NOTES
Feb., 1963
509
TABLE I THERMODYXAMIC DATAFOR SOLUTION OF M(DMG)2IN VARIOUS SOLVENTS solvent Reaction: M( DMG)n (solid) -+ R/I(DMG)t,(soln.)
______-_
Solvent
8a.b
Water Chloroform Benzene n-Heptane ‘I Taken directly from
Ni(DMG)z----A H ~ Z W ~ ’ ~AG%d
AS029~~
-
-__Cu(DMG)z----,y.b
A~0299a2C
9.0 8.15 2.9 1.05 X 1 0 P 5.68 x 0.0 4.8 4.55 0.8 4 . 6 2 x 10-4 1.24 X 1.3 6.6 5.63 3.3 7 . 5 3 x 10-5 4.1 X 10.0 14.8 9.01 19.4 1.84x 10-7 16.6 2 . 5 x 10-7 Fleischer and Freiser.2 Solubilit,y, mole/l. kcal./mole. cal./mole deg.
I
om--
2.53 H---0
IU \I 32
\o-----
H-0 2.70 Fig. 1.-Structure of one monomsr of Cu(DMG)2 dimer. Oxygen 011forms a Cu-0 bond, 2.43 A., Lo the monomer below. Two halves of the dimer are related by a center of symmetry so that 011 of the lower monomer forms a Cu-0 bond to Cu of the monomer shown. Distances imply that the 0-H-0 with length 2.70 A. is unsymmetrical in the manner shown. No implication as to short hydrogen bond is intended. The difference in the N-01 and the N-0111 bond distances is of doubtful significance. For other distances and angles, see ref. 4.
lI
gad
3.06 3.97 5.99 9.19
ASQ*,*d
-10.3 -9.0 13.5 24.9
for a distance of 2.43 k. is available, but a bond energy of about 44 kcal./mole is well within reason. It is iloteworthy that the Cu-0 bond energy in the dimer is so large that Cu(DMG)z could not dissolve as monomers in non-complexing solvents without a large gain in the energy of hydrogen bonding in going from the dimer to the monomer. It seems possible to check on the hydrogen bond reorganization, especially in chloroform in which Cu(DMGjr is rather soluble. Finally, the dimer structure in the crystal may lead to interesting magnetic behavior, as in copper acetate dihydrate where a t low temperatures the two electron spins of the dimer become paired to give a singlet with diamagnetic behavior.14 Both of these interesting possibilities will be given study. (14) B. Bleaney and K. D . Bowers, Phal. Mag., 43, 372 (1952); Proc. Roy. SOC.(London), A214, 451 (1952); J. N . Niekerk and F.R. Schoening, Acta Cryst., 6, 227 (lY5.3).
MOLTEN SALT SYSTEM GALLIUM MONOIODLDE-GALLIUM TRITODIDE. 11. VAPOR PRESSURES A S D SPECIES’ BYE. F. .RIEBLIXG~AND C. E. ERICKSON
57 kca1./mole.lL It seems not unlik.ely that the longer 0-H-0 bond will have a bond energy as high as 25 kcal./mole, and perhaps higher. With more certainty it can be estimated that hydrogen bonds of 2.70 and 2.53 A. have bond energies of about 5 and 1%kcal./ mole, respectively.lz Hence, the change in 0-H-0 bonds in going from crystalline Cu(DMG)* to mono12 -- -33 mers may contribute about -2 X 25 5 kcal./mole to AHoof solution. Since we have estimated that the enthalpy of solution of Cu(DMG)Z is about 11 kcal./mole greater than it would be if monomers were dissolving, this requires that the bond energy of Cu-0 bonds in the crystal be about 44 kcal./mole. This seems not unreasonable. The heat of solution of Cu+* in water is 508 kcal./ m0le.13 Presuming that there are six water molecules/ Cu++ ion, the average Cu-0 bond energy is about 84 kcal./mole. It may well be in this case that there are four close Cu-0 bonds of about 2.0 k.,two more distant ones of about 2.6 A. as in other Cu(I1) complexes. I n the Cu(DMG)* dimer, there are four close distances, Cu-N, and only ?ne further bond, the Cu-0 bond whose distance is 2.43 A.,4intermediate in distance, and presumably intermediate in energy, between the strong primary valence bonds and the weaker bonds of length 2.6 A. N o reliable way of estimating the bond energy
+ +
(11) T. C. Waddington, Trans. Faraday SOC..64, 25 (1958). (12) E.R.Lippincott and R. Schroeder, J . Chem. Phys., 23, 1099 (1955). (13) L. L. Quill, “The Chemistry and Metallurgy of Miscellaneous Materials.” MoGraw-Hill Book Co., Ino., New York, N. y.,1950.
Ralph
a.
Wraght Laboratory, School of Chemastry,
Rutgers, The State University, New BTunswtck, New Jersey
Ii’eceived J u l y 65, 1963
Vapor pressure determinations for molten compositions in the GaI-Gaz14-Ga& phase system were obtained with a transpiration technique. The vapor pressure information was used to calculate activities, activity coefficients, and their deviations from ideality for gallium(II1). These deviations can be related to the presence of complex ions3 in the molten salt mixtures because additional supporting evidence is available.4 The transpiration method was chosen because a considerable range of pressures was anticipated, because appropriate microanalytical techniques were available, and because a knowledge of the vapor composition was required for amore completeinterpretation. Experimental The transpiration apparatus was similar to that used by Sime.6 As a carrier gas, argon ( a t about 1000 mm. pressure) was allowed to flow from a trap a t the temperature of liquid oxygen, through the transpiration chamber, through an interchangeable calibrated (1) This paper is based on a thesis presented by E. F. Riebling in partial fulfillment of the requirements for the P h . D . degree, Rutgers, The State University of New Jersey, June, 1961. (2) Research and Development Division, Corning Glass Works, Corning, New York. (3) H. Bloom and J. O M . Bockris, “Molten Electrolytes,” in “Modern Aspects of Electrochemist,ry,” Vol. 2, ed. by J. O’M. Bockris, Academic Press, New York, N. Y., lMY, p. 215. (4) E. F. Riebling and 0 . E. Eriokson, J . Phys. Chem., 67, 307 (1963). ( 5 ) R. J. Sime, Doctoral Dissertation, University of Washington, 1959.
NOTES
510
flow-meter, and into a collection trap, maintained a t the temperature of liquid nitrogen. Salt samples were prepared in Pyrex capsules in a fashion similar to that for the density and conductivity experiments.‘ All samples were opened and the solids were transferred to the transpiration chamber in a drybox. The solids showed a marked transition in color between GaIz.oaand GaIz.li. Solids of I/Ga ratio equal to or greater than 2.17 were always a yellow-green shade. Solids of I/Ga ratio equal to or less than 2.06 were usually orange-brown if cooled slowly and orange, transparent glasses if cooled rapidly. These glasses could be induced t o crystallize if they were reheated. The apparently higher viscosities’ (and hence longer relaxation times) for melts within these latter composition limits, when associated with rapid cooling, could be partially responsible for the suppression of nucleation and the occurrence of glass formation.6 The texture of the solids also showed a marked change with composition in that salts of I/Ga ratio a t least equal to or less than 2.20 possessed a distinct fibroustype cleavage that was similar to that of asbestos. Compositions of I/Ga ratio greater than 2.20 possessed a granular cleavage. The amount of salt collected during a run was of the order of 1-57, of the 3-g. charge that had been placed in the transpiration chamber. A fresh charge was used for each run. The micro volumetric technique for iodide ion involved an argentimetric titration with dithizone as the indicator in a water-acetone mixt ~ r e .Micro ~ determinations of gallium(II1) ion were performed with standard EDTA solutions using gallocyanine as the indicator a t a p H of 2.8 in an acetic mixture.8 Standardization of these procedures indicated errors of the order of - 1 t o -2% in the 1-10-mg. range. The experimental vapor pressure of pure molten Ga& was markedly dependent upon argon flow rate. Flow rates of 1-4 cc./min. (calculated a t room temperature) resulted in equilibrium saturation of the carrier gas while larger flow rates resulted in considerable undersaturation of Ga18 in the carrier gas. Flow times between 30 and 180 min. were used. A 1-in. diam. hinged tube furnace provided a 3-in. constant temperature zone ( f 2 ” ) in which the chamber portion of the transpiration tube was placed. A chromel-alumel thermocouple, placed next to the chamber, was used to measure the temperatures.
Results T’apor pressures were calculated from Pln,
=
mE ma ma
+
x
PT
(1)
where m, = millimoles of salt collected and ma = millimoles of argon carrier gas used (from ma = pv,/RT, where 21, = calibrated 2-1. volume). The design of the transpiration chamber, with small openings, reduced the effect of thermal diffusion so that it contributed only about 1% to the total of 0.03 to 0.6 mmole of salt collected. m, represents the average between the experimentally determined mI - a and m G a + 3 . This procedure was adopted because the experimental I /Ga ratio for pure triiodide vapor was usually between 2.93 and 2.98 and because the analytical techniques for each ion mere about equally reliable. The experimental transpiration vapor pressures for pure molten Gad6 were calculated on the basis of a monomer and agreed with the pressures measured by F i ~ c h e r . ~The uncertainty for the higher pressures (20 to 100 mm.) was generally about 1 to 2% whereas for lower pressures (1 to 20 mm.), i t amounted to approximately 2 to 5%. For melt compositions other than the triiodide, the average vapor phase I/Ga ratio varied between 2.90 and 3.10. Thus, the predominant vapor species above all melts studied appears to be Ga13. This conclusion is (6) W.A. Weyl and E. C. Marboe, Glass Ind., 41, 549, 620 (1960). (7) E. E. Archer, Analyst, 88, 571 (1958). ( 8 ) G. FV. C. Milner, tbzd., 80, 77 (1955). (9) W. Fisoher and 0, Jubermann, 2. anoru. allgem.. ChFm,, 287, 227 (1936).
Vol. 67
supported by the yellow-green color of all solids condensed from the vapor as well as their ability to dissolve readily in water to give clear solutions. This behavior contrasts with solids known to contain gallium(1) (I,’Ga < 3.0). They always gave turbid solutions coiitaining gray, flocculent particles that required the addition of sulfuric acid to complete dissolution (this has also been observed by CorbettIO). Table I contains the constants for the experimental vapor pressure equations. The entropies and enthalpies of vaporization were calculated from a and b of the vapor pressure equations. The partial molal free energies (or “excess free eiiergies”) XTere calculated from A ~ I I I= RT In (PIIIIPOIII) = RT In
a111 =
4.574 T log
a111
(2)
where PO111 is the vapor pressure of pure triiodide and P I I is ~ the vapor pressure over another composition a t the same temperature. Since the vapor compositioii was found to be uniform, regardless of the compositioii of the melt, the thermodynamics of vaporization always refer to the same substance, namely Ga13 monomer. The excess free energy represents the change in the free energy of one mole of GaI3 in the melt on the addition of enough gallium to attain the specified composition. A p l I I , calculated for 250’ from the vapor pressure equations, varies with temperature and is shown in Table 11. A knowledge of A ~ I I I(or “excess free energy”) as a function of temperature allowed the calculation of equations from which the activity coefficient ( ~ I I I ) ,AS111 (or “excess entropy”), and ARIII (or “excess enthalpy”) could be obtained. The excess enthalpy and excess entropy, Table 11, can also be obtained from data in Table I by subtracting from the enthalpy and entropy of vaporization of the triiodide the corresponding values of the other compositions. As is true for excess free energy, these quantities refer to a single mole of gallium triiodide in the liquid phase. TABLE I VAPOR PRESSURE EQCATIOSS FOR Ga13 log P,,
ABOVE
Gal,
k1ELTS
= a - b X 103/T(O K . ) AS”&P AHvap
I/Ga
a
3.00 2.50 2 . 2oa 2.00 1.85 Only for T
a
?
9.033 9.028 7.342 6.294 6.608 > 225”.
f3.782 +3.794 f3.022 +2.888 4-3.293
(kcal.)
(cal. mole-‘ deg. -1)
28.1 28.1 20.4 15.6 17.1
17.3 17.4 13.8 13.2 15.1
TABLE I1 THERMODYNAMIC FCNCTIOXS FOR GALLIUM(III) IN GaI, MELTS A&II 250’
I/Ga
0111 250’
2.75 2 50 2 20b 2.00 1.85
(kcal.)
0 . 96a -0.05“ .94 - .07 .58 - .57 093 -2.47 ,032 -3.57 a T = 244’. Only for T > 225”. (10) J. D. Corbett and (1958).
AHIII (kcal.)
.... -0.1 +3.4 4-4.11 +2.20
R. K. McMullan, J . Am.. Ghem.
A h (cel. mole-’ deg. - 1 )
.....
-
0.06
+ 7.6 4-12.6 i.11.0
SOC.,77, 4217
Feb., 1963
SOTES
A ~ I I and I A3111 are not dependent on temperature as long as log P and 1 / T are linearly related.
The reported experimental heat of vaporization for gallium triiodide (17.3 kcal./mole of Ga13) is in close. agreement with Fischer’s value of 18.1 kcal., the average of the two being 17.7 kcal. Subtraction of this value from the known heat of sublimation for the monomeric species, 23.0 kcal.,ll yields an estimated heat of fusion of 5.3 kcal./mole of monomer or 10.8 kcal.,/mole of dimer. While the vapor has been shown to be monomeric, the high experimental entropy of vaporization (28 cal. mole-I deg.-l) reported here (Trouton’s rule would predict ASvap = 22 cal. mole-’ deg. for ti normal liquid) suggests the dissociation of a dimeric liquid during the vaporization process. The pressure of Ga13 shows two different kinds of behavior. I n the range Ga13-Ga12.5the pressure is little affected by the change of composition. Iiti fact, if one assumes simple mixing of gallium(1) and gallium(111)) the activity coefficient is a little greater than unity. The constancy of both the enthalpy and entropy of vaporization implies that the process of vaporization is essentially the same over the entire range. The vaporization process seems to involve only Ga216 molecules in the melt going to GaI, molecules in the vapor, without regard to the presence of ioiis. The electrical conduction process, on the other hand, seems to involve the ions, without regard to the molecular species present. Ga216behaves very much like a solvent throughout this range.4 In the range GaIz.z-GaIl.8sthe pressure is very much lower and indicates the occurrence of a different process. The activity coefficient of gallium(XI1) drops as low as 0.076 at 250’ for Ga11.85rrepresenting an extremely negative deviation from Raoult’s law. The positive values of the excess enthalpy and excess entropy in the same composition range also suggest that the essential vaporization process is different, and that gallium(IIJ.1 is present in different form. The vaporization process for the more ionic melts close to Ga214is more nearly (3)
The simplest assumption regarding the nature of the liquid portions of the GaI-Ga13 phase system is to suppose that a t Ga13 the principal species present is GaJ6 and that as the gallium content increases the following reaction takes place 2Ga0
+ 2Ga216+3Ga+ + 3Ga14-
THE COSDUCTASCE OF HYDROCHLORIC ACID AT 50’ BY BARBARA M. C O O EAND ~ R. H. STOKES~
Discussion
Ga14-(1) --f Ga13(g) -t I-(1)
51 1
(4)
The nominal composition of Ga214 is Ga+Ga14- while Ga12.33would contain one-half mole of Ga216for each mole of Ga+GaI4-.
Department of Inorganzc and Phgszcal Chemtstry, Unzzerszty of New England, Armadale, NEU South Wales, Australta Receaved June 14, 1989
This note supplements data for hydrochloric acid at 25” given in a previous paper3 by similar measurements at 50”. The equipment and procedure followed closely that used for the 25” work, the most important point being elimination of concentration inequalities produced by the Soret effeci,b in the cells during the warming-up period. The cell (design was modified so that the contents of the measuring section could be run out into a mixing chamber by merely tilting the cell without removing it from the thermostat; this obviated possible evaporation losses which might have occurred had the earlier pattern with a mixing bulb in the side arm been used at this higher temperature. The stock solution s analyzed by ineasuring its conductance a t 25” as cribed in ref. 3. Three cells of constants from 3.5 to 48.5 cm.-’ were used. That of constant 48.5 cm.-l had a central tube approximately 7 cm. long and 4 mm. in diameter between the electrode bulbs. Since this tube was too narrow to permit the pouring of the solution into the mixing chamber (which was attached to one of the electrode bulbs) it was necessary to bridge the two filling tubes (one attached to each electrode bulb) by a piece of glass tube, to provide for movement of air between the bulbs. When the surfaces of this bridge and the filling tubes were wet with electrolyte solution, an appreciable surface leakage of current occurred. This was overcome (at the suggestion of Dr. J. S. Agar) by coating the inner surface of the cell with a water-repellent silicone layer. This cell was used mainly for determining the cell constants of the other cells a t 50”) since its geometry was suitable for the calculation of as,,from the measured value of a26.j The results are given in Table I. The scatter of the measurements from a smooth curve is somewhat greater than at 2 5 O , averaging +0.014%. The two extreme deviations of +O.O40j, and -0.04y0 occur in the range below 0.001 &I, Just as at 25”, it was found that the equation of Pitts6 gives the best account of the data, which it represents up to 0.02 &If within the experimental error quoted above, the parameters used being .io = 580.90 ohm-1 equiv.-l and a = 3.67 A. The ion size parameter is the same as that which gave the best fit to the 25“ data.3 The equation of Fuoss and Onsager’ gave an extrapolation-function showing definite upward concavity over the same concentration range, though a linear extrapolation to Ao = 581.0 (1) The work reported here summarizes a thesis presented by Barbara LI. Cookin partial fulfillment of the requirements for the degree of Bachelor of Science with Honours In the University of Kew England in Pebruary,
Acknowledgments.-We wish to thank the Aluminum Company of America for their generotus loan of the gallium. We also wish to acknowledge the encouragement of Dr. P. A. van der Meulen, in t’he form of the A I . G. and R. G. Wright Fellomhip to E. F. R. for the years 1958-1961,
1962. (2) To n h o m correspondence should be addressed. (3) R . H. Stokes, J . 1’1gs. Chem., 66, 1242 (1961). ( I n the definitions following eq. 6 of this papt’r, AD should be inserted after the equals sign in the definition of Ps ) (4) R. H. Stokes, zbzd., 66, 1277 (1961). (5) R, .4.Robinson a n d R. H. Stokes, “Electrolyte Solutiona,” 2nd E d , Butterworths, London, 1959, pp. 97-99. (6) E. Pitts, Proc. R Q ~Sac. . (London), A217,43 (1953). (7) R M. Fuoss and I,. Onsager, J . Phgs. Chem., 61, 668 See
(11) F. J. Smith and R, F. Barrow, T r a m . Puradag Sac., 64, 826 (1958).
also R. M aFuoss a n d F. Accascina, “Electrolytio Conductance,” Interscience Publishers, S e w York, N. Y . , 1959.
(1957).
