April, 1958
HEATSOF FORMATION AND HYDRATION OF ANHYDROUS ALUMINUMCHLORIDE
419
sodium nitrate as observed by Van Artsdalen. On the basiso of the nearly equal ionic radii of calcium (0.99 &.) and cadmium (00.97 A.) and of Ptability constant barium (1.35 A.) and lead (1.21 A.), forces other Complex species 250' 275' 300' than purely electrostatic appear to be important. PbCl + 18 8 6 Van Artsdalen's basis for the argument that the PbClz 2 3 3 bonds of the complex are more of the chemical PbCla2 1 1 type because the predominant species are the PbBr 18 13 11 ones with two and four halide ions is not substanPbBrz 5 2 2 tiated in this work. In view of the similarities PbBra1 1 2 between this solvent and water with rcspect to the 20 .. 24 CdX stability of the metal halide complexes of lead and CdXz 5 .. 5 cadmium, one might expect some degree of asCdXssociation between these metal cations and the a Stability constant = IC,, = ( MXn2-")/[(MX,-~S-n) nitrate ion of the solvent.1° Therefore, an inter(X-)] where X- is either C1- or Br-. pretat,ion of the d a h on the basis of stable complex ions of the type M X n 2 - " where n is one, two atid of the complex with an increase in temperature in three, does not indicate whether the complex is of the case of lead. the ionic type or of the chemical type since, among The slight difference in stability of the chloride other things, the role of the solvent is not known. and bromide complexes is interesting in view of the (10) A. I. Biggs, N. H. Parton and R. A. Robinson, J . A m . Chem. ten-fold difference in stability of the cadmium Sac., 77, 5844 (19553, give a value of 0.67 for the stability constant of bromide and cadmium chloride species in molten the PbNOr complex in aqueous solution. TABLE IV
STABILITY CON ST ANTS^ FOR METAL-HALIDE COMPLEX IONS IN FUSED KNOs-NaN03 EUTECTIC
+
+
+
HEATS OF FORMATION AND HYDRATION OF ANHYDROUS ALUMINUM CHLORIDE BY JAMES P. COUGHLIN Minerals Thermodynamics Experiment Station, Region I I , Bureau of Mines, United Xiaies Department o j ihe Interioi, Be,Xelrp 4 , California Received January 6 , 1968
Heat of solution measurements of crystalline anhydrous aluminum chloride, aluminum chloride hexahydrate and aluminum were conducted a t 303.15"K. in 4.360 rn hydrochloric acid solution. Based upon these data, the following heats of formation from the elements at 298.15"K. werd derived: -168,570 rrt 200 cal./mole for anhydrous aluminum chloride and -643,600& 210 cal./mole for aluminum chloride hexahydrate. I n addition, the heat of hydration of the anhydrous chloride to the 80 cal./mole. hexahydrate a t 298.15'K. was evaluated as -65,130
Recent articles from this Laboratory dealt with the heats of formation of several aluminum compounds.ls2 In continuation of t'his activity the present paper reports the heats of formation of anhydrous aluminum chloride and aluminum chloride hexahydrate. The previously accepted value of the heat, of formation of the hexahydrate3is based upon work of Sabatier4 in 1889, and that of the anhydrous compound was obtained indirectly by combining thermochemical values in the literature. Materials.-The two samples of aluminum used in the measurements were in the form of thin lathe turnings of 99.995 and 99.998% pure metal as described in an earlier paper.' Anhydrous aluminum chloride was prepared from the same high-purity metal by direct reaction with dry chlorine gas a t 500-600" in an all-glass apparatus. The product was resrtblimed in a stream of dry chlorine gas at 280-300°, transferred to the sample bulbs in a closed system, and finally glass-sealed with minimum exposure to air. One bulb of sample was used for analysis. The bulb was broken under water in a calibrated, 2-1. volumetric flask. The resulting solution wag diluted to the mark (account being taken of the volume of glass in the broken sample bulb) and ____(I) J. P. Coughlin, J . Am. Chem. S O ~ 78, . , 5479 (1950). (2) J . P. Coughlin, ibid., 79, 2397 (1957). (3) F'. D. Rossini, D. D. Wagman. W . H. Evans, 9. Levine and I. Jaffe, Natl. Bur. Standards Circular 500, 1952. chim., France, 1, 88 (1889). (4) P. Sabatier, Bull. SOC.
aliquots were removed for analysis. Aluminum was determined by precipitation as hydroxide, followed by ignition to oxide, and chlorine by precipitation as silver chloride. The results were 20.28Oj, aluminum and 79.627" chlorine, as compared with the theoretical 20.23 and 79.77%. The aluminum chloride hexahydrate sample was reagontgrade quality. Analysis showed 11.18% aluminum and 44.09% chlorine, with 44.737&water by difference (theoretical: 11.17, 44.06 and 44.77%, respectively). An attempt was made to make up the slight water deficiency by sealing a sample in glass with the required amount of water and aging a t several different oven temperatures; however, the added water was never absorbed uniformly, and so the substance was used without this treatment. The hydrochloric acid solution (4.360 molal or HCl. 12.731HzO) wasprepared by dilution of reagent-grade acid in the manner previously described.'
Method and Results.-The apparatus for the heat of solution measurements is that described by Sou$hard,6 with minor improvements reported earlier by the author.6 The results are expressed in defined calories (1 cal. = 4.1840 abs. joules), and all niolecular weights are based upon the 19541955 Report on Atomic Weights.' All sample weights were corrected to vacuum. In all measureinent>s,1936.2 Q. of 4.360 m hydro(5) J. C. Bouthard, I n d . Ens. Chem., Sa, 442 (1940). (6) J . P. Coughlin. J . A m . Chem. S o c . , 77, 868 (1955) (7) E. Wiohers, zbzd., 7 8 , 3235 (1956).
420
JAMESP. COUGHLIN
Vol. 62
TABLE I HEATOF FORMATION or ANHYDROUS ALUMINUM CHLORIDE (Mol. wt. =: 133.35)
+
(1) (2) (3) (4) (5)
Al(c)
+
Reaction
AHroa.l~,oal.
+ + + +
-127,050 i120
M(C) 3 H + ( d ) A ~ + + + ( s o ~ )3/2Ha(g) 3(HC1.12.731HzO)(l) 3 H + ( d ) 3c1-(~01) 38.193H20(~01) AlC&(c) = Al+++(sol) 3Cl-(sol) 38.193"20(1) = 38.193HnO(sol) 3(HC1.12.731H~0)(1)= AlClr(c) 38.193H20(1) 3/2Hz(g) At 298.15"K., LWS = -51,870 i 140 cal.
+
O f 10 - 7 2 , 5 1 0 f 50 - 3 , 0 5 0 i 20 -51,490 f 140
+
chloric acid was employed as the solution medium. TABLE I1 The amounts of samples used were 0.5396 g. of alu- HEATOF SOLUTION OF ANHYDROUS ALUMINUM CHLORIDE minum (0.02 gram-atom) and corresponding quanSample wt., g. Filling order AHsos.u, cal./mole tities of all other materials, as they appear below in 2.2057 6 -72,440 reactions 5 and 11. 2.4019 10 -72,510 It was not necessary to determine A C p of the re2.5061 3 -72,520 actions under study, as all heat measurements were 2.6213 4 -72,570 made within 0.05 of 30" and required no correction. 2.8957 5 -72,510 A small correction was involved in bringing the electrical calibrations to exactly 30". This Mean -72,510 i50 amounted t o -20 cal./mole in the case of anhydrous aluminum chloride and was negligibly small using heat capacity data of Kelley*s9and Rossini. lo for the other substances. (The corrections applied (The apparent molal heat capacity of hydrogen to the measurements on aluminum metal, reaction chloride in 4.360 m solution was taken as -17.3 cal./deg., by extrapolating Rossini's data.) 1 , were discussed earlier. ' ) The heat of formation from the elements Anhydrous Aluminum Chloride.-Table I gives the skeleton equations for the reactions measured Al(c) 3/2Ch(g) AlCls(c) t o obtain the heat of formation of anhydrous AH288.15= -168,570 i 200 cal./mole (6) aluminum chloride. Reactions 1 and 2 were measured consecutively in the same acid solution. was obtained by combining the heat of reaction 5 Reactions 3 and 4 were measured consecutively in with the heat of formation of HC1.12.731Hz0from a fresh portion of the acid. Thus the final solution NBS Circular 500. Aluminum Chloride Hexahydrate.-The reacfrom reactions 1 and 2 is identical with that from tions measured t o obtain the heat of formation of reactions 3 and 4; consequently, AH5 = AH1 aluminum chloride hexahydrate are shown in AH2 - AH3 AH4. The time required for completion of the reactions Table 111. The sequence of reactions followed the in Table I (and also those in Table 111) was less pattern outlined for the anhydrous compound, and than 15 minutes, except for the reaction of alumi- exact stoichiometry again was maintained so that AH8 - AHg - AHlo. num metal with the acid which required 36 t o 82 AH11 = A H , Reactions 7, 8 and 10 are discussed above as reminutes, depending upon the thickness of the lathe actions 1 , 2 and 4. The data for obtaining the heat cuttings. Reaction 1 was discussed in an earlier paper.' of reaction 9 are in Table IV. Four measurements The molal heat of solution of aluminum in 4.360 m were made of the original material (the composiof which was taken as A1C13.5.99Hz0)and four hydrochloric acid (including all corrections) is tion of material from which small amounts of water - 127,050 f 120 cal. at 303.15"K. A single measurement was made of reaction 2, the heat of mixing had been removed. The results were plotted of approximately 15 ml. of the original hydro- against per cent. water and extrapolated to the chloric acid with the final solution of reaction 1, theoretical water content. The equation merely to confirm that the heat is virtually zero. LWsoa.ia -143.45 2.5 X (% HnO) (12) The data for reaction 3 appear in Table 11. I n fits the data with a precision uncertainty of k0.274 all, ten glass bulbs were filled with sample and and yields -31.52 i 0.07 cal./g. or -7,610 f 17 sealed. Three were unsuitable for the measure- cal./mole for aluminum chloride hexahydrate. The ments, because they were found to contain amounts uncertainty is increased to A40 cal./mole to allow of sample over 1 g. in excess of the desired quantity for i 0 . 0 5 % possible error in the determinations of (2.6670 g.) ; two other bulbs were lost in apparatus water content. failures. Within experimental error, there is no The heat of reaction 11 was corrected t o trend in the results in Table I1 because of sample 298.15"K., using data of Kelley8g9 and Rossini.1° size or filling order of the bulbs. Employing literature values3 of the heat of formaThree measurements of reaction 4 (- 79.9, - 79.9 tion of water and hydrochloric acid, the following and -79.6 cal./mole) were combined w t h three reaction heats are obtained measurements of subsequent reaction 10 (-80.5, (8) K. K. Kelley. U.S. Bur. Minea Bull. 477, 1950. -79.0 and -79.6 cal./mole) t o give an over-all (9) K. K. Kelley, personal communication. The heat capacities of average of -79.8 cal./mole of water for both reac- anhydrous aluminum chloride and aluminum chloride hexahydrate tions. were estimated as 19.8 and 77.0 cal./deg. mole. The heat of reaction 5 was corrected to 298.15"K., (10) F. D. Rossini, J . Research NatE. Bur. Standards, 4, 313 (1930).
+
-
5
+
+
+
THERMAL DIFFUSIONIN DENSEGASES
April, 1958
42 1
TABLE I11 HEATOF FORMATION OF ALUMINUM CHLORIDE HEXAHYDRATE (Mol. wt. = 241.45) Reaction
+
(7) (8) (9) (10) (11)
Al(c)
+
Al(c) 3Hf(so1) = 3(HC1.12.731HzO)(l) = AlCla.GHzO( C) = 32.193H2(1) = 3(HC1.12.731H20)(1) A t 298.15”K., AH11 =
AH808.16,
+ + +
Al+++(sol) 3/2Hz(g) 3H+(sol) 3cl-(s01) 38.193Hzo(s01) A l + + + ( d ) 3C1-(~01) 6HzO(sol) 32.193HzO(SOl) AlC1~.6HzO(c) 32.193H20(1) 3/2Hz(g) -117,000 f 130 cal.
+
+
+
AlCL(0)
TABLE IV HEATOF SOLUTION OF HYDRATED ALUMINUM CHLORIDE HnO, %
AHaor.16, cal./g.
44.73 44.73 44.73 44.73 44.71 44.64 44.62 44.61
-31.63 -31.64 -31.59 -31.67 -31.77 -31.97 -31.74 -31.89
+
AHaor.16
- 2.5(%
AH303.16
HnO)
cal.
-127,050 f 120 O f 10 - 7 , 6 1 0 f 40 -2,570* 20 - 1 1 6 , 8 7 0 i 130
+ 6Hz0(1) = AlCls*6HzO(c) (15) = -65,380 f 80 cal.
LWZSS.IS = -65,130 f 80 cal.
-143.45 - 143.46 -143.41 - 143.49 - 143.55 - 143.57 - 143.29 -143.41
Discussion
Previously accept.ed values3 of the heats of reactions 6 and 15 (the heats of formation and hydration of anhydrous aluminum chloride) are - 166,200 and - 65,000 cal., respectively. The present work agrees with the value for the hydration reaction but differs on the heat of formation of the anhydrous compound by 2370 cal. From examination Mean -143.45=k0.07 of the bibliography of NBS Circular 500,3it appears Al(c) 3/2Clz(g) 6Hz0(1) AlC13.6HzO(c) (13) that the “best” values of the heats of solution of AH2g8.16 = - 233,700 f 200 cal., aluminum metal, the anhydrous chloride and the AKc) 3/2clz(g) 6Hz(g) 3oz(g) = hexahydrated chloride of necessity were taken from AlCla*6HzO(c) (14) the work of a t least three different investigators. AHzs8.l~ = -643,600 i 210 cal. This involves possible errors due to variations in The heat of reaction 15 wasevaluated a t 3 0 3 ~ 5 ° K . sample purity, solution concentration, temperature by combining the data for reactions 3, 4, 9 and 10. of measurement, and variation in systematic errors Correction t o 298.15”K. was made, using an esti- in apparatus, all of which are avoided in the new mated AC, = -51 cal./deg. values presented here.
+
+
+
+
+
THERMAL DIFFUSION I N DENSE GASES BY J. E. WALTHER AND H. G. DRICKAMER Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, Illinois Received November 4, 1067
Thermal diffusion measurements have been made on a series of binary mixtures of gases to 500 atm. (Several systems were studied to 1000 atm.). Mixtures far from the critical temperature showed only small ressure effects. For systems where one component is near its critical temperature, a large negative value of the thermardiffusion ratio 01 is obtained. Neither present kinetic theories, nor the thermodynamics of irreversible processes offer a satisfactory explanation, but i t is possible t o get some insight into the phenomenon from each theory.
There have been numerous studies of thermal diffusion in gases at or near atmospheric pressure2; the theory is well developed and gives good agreement with expCriment. There also have been rather frequent investigations of the phenomenon in liquids and it can be described with reasonable qualitative accuracya in terms of activated motion. There are only a very few investigations in the dense gas region, and these have been over a very limited pressure range,4 or have used the thermal (1) This work waa supported in part by the A.E.C. (2) J. 0. Hirsohfelder, C. F. Curtiss and R. B. Bird, “Molecular
diffusion column.b The theory of the column is at best only semi-quantitat.ive and, in dense gases, particularly near the critical point, its applicability is very doubtful. Therefore thermal diffusion measurements have been made, in a single stage cell, on a series of binary gas mixtures. In most cases the pressure range was to 500 atm.,’although a few data were obtained to 1000 atm. I n all cases the gases were pure grade commercial products used as purchased. The defining equation for the thermal diffusion ratio in a binarg system is
Theory.of Liquids and Gases,” John Wiley and Sons, Ino., New York, N. Y.,1954, p. 582 ff. (3) L. J. Tichrtcek, W. 8. Kmak and H. G. Driokamer, THISJOWR( 5 ) N. C. Pierce, R. B. Duffield and H. G. Driokamer, J. Chsm, NAL, 60, 660 (1956); E. L. Dougherty, Jr., and H. G. Driokamer, ibid., Phus., 18, 950 (1950); E. B. Giller, R. B. Duffield and H. G. Dricka59,443 (1955); J. Cham. Phus., 23, 295 (1955). mer, ibid.. 18, 1027 (1950); F. E. Caskey and H. G. Drickamer, (4) E. W, Becker, Naturforechung, Oa, 457 (1950). $bid., 88, 153 (1953),
