems1
by A. S. Dworkin, H. R. Bronstein, and M. A. Bredig Chemistry Division, Oak Ridge National Laboratow, Oak Ridge, Tennessee
(Received December $1, 1967)
The phase diagrams of the alkaline earth metal-metal halide systems Sr-SrClp, Sr-SrBrz, Sr-SrL, Ba-BaBrz, and Ba-BaIz were determined. Salt-metal miscibility is much greater than in the calcium systems and increases with the size of either the metal or halide ion. The critical solution (consolute) temperatures for the five systems above are 1106, 1064, 1056, 960, 9 1 4 O , respectively, with the critical composition of approximately 52, 62, 66, 65, and 7 5 mole % metal, respectively. Most of the experimental results reported by Emons, et al., and their assumption of polymeric metal ions Mn2+(n > 2) are shown to be in error. Rather an equilibrium M 2 2 + e 2MZ+ 2e- is indicated as proposed earlier in connection with electrical conductance measurements.
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The following is part of an account of the extension of our general study on mixtures of the electropositive metals with their molten halides2to many of the alkaline earth metal-alkaline earth halide systems. Measurements of the electrical conductance carried out in parallel with the phase equilibrium studies have been discussed earlier.3 In the alkali metal-metal halide systems, miscibility of the metal with the salt had been found to increase with the atomic number, or the size, of either the metal or the halide ion, Le., apparently with the simultaneous decrease in the cohesive, or interionic, forces in both components.2 It was of interest to ascertain that this was indeed a general trend for this type of metal-salt miscibility by measurements on alkaline earth systems. The Journal of Phyaical Chemistry
Reported consolute temperatures, taken as a measure of miscibility, namely, l5 Melts prepared with small additions of BaO to the bromide or iodide showed, after solidification, many of the same lines in the X-ray patterns as the solidified salt phase from the BaXz-Ba experiments with undistilled Ba. The influence of the impurities upon the miscibility of metal and salt appeared to be surprisingly small, and the temperature of the halt reflecting the freezing of the metal was virtually unaffected. An example of this is shown in the Ba-BaClz diagram of Figure 1,where our measurements with undistilled Ba a t two concentrations are compared with Hinkebein's measurements.* SrBrz has a transition just below the melting point which involves even more heat than the melting process.Ib Since this transition also shows considerable supercooling it was impossible to distinguish the corresponding halt from the eutectic in the Sr-SrBrz system. For the lower concentrations (up to 20 mole % Sr), two large halts were observed a t virtually the same temperature, the second one exhibiting supercooling. No attempt was made, therefore, to indicate the transition line for the Sr-SrBrz system in Figure 1. 9
Compn, mole % salt
Consolute Compn, mole % Temp, "K metal
I n the Sr-SrIz system, the line due to the transition in Sr metal could not be distinguished from the eutectic in the scale of Figure 1. However, it is indicated in Table I and could easily be found in the actual measurement. The critical solution temperature (To)drops from 1018 to 960 to 914" for the chloride, bromide and iodide-barium systems, respectively. This means that with increasing size of the halide ion the miscibility of barium metal with its molten halides increases, similarly to the miscibility of the alkali metals with their halides, but the trend appears to be the opposite in the systems of Ca metal with its Salk6 In the Sr systems, Todrops from 1106 to 1064 to 1056" for the chloride, bromide, and iodide, respectively, that is, somewhat intermediate between the Ca and Ba systems. The so-called "iodide effect," namely the failure of T, to drop, ie., of the miscibility to increase as much in going from the bromide to the iodide systems as would be expected from the change in going from the chloride to the bromide systems, is observed, but is not as strong as in the alkali metal systems. The critical composition changes from about 55 to 65 to 75 mole % metal for the barium chloride, bromide, and iodide systems and from about 52 to 62 to 66 mole % metal for the strontium, chloride, bromide, and iodide systems, respectively. This relative increase in solubility of the metal in the molten salt with increasing anion size may be attributed to the increase in the volume ratio of the two components, a reasonably well understood relationship observed in other molten salt-metal systems and in binary systems in general. (13) B. Neumann, C. Kroger, and H. Jtittner, 2. Elektrochem., 41, 725 (1935). (14) P. Ehrlich, et al., 2. Anorg. AElgem. Chem., 2 8 3 , 58 (1956); 288, 148, 156 (1956). Portions of some of the phase diagrams proposed there are open to serious question. For example, some heats of fusion calculated from the reported freezing point depressions, dT,,,/dNhlHz, of alkaline earth halides are much lower than those measured calorimetrically (cf. ref 15). (15)A. S. Dworkin and M. A. Bredig, J.Phys. Chem., 67,697 (1963). Volums 78, Number 6 June 1968
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A. S. DWORKIN, H. R. BRONSTEIN, AND M. A. BREDIG
The barium systems in Figure 1 clearly show the kind of curvature of the liquidus curve for the crystallization of salt that is the result of increasing positive deviation from ideal solution behavior and is a rather general characteristic of systems with limited miscibility in the liquid state. I n this, the barium-salt solutions resemble the potassium-salt solutions as they do in other respects, e.g., electrical cond~ctivity.~In the strontium systems, the limited solubility of metal in salt does not allow the clear observation of curvature of the liquidus curve. The estimated solubility of Sr in solid SrClzis shown in Figure 1 based on cryoscopic consideration~~ and the similarity in crystal structure between SrClz and CaFz. Both have a fluorite type of structure, and solubility of the metal in the salt has been reported previou~ly.~ In both these cases the observed curvature of the salt liquidus may also, a t least partially, be connected with some residual tendency, much weaker than in the sodium and lithium and the strontium and calcium systems, of F-center-like electrons to associate. The corresponding concentration-dependent equilibrium 2e2Ba2+ e Baz2+(analogous to 2e2K+ e Kz) had first been suggested by the results of the electrical conductance measurements in the sodium systems.2f3 I n a series of publications, Emons, et uZ.,16 also have reported work on the equilibrium phase diagrams of the alkaline earth metals with their molten halides. With the exception of the Ba-BaC12 system known since 1952 from the work of Schafer and Niklas,’ their results disagree in most respects with those of recent investigators618v17 with which our work when it happens to overlap is in essential agreement. In a recent review, Richter and Emonsl8 have corrected some of the gross errors in the earlier values of Emons, et uZ., for eutectic compositions. Probably in cognition of the results of electrical conductance measurement~,~:~9 metal solubilities in the calcium systems have been drastically revised downward by factors 3 or 4,namely, from 8, 14, and 19 to 2.5, 3.0, and 4.5 mole % in the chloride, bromide, and iodide systems, respectively. KO mention is made of these large discrepancies with their prior work. Most surprisingly, however, the eutectic temperatures, 10 to 54” lower than ours, have remained the same as in the original, incorrect phase diagrams. For the SrSrBrz and Sr-SrIz systems, the values for the eutectic concentrations have been modified slightly, but are still
4 to 5 times greater than ours. No experimental points are given, so that it is difficult to evaluate the low-temperature portions of Emons’ phase diagrams, especially the liquid-solid equilibria, in the light of reproducibility attained. As far as the liquid-liquid phase equilibria are concerned, the quenching technique used, namely, the determination, a t room temperature, of the presumed equilibrium solubilities after more or less rapid cooling from the test temperature, has long been proved to be entirely unreliable.2 The interpretation of freezing-point lowering in terms of various polymeric metal ion species Mn2+ by Richter and Emons18 (Table 3 in ref. 18) is inherently in gross disagreement with their own (revised) experimental data (Table 1 in ref 18). In fact, for the comparison of actual heats of fusion with “theoretical” ones calculated from freezing-point lowering (“AH8 = (RToT/AT) In xsaltf’) , I * the original, incorrect phase diagramsle are still used (Table 3 in ref 18). The suggestion, based on this comparison, that various clusters of metal atoms solvated with one metal ion, such as Ca32+and Sr2+, occur is thus not only totally a t variance with our results but even with Emons’ own results as well. A valid calculation of the number of particles v formed on dissolution of metal in salt (from v = (T, - T e u t ) X AH,/Na&Tm2) is not possible from the revised data (Table 1 in ref 18) either, since incorrect eutectic temperatures are still listed. As one example, for calcium metal in calcium bromide, the revised data (Table 1 in ref IS), N M = 0.03, T , = (732 273)”K, Teut = (675 273)”K, and AH, = 7000 cal/mole of CaBrz, yield v = 6. A dissociation of calcium atoms into six new particles is obviously unreasonable. Our own data as discussed elsewhere3 give v = 1.5 which we attribute to 2e- following the an equilibrium CazZ+e 2Ca2+ dissolution of calcium atoms according to Ca Ca2+--t Ca22+, Similar equilibria appear to exist in the other systems, but quantitative evaluation is in general limited not only by the accuracy of the data but also by the possible occurrence of solid solutions.
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The Journal of Physical Chemistry
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(16) H. H. Emons, et al., 2. Anorg. AZZg. Chem., 323, 114 (1963); 2. Chem. 2 , 313, 377 (1962).
(17) J. A. Van Westenburg, Ph.D. Thesis, Iowa State U., 1964; University Microfilms, Ann Arbor, Mich. 64-9291. (18) D. Richter and H. H. Emons, Z . Chem., 6,407 (1966). (19) H. H. Emons and D. Richter, Z . Anorg. AZZa. Chem., 339, 91 (1965); 353, 148 (1967).
