127
Thermodynamic Properties of CuAIC14
Thermodynamic Properties of, the Molecular Complex CuAICI4 Wllilam C. Laughlin and N. W. Gregory' Department of Chemistry, University of Washington,Seattle, Washington9 8 195 (Received August 4, 1975) Publication costs assisted by the National Science Foundation
The vaporization of cuprous chloride is considerably enhanced in the temperature range 189-390' by the presence of aluminum chloride vapor. Evidence is presented to support the conclusion that the molecular species CuAlC14 is an important constituent of the vapor phase. Thermodynamic properties of this molecule have been derived from transpiration data. For the reaction CuCl(s) AlC13(g) CuAlCMg), AH'640K = -0.8 kcal mol and AS'640K = -2 cal mol-l deg-l. The values M ' 4 7 3 K = 34 kcal mol and A S ' 4 7 3 ~ = 48 cal mol-l deg-1 are derived for the process CuAlC14(s) = CuAlC14(g). The vapor phase complex does not show a strong absorption band in the wavelength range 200-600 nm, in contrast to Cu~C13.
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Introduction Relatively little is known about the cuprous chloridealuminum chloride system. A melting point study by Kendall, Crittenden, and Miller' indicates the existence of only one solid state intermediate compound, CuAlC14, with a congruent melting point of 235'. The catalytic activity of CuAlC14 in the preparation of olefines has been a matter of considerable interest and has generated a substantial patent literature.213We have been interested in the nature of molecules formed in the vapor phase of such Cuprous halide vapor molecules have a strong absorption band in the region 200-250 nm and can be detected spectrophotometrically a t relatively low concentration^.^ In the present work we have demonstrated by transpiration experiments that the presence of aluminum chloride produces a considerable increase in the amount of copper in the vapor phase in equilibrium with cuprous chloride in the temperature range 180-390'. This is attributed to formation of CuAlC14 molecules and thermodynamic constants for this mixed metal dimer species have been derived. The vapor phase shows no new absorption band in the range 200-600 nm or appreciable enhancement of the characteristic cuprous chloride absorption. It is suggested that the vapor complex is fundamentally an ion pair, much like NaAlC14, rather than a molecule in which the bonding between copper and chlorine is similar to that in Cu3C13, the principal species in the saturated vapor of pure cuprous chloride in this temperature range. Experimental Section The experimental method used for the transpiration studies was basically that described by Richards.8 Argon a t ca. 1000 Torr and at flow rates between 9 and 72 cm3 min-l served as a carrier gas. The argon was made to flow either directly over CuCl-AlC13 mixtures or first over a solid sample of AlC13, heated in a compartment adjacent to the main reactor furnace to a temperature appropriate to introduce the desired partial pressure of aluminum chloride, and then over a sample of pure CuCl or over condensed mixtures of CuCl and Partial pressures in the equilibrium vapor were deduced from the relative number of moles of Al, Cu, and Ar in the transported sample. Results showed no systematic dependence on flow rate in the indicated range. The amounts of aluminum and copper transported were determined by atomic absorption analysisg of material
-
which condensed outside the reaction zone. The pressure of argon was measured manometrically; the number of moles of argon flowing through the reactor was determined by evaporating the sample of solid argon, collected in a liquid nitrogen cooled trap, into a calibrated volume and measuring the pressure of the gas at room temperature. Ideal gas behavior was assumed. The CuCl sample was prepared by reaction of copper at 500' with chlorine; the latter was generated by thermal decomposition of CuC12, formed by vacuum dehydration of CuC12.2H20 (Baker Chemical, 99.1%). Samples of CuCl were subsequently sublimed under vacuum twice before being transferred into the transpiration reactor vessel. Aluminum trichloride was prepared by reaction of aluminum wire (Baker's Analyzed Reagent) and HC1 (generated from Mallinckrodt Analytical Reagent NaCl and Allied Chemical 95.5-96.5% HzSO4). The product initially formed was resublimed twice under vacuum, with a sample finally collected in a side-arm extension of the transpiration apparatus where it could be introduced into the system by fracturing a Pyrex breakseal.
Results and Discussion The transpiration data are summarized in Table I (see paragraph at end of text regarding supplementary material). In all experiments CuCl(s) was present as a separate solid phase. The contribution to the total number of moles of copper in the vapor phase expected from the vapor pressure of CuCl(s) was predicted from the data of Shelton.lo Relative to the total collected this amount was negligible for the majority of runs but constituted as much as 85%in experiments in the high temperature range and in which the partial pressure of AlC13 was very low. The additional copper in the vapor was attributed to the presence of the mixed metal dimer CuAlCld(g). The indicated number of moles of this species was subtracted from the total number of gram atoms of aluminum transported to give the number of gram atoms of aluminum in the form of the monomer and dimer of aluminum chloride. The relative amounts of AIC13(g) and &Cls(g) were calculated using data from the JANAF Tables.ll Partial pressures of the various components were calculated using Dalton's law, Pi = XiPt; Pt (the total pressure) and nt (the total number of moles of gas) were taken as the values measured for argon, since n~~
>> nCusCls + nCuAIC14 + nAlCla + nAl2Cls.
The Journal of Physical Chemlstry, Vol. 80, No. 2, 1976
120
William C. Laughlin and N. W. Gregory
Table I (supplementary material) includes several series of experiments in which CuCl(s) a t a given temperature (very nearly the same if not identical) was allowed to equilibrate with various partial pressures of AlC13. I t will be observed, for example, that around 640 K six experiments, in which the pressures differ by factors as large as 70, were conducted. The amount of copper transported as the complex showed only a first power dependence on the AlC13 pressure which indicates that complex molecules containing more than one aluminum atom were not present at significant concentrations. A similar test of the number of copper atoms in the complex molecule was not feasible; however, in five experiments deposits of CuC1, the complex, and AlCb appeared reasonably well separated and were analyzed separately. While the separation was not perfect the analysis generally confirmed the expected compositions. The material thought to be the complex gave Al/Cu ratios of 0.91, 0.97, 0.95, 0.94, and 0.98, respectively. The slight excess of copper over that expected for the formula CuAlC14 could be due to (a) the concomitant deposition of some CuCl with the complex, (b) a small amount of decomposition of CuAlC14 after condensation in the warm condensation region, losing AlC13, or (c) the deposition of small amounts of species such as CuZAlC15 or Cu3AlC16. The observed ratios are close to unity, however, and we have assumed that the deviation was caused by (a) or (b) and hence that the principal vaporization reaction is CuCl(s)
+ AlCl&)
= CuAlCld(g)
(1)
This conclusion also seems consistent with the reported composition of other similar complexes of aluminum chloride.'*J3 Alternatively, of course, (1) could be written with I/zAl&lG(g), the dominant molecular form of aluminum chloride vapor at moderate to high pressures, as the reactant. Equilibrium constants derived for (1)are shown in Table I (supplementary material) and are displayed graphically in Figure 1. At temperatures above 500 K, a liquid phase of unknown composition but presumed to be the complex was observed in the reactor along with CuCl(s) in several of the experiments. In the cases marked with an asterisk (Table I, supplementary material) the final equilibrium vapor mixture was generated by decomposition of this condensed complex, i.e. no aluminum chloride was introduced into the carrier gas before it entered the reaction zone. In the other experiments (in which a liquid phase may or may not have been present) equilibrium was approached by the reaction of an initial excess pressure of aluminum chloride with the copper chloride (i.e., (1)proceeding to the right). Values of K1 in these two cases show good consistency. A least-squares treatment based on an equation of the form log K1 = AT-' B gave the line shown in Figure 1 with constants corresponding to an enthalpy change of -0.78 f 0.06 kcal mol-' and an entropy change of -2.0 f 0.1 cal mol-] deg-1 for the mean temperature of 560 K. The indicated uncertainties represent the least-squares analysis of the spread of the experimental data. Using JANAF values for the standard entropies and enthalpies of formation of CuCl(s) and AlC13(g), our result leads to 113.7 cal mol-l deg-l and -172.8 kcal mol-l for the standard entropy and enthalpy of formation, respectively, of CuA1Cl4(g)a t 560 K. Information on the vibrational frequencies and the structure of the CuAlC14 molecule has not been found. We have used data for similar molecules to see if the experi-
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The Journal of Physical Chemistry, Vol. 80, No.2, 1976
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15
16
17
18
19
20
21
22
I
23
IOOO(T~K)-'
Figure 1. Temperature dependence of the equilibrium constant for ordinate scale on left),and of the vapor pressure of reaction 1 (0, CuAIC14 in equilibrium with its solid phase and CuCl(s) (e, ordinate scale on right). mentally derived value for the entropy is reasonable for such a molecule. The combined translational and rotational contribution is expected to be 77 cal molw1deg-l, including a rotational contribution of 31.8 based on an assumed structure consisting of a tetrahedral AlC14 group with the copper atom bridging through coordination with two chlorine atoms (forming a planar AlClzCu ring) with overall symmetry Cz0. The bond distances were assumed the same as the mean values reported in a crystal structure study of NaA1C4.14 From an analysis of the Raman spectrum of liquid NaAlCb, Balasubrahmanyam and Nanis15 have assigned frequencies for the tetrachloroaluminate ion of 145(2), 183(3), 349, and 580(3) cm-l. If these values are used for nine of the vibrational degrees of freedom for the CuAlC14 molecule, the remaining three degrees of freedom would have to contribute 13 cal mol-' deg-', the equivalent of three frequencies of the order of 123 cm-l, which appears generally reasonable.16 An estimated valuell of AC, for (1) of -2.0 cal mol-' deg-l leads to a predicted heat of formation of CuAlCld(g) of -173 kcal mol-l and an absolute entropy of 95 cal mol-l deg-l at 25O. In experiments below 500 K and with a sufficiently high partial pressure of aluminum chloride, the solid complex condensed in the reactor. In these cases the partial pressures of CuAlC14 reflect the saturation vapor pressure of the solid. Partial pressures of AlC19 were varied significantly above the saturation limit without observable effect on the apparent vapor pressure of the solid complex. The pressures of AlC13 generally remained above values expected for equilibrium with the excess of CuCl present and hence it appears that the solid complex formed a protective coating over the CuCl(s) crystals which prevented rapid equilibration through reaction 1. The P C ~ A I C ~ ~ Iratios PAIC~~ (Table I, supplementary material) in parentheses are seen to be abnormally low. In two instances (marked with asterisks) AlC13 was not introduced into the carrier gas prior to its entry into the reactor and after substantial amount of the solid complex had been formed in earlier runs; then the solid complex, by decomposition, generated the aluminum chloride vapor found in the exit gas and the resulting data gave pressure ratios in good agreement with values of K1 projected from the results a t higher temperatures (see Figure 1).
Second Virial Coefficient of
Nonpolar
129
Substances
The consistency of the derived sublimation vapor pressures at various aluminum chloride pressures suggests that the CuAlCk solid phase remained at fixed composition. Data from the 11runs in which CuAlCld(s)was present are tabulated below the dotted line in Table I (supplementary material); the vapor pressure dependence on temperature is also shown in Figure 1. A least-squares treatment, assuming the simple vaporization reaction CuAlC14(~)= CuAlCld(g)
(2)
led to a predicted enthalpy of 34.3 f 1.0 kcal mo1-l and an entropy of 47.8 f 2.4 cal molm1deg-l for (2) at a mean temperature of 473 K. With an estimated ACp of -6 cal mol-l deg-' for (2), a combination of results for (1) and (2) with JANAF values for CuCl(s) and AlC13 leads to a predicted standard enthalpy of formation of -208 f 3 kcal mol-' and an absolute entropy of 44.5 i 4 cal mol-1 deg-l for CuAlCld(s) at 25O. The entropy value is not greatly different from the sum of the entropies of CuCl(s) and AlC13(s), 47 cal mol-l deg-l, as generally expected for such complexes. The temperature dependence of the saturation pressures of CuAlClr(g) when a liquid phase is present along with CuCl(s) reflects not only the variation of the vapor pressure with temperature but also the effect of the change in solubility of CuCl in the liquid complex with temperature. While the calculated pressures of CuAlC14 above 500 K (Table I, supplementary material) do appear to converge, a t a given temperature, on a limiting value presumed to characterize the saturated liquid, this upper limit could not
be reliably fixed, Le., one could not be certain in every case when a liquid phase was actually present, or if the liquid phase had reached saturation equilibrium with CuCl(s). Hence we have not attempted to calculate the properties of the liquid phase. Acknowledgment. This work was supported by a grant from the National Science Foundation, GP 37033X. Supplementary Material Available: Table I, experimental transpiration data (2 pages). Ordering information is available on any current masthead.
References and Notes (1) J. Kendall, E. D. Crittenden, and H. K. Miller, J. Am. Chem. SOC.,45, 963 (1923). (2) Eg. B. H. Johnson (Esso Research and Engineering Co.) U S . Patent 3475347 (Cl. 252-429; Bolj) Oct 28, 1969. (3) Eg. D. G. Walker (Tenneco Chemicals, Inc.) Ger. Offen. 2057162 (C1.C Olgb) June 3, 1971. (4) W. C. Laughlln and N. W. Gregory, lnorg. Chem., 14, 1263 (1975). (5) W. C. Laughlin and N. W. Gregory, lnorg. Chem., In press. (6) C-F. Shleh and N. W. Gregory, J. Phys. Chem.. 79,828 (1975). (7) D. L. Hilden and N. W. Gregory, J. Phys. Chem., 76, 1632 (1972). (8) R. R. Richards and N. W. Gregory, J. Phys. Chem., 68, 3089 (1964). (9) Perkin-Elmer Model 303. (10) R. A. J. Shelton, Trans. FaradaySoc., 57, 2113(1961). ( I 1) "JANAF Thermochemical Tables", Revised Edition, Thermal Laboratory, Dow Chemical Co., Mldland Mich. (12) G. I. Novikov and F. G. Gavryuchenkov, Russ. Chem. Rev., 36, No. 3, 156 (1967). (13) C. R. Boston in "Advances in Molten Salt Chemistry", J. Braunstein, G. Manantov, and 0. P. Smith, Ed., Plenum Press, New York, N.Y., 1971. (14) N. C. Baenziger, Acta Crysfallogr.,4, 216 (1951). (15) K. Balasubrahmanyamand L. Nanis, J. Chem. Phys., 42, 676 (1965). (16) L. Brewer, G. R. Somayajulu, and E. Brackett, Chem. Rev.. 63, 111 (1963).
The Second Virial Coefficient of Nonpolar Substances R. M. Gibbons The Brifish Gas Corporatlon, London Research Sfaflon, London SWS, Eng/and (Received April 15, 1975) Publication costs assisted by The Britlsh Gas Corporatlon
Continued fractions have been developed for the second virial coefficient for the Barker Bobetic potential which contain, without significant error, all the thermodynamic information contained in the exact expression. Constants are obtained for the BB potential for Ne, Ar, Kr, and Xe. The general problem of obtaining approximate expressions for the results of statistical mechanical theories is discussed and compared with the analogous problem of using analytical mathematical functions on digital computers.
Introduction Empirical correlations for the second virial coefficient, B , are still widely used t o d a ~ , l -even ~ though there are a number of intermolecular potentials which describe the second virial data for many substances better than any empirical c ~ r r e l a t i o n . ~ The - ~ reason for this is the complexity of the calculations to evaluate B from the intermolecular potentials, which precludes the use of these models for routine calculations of B. It is the purpose of this note to show how simplified expressions for the values of B calculated from intermolecular potentials may be obtained which are
simple enough to use routinely. Since this is a general problem with all statistical mechanical theories, we start by discussing what is required of such simplified forms before discussing the application of the method to the calculation of B for the Barker Bobetic (BB) potential? which was chosen as an example because it is one of the best potentials for argon which has yet been devised. We conclude with a short discussion of the application of the method to other statistical mechanical theories. The integral for B for a realistic potential is typical of statistical mechanical theories of fluids in that the answer is provided as an intractable integral which can only be The Journal of Physical Chemistry, Vol. 80, No. 2, 1976
