716
A. V. DEO AND I. G. DALLALANA
involves the separation of repulsive charges as well as the migration of a very mobile proton, we are not surprised that kl = 1.4 X lo6 see-l for this reaction is a great deal larger than the 25' first-order rate constant kl* to sec-I reportedz4for the much studied reaction
-
ki*
Cr (HzO)
-+
Cr (HzO)b3+
+ HzO
that bears it a superficial resemblance. The iondipole attractive interaction in this latter reaction as well as the lower mobility of HzOthan H+ could account for a markedly lower kl* than kl. (24) M. Eigen and R. G. Wilkins in "Mechanisms of Inorganic Reactions," Advances in Chemistry Series, No. 49, R. F. Gould, Ed., American Chemical Society, Washington, D. C., 1965, p 64.
An Infrared Study of the Adsorption and Mechanism of Surface Reactions of 1-Propanol on y-Alumina and ?-Alumina Doped with Sodium Hydroxide and Chromium Oxide by A. V. Deo and I. G. Dalla Lana Department of Chemical and Petroleum Engineering, University of Alberta, Edmonton, Alberta, Canada (Received September 9 , 1 9 6 8 )
Infrared studies of the adsorption of 1-propanol on y-alumina and the effect of doping the y-alumina with sodium hydroxide and/or chromium oxide in the temperature range from room temperature to 400" are described. Four different types of surface species were observed : (i) strongly physically hydrogen-bonded propanol to the surface hydroxyl groups; (ii) an ahminum propoxide type structure chemisorbed on Al" ions; (iii) a carboxylate structure in which both surface hydroxyl and A13+ ions are involved; and (iv) a conjugated hydrocarbon species bonded probably to the A13+ ions. Dehydration mainly occurred on the pure y-alumina surface, particularly on the high-frequency hydroxyl groups. On the addition of sodium hydroxide, the dehydration reaction was suppressed and the dehydrogenation reaction became dominant. The addition of chromium oxide apparently creates more dehydrogenating sites. The dehydrogenation possibly proceeds via a mechanism involving both carbonium and carbanion ions. The results are correlated by both infrared spectra and mass spectral analysis of the gaseous products.
Introduction One of the earliest studies of the catalytic dehydrogenation of primary alcohols was reported by Ipatieff in 1936. Komarewsky2 used this method to prepare symmetrical ketones. I n a similar study, with 1propanol as the feed to a fixed-bed reactor using a sodium hydroxide-treated chromia-Alundum catalyst, D a h Lana,3 et al., proposed a chemical model for the multiple reaction sequence. By isolating and identifying chemical intermediates and by examining the chemical reactions of several intermediates under identical conditions, they showed the chemical sequence involved dehydrogenation of the 1-propanol to propionaldehyde followed by parallel condensation of the aldehyde to either its aldol or to propyl propionate under the elevated temperature a t 400". They also encountered some thermal reactions.4 The present paper discusses some mechanistic aspects of this reaction system, obtained by studying the infrared spectra of 1-propanol adsorbed on the catalyst and of the interThe Journal of Physical Chemistry
mediate surface species formed thereof. Some attention is also devoted to the catalytic influence of yalumina on 1-propanol. Infrared spectral studies of the adsorption of primary alcohols on alumina have been previously reported. Babushkin and Uvarov6 studied the adsorption of ethanol on alumina at 20" and found that the alcohol was adsorbed in fragments such as OH, -CH2-CHZ--, and also as an ethoxy group bound directly t o an aluminum atom on the surface, Al-O-CH,-CHZ. (1) V. N. Ipatieff, "Catalytic Reactions a t High Pressures and Temperatures," The Macmillan Go., New York, 73. Y., 1936. pp
411-45 1. ( 2 ) V. I. Komarewsky and J. R. Coley, J . Amer. Chem. S O C . ,63, 700,3269 (1941): Advan. Calal., 8, 207 (1956). (3) I. G.Dalla Lana, K. Vasudeva, and D. B. Robinson, J . Catalysis, 6 , 100 (1966). (4) I. G. Dalla Lana, 5. E . Wanke, and A. V. Deo, paper presented
a t the Second Symposium on Catalysis, Hamilton, Ontario, Canada, June 16, 1967. (5) A. A. Babushkin and A. V. Uvarov, Dokl. A k a d . N a u k S S S R , 110, 581 (1956).
REACTIONS OF P PROPANOL
ON
717
7-ALUMINA
00
FREQUENCY ( c m - 1 ) Figure 1. 1-Propanolon y-alumina.
Heated in 1 om of PrOH a t
Tofor 1 hr
Roev and Terenin6 studied the mechanism of decomposition of methanol and ethanol on chromium oxide catalyst at 20 and 150" and observed that water molecules and hydroxyl groups split off from the alcohols with the formation of unsaturated compounds. The elimination of an H atom also resulted in the formation of surface components of the type Cr-0-R. Greenler' studied the adsorption of methanol and ethanol on y-alumina in the temperature range from 35 to 430" and identified three surface species, vix., an easily removed liquid phase, a surface alkoxide structure, and an ester-like surface structure. More recently, KageP studied the adsorption and surface mechanism of CI to C4 normal alcohols on y-alumina in the temperature range from 25 to 500" and observed the same three species as reported by Greenler. H e proposed a mechanism for these adsorption processes. The present work restricts the study t o 1-propanol but investigates the effect of doping y-alumina with sodium hydroxide and chromium oxide, in the temperature range from room temperature to 400". Sodium hydroxide is generally believed to suppress the catalytic dehydration activity of y-alumina, and chromium oxide itself behaves as a dehydrogenation catalyst. Their
and pumped off at
T"for 1 hr.
use as doping agents in this work parallels the earlier studies reported by Dalla Lana, et aL3 Experimental Section A y-Alumina powder, A I O ~ - Cwas , ~ used to prepare thin infrared-transparent wafers by pressing the fine powder in a 2-in. diameter die a t a pressure of 12 tons/in.2. To dope the y-alumina catalyst with sodium hydroxide and/or chromium oxide, 2% aqueous solutions were mixed with alumina into a thick slurry using a minimum amount of water. The excess water was removed by evaporation from the slurry at 110" and the resulting cake was crushed into fine powder, which could then be pressed into thin wafers. The catalyst wafer was then placed in an in situ infrared celllo with sodium chloride windows. The pretreatment of the pure y-alumina and the one doped with sodium hydroxide involved degassing at, 450" for 2 hr, heating (6) L.M.Roev and A . N. Terenin, Dokl. A k a d . N a u k S S S R , 1 2 4 , 373 (1954). (7) R. G.Greenler, J. Chem. Phys., 37, 2094 (1962). (8) R. 0.Kagel, J. Phys. Chem., 71, 844 (1967). (9) Cabot Corporation, Boston, Mass. (10) J. B. Peri and R. B. Hannan, J. Phys. Chem., 64, 1526 (1960).
Volume 78,Number 8 March 1959
A. V. DEO AND I. G. DALLALANA
718
,
\-A
A
I
I
00
FREQUENCY ( cm-1 ) Figure 2. 1-Propanol on NaOH treated ?-alumina. at 400" for 1 hr and pumped off a t 300" for 1 hr.)
Heated in 1 em of PrOH a t Tofor 1 hr and pumped off at Tofor 1 hr. ( X : heated
in oxygen at 10 cm pressure for 2 hr at 450°,and again degassing at this temperature overnight. I n the case of chromia-doped catalyst, the oxygen treatment was followed by heating in hydrogen at 10 cm pressure for 2 h r a t the same temperature to ensure that the chromium was in a reduced state. After cooling to room temperature, the base line spectrum of the catalyst was recorded on a PerkinElmer Model 621 infrared spectrophotometer, using a similar evacuated cell without a catalyst wafer in the reference beam. Because of the low transmission of the catalyst wafers, the reference beam had to be The Journal of Physical Chemistry
attenuated and the slits opened to compensate for the loss in energy, the slit opening being directly proportional to the square root of the reciprocal of transmittance. The 1-propanol (Fisher Co.) was purified by distillation followed by repeated freezing and thawing on the vacuum rack. One centimeter of absolute pressure of propanol vapor was introduced in both cells and the spectrum was recorded differentially to eliminate interference from spectrum of gaseous propanol. To check that the spectrum of the adsorbed species alone was obtained, the procedure was repeated but with removal
REACTIONS OF PROPANOL
ON
719
y-ALUMINA
/
"
A
0 3800 3700 3600 3500
"
A
I
2950
2850 2750
t
1700 1600 1500 1400 1300 1200
1100 1 DO
FREQUENCY ( c m - 1 ) Figure 3. 1-Propanol on NaOH treated y-alumina doped with chromium oxide. Heated in 1 cm of PrOH at Toand pumped off at 2'' for 1 hr. ( X : heated st 400" for 1 hr and pumped off at, 300' for 1 hr.)
of the catalyst from the sample beam and again recording the spectrum with vapor alone in both cells. After recording the spectrum of the adsorbed substance, excess alcohol was pumped off a t room temperature for 1 hr. Physically adsorbed alcohol was removed by degassing a t 100" for 1 hr. High-temperature heating up to 400" was maintained for a further 1 hr with the vapor of propanol a t 1 ern pressure in the cell. The subsequent degassing was completed in steps of loo", the spectrum being recorded after each step. The spectra so obtained are shown in Figures 1 to 3. They show selected spectra for each set of experiments and the spectra are shifted along the transmission scale
for convenience and clarity in reading. Although Figures 1 to 3 appear rather crowded in detail, the presentation of all related spectra in a single figure (rather than in several figures) facilitates the observation and comparison of changes in infrared spectra. The region between 1300 and 1000cm-l, where the transmission of the catalyst drops sharply, is shown as a difference between the base line and the spectrum in the adsorbed state, whereas in the region a t higher frequencies, the spectra are shown as recorded. After heating the propanol in the presence of the catalyst wafer, the infrared spectrum of the vapor was recorded to study the products formed. These results Volume 79,Number B March 1060
A, V. DEOAND I. G. DALLALANA
720 were checked against the related mass spectra, for the same vapor phase as well as for the species removed by pumping. The mass spectra were obtained with an AEI (RlS2) mass spectrometer.
Results and Discussion I . I-Propanol on Pure y-Alumina. Figure 1 shows the results of adsorption of propanol on alumina at different temperatures, along with the base line. The base line shows three broad bands at 3785, 3720, and 3680 cm-I which are attributed to the three different types of surface hydroxyl groups as reported by Peri.lo The model put forward by Perill for the surface sites on y-alumina will be used in the discussion of the results. The model assumes that even after sufficient dehydration, when the water molecules physically adsorbed are largely eliminated, the surface of alumina still retains hydroxyl groups with neighboring sites of oxide and aluminum ions. Moreover, the number of oxide ions surrounding the hydroxyl groups differentiates them from one another and imparts different acidity to them. Spectra obtained at room temperature and 100" (and up to e170") are more or less similar t o the propanol spectrum in the vapor phase except for a few distinct changes and sharper bands. The high-frequency 3758 cm-l band disappears completely while the other two hydroxyl bands are still evident. The large broad band around 3500 em-1 is due to hydrogen bonding of the alcohol molecules to the surface hydroxyl groups. Because of the possibility of hydrogen bonding between the alcohol molecules themselves, though normally negligible in the gas phase at low pressures, as well as with the surface groups, the hydroxyl group interactions are difficult to interpret but may be used as an indirect evidence of hydrogen bonding with the surface. There are no changes in the C-H stretching and bending regions; however, the OH in-plane bending band at 1220cm-1 shifts to a higher frequency of 1250 cm-l. More pronouncedly, the -C-OH stretching band also shifts upwards from 1060 to around 1150 cm-1. These shifts to higher frequencies and the presence of the OH in-plane bending band suggest that the H of the hydroxyl group in the alcohol is free and that the alcohol molecules are hydrogen bonded to the surface hydroxyl through the oxygen in the alcohol. Moreover, this band structure corresponds almost exactly to that reported'!* for an aluminum alkoxide (in the present case aluminum propoxide). Although the 1150 and 1250-cm-l bands might mask one another, additional supporting evidence was observed. The large broad band at roughly 3500 cm-I is characteristic of hydrogen bonding. Furthermore, pumping at higher temperatures after room temperature adsorption of alcohol resulted in rather easy desorption of the hydrogen-bonded alcohol molecules. The minor differences are due to the number of hydrocarbon chains, Tha Journal of Physical Chemistry
aluminum propoxide with three hydrocarbon chains, while the surface structure will have only one. From these results, one may surmise that, up to 170") two types of adsorbed species (I and 11) are formed, with their structures as follows. up to -170'
I
Structure I is the strongly hydrogen-bonded physically adsorbed species. Considering either ITa where the H atom is dissociated as such, or I I b where the hydrogen ion H+ is removed, structure IIb seems to be the more plausible one. The H+ ion thus removed is free to migrate t o oxide surface sites and form Hz. Significant changes occur in the spectra of adsorbed species on heating above 170". The broad band around 3500 cm-1 attributed to the hydrogen bonding with the surface now increases to a larger extent than that encountered a t room temperature and up to ~ 1 7 0 " . Correspondingly, a new band a t around 1650 cm-l (not shown in figure) appears due to I-I-0-H bending of adsorbed water. This water band and also the increased broad band around 3500 cm-I are easily removed on pumping at room temperature. Four new bands are observed a t 1565, 1480, 1450, and 1303 cm-l. These are stable up to 400" with degassing. The bands a t 1565 and 1480cm-' correspond to asymmetric and symmetric stretching vibrations of the carboxylate ion; the bands at 1450 and 1303 cm-1, which decrease in intensity on pumping at higher temperatures, seem t o be either characteristic of the propionate structure or associated with the carboxylate structure. The sources of these two bands were not confirmed. Now, since the alkoxide structure is not observed above 200' but the carboxylate structure is, it may be expected that the alkoxide must have formed the carboxylate. Greenler' has shown by studying the adsorption of CH8OD and CD30H on y-alumina that the hydrogen removed from the surface carboxylate (formate) comes from the methyl group, and taking into consideration the bridge structure proposed by Eischens and Pliskin12for chemisorbed formic acid on nickel, one can write the carboxylate structure (I1) (12)
B* J. P h y s . 697 220 (lgfj5). R. P. Eischens and W. A. Pliskin, Aetes Congr. Intern. Cfltnl?/sc, J'
paris (1960).
REACTIONS OF PROPANOL
ON
T-ALUMINA
of the adsorbed species (111) as above -170"
to 400'
I I -CI
-C-
-C-
-c-
I
I
I
o--AI-o--;u
(confirmed by m.8.)
The hydrogen evolved in this reaction was detected by mass spectrometer. The shift to the lower frequency side of 1565-cm-' band on degassing at higher temperatures may be due to a change in A1-A1 distance with the corresponding change in the carboxylate bond angle. The infrared spectrum of the gaseous products formed above 200" shows that propylene is the major reaction product. The band at 1 6 5 0 ~ m -for ~ the adsorbed species suggests that water is also formed during the reaction above 200". This was confirmed by mass spectral analysis of the gaseous product formed at 400", which was found to consist mainly of propylene, water, and less than 1% hydrogen, This reaction, occurring above 20O0, is explained on the basis that structure I, hydrogen-bonded propanol, dehydrates giving propylene. I
.
1
(confirmed by m.s.)
.
A3+
1
0
I
AI
I I . 1Propanol on Sodium Hydroxide Treated yAlumina. Figure 2 shows results frem a similar set of studies of adsorption of propanol on sodium hydroxide treated y-alumina at different temperatures, along with the corresponding base line. The base line shows only two well-separated hydroxyl bands a t 3740 and 3680 cm-1 instead of the three encountered in the case of pure alumina. Perilo has reported that the band a t the lower frequency of 3680 cm-l appears to be the more acidic of the three hydroxyl groups, shown by the greater ease with which it exchanges hydrogen. As R. result, the disappearance of the high-frequency 3785cm-1 band and not the low-frequency more acidic 3680-cm-l band is rather unexpected. The results from the adsorption of propanol on y-alumina (Figure 1) show that the high-frequency 3785 cm-1 hydroxyl band disappears completely while the other two remain more or less separated, and on desorption at high temperatures, this high-frequency band reappears last.
721 The disappearance suggests that sodium hydroxide reacts with the high-frequency surface hydroxyl to form AI-0-Na. Adsorption spectra at room temperature, lOO", and 200" are more or less similar to the one on pure alumina. The following three differences were noted: (i) the hydroxyl region is more distinctly outlined suggesting a lesser degree of hydrogen bonding; (ii) in the C-H stretching region, 2940 and 2880-cm-l bands are resolved into doublets, In conjunction with this, the fact that the 1380-cm-'band, which is always smaller in intensity than the 1455-cm-' band as is the case on pure alumina, has reversed the intensity relation, suggesting an 0-R bonding;13and (iii) the small band a t 1562 em-' gives an indication of the beginning of formation of carboxylate species. From this, one can conclude that up to 200" the major surface species formed is the alkoxide (11) with very little of the hydrogen-bonded species (I). At 30O0, the asymmetric and symmetric stretching bands of the carboxylate ion become more pronounced but the bands due to the alkoxide species still exist, suggesting an abundance of this latter species. Up to this high temperature, the spectrum of the gaseous phase does not change from that of propanol showing that no reaction has taken place. The spectrum marked X was taken after heating the catalyst with 1 cm of propanol a t 400" for 1 hr and then pumping off a t room temperature, 100, 200, and 300" for 1 hr each. Two new bands are observed a t 1650 and 1617 cm-l, while the other bands due t o the alkoxide species were much reduced. These new bands show the presence of conjugated double bonds; however, these bands disappear after pumping at 400" and then only the carboxylate bands remain. The spectrum of the gaseous product shows the band structure is similar to unsaturated compounds. This was confirmed by a mass spectral analysis of the gaseous product, which showed the presence of propylene and mainly conjugated hydrocarbons with the molecular formulas CeHlo (confirmed by exact mass measurements) and 7% hydrogen. Water was not formed in this reaction, indicated by the fact that the hydroxyl region did not increase in intensity and the absence of the 165O-cm-' band which disappears on pumping at room tempcrature. From studies by other workers on silica and alumina, both having surface hydroxyl groups, the greater reactivity of alumina is attributed to the electronabstracting Lewis acid centers present 011 the alumina surface as aluminum ions. The results of adsorption of propanol on y-alumina and the mechanism proposed here suggest that the surface hydroxyl groups more than compete with the aluminum ions resulting in dehydra(13) K . Nakanishi. "Infrared Adsorption Spectroscopy," IroldcnDay, Inc., San Franaisco, Calif., 1982, p 22. Volume 75, Number 3 March 1060
A. V. DEO AND I. G. DALLALANA
722 tion of the alcohol. Once this tendency is suppressed by doping with sodium hydroxide, the main reaction involves aluminum ions and thus the dehydrogenation reaction becomes dominant, Sodium hydroxide, it seems, does not take part in the reaction but essentially suppresses one type of active site and leaves the other unaffected. The following mechanism for the formation of unsaturated hydrocarbon then seems plausible. IIb
tion and the formation of conjugated hydrocarbons may be explained by considering the carbanion ion mechanism suggested by earlier workers.
carbonium ion
+
carbanion ion
-
C,H,
Such a carbanion ion will then react with another carbonium ion, as was shown to form on the sodium hydroxide-treated y-alumina surface, resulting in the formation of C6Hs.
Conclusions
From two alkoxide groups, two carbonium ions will be formed, in which either one of two neighboring A13f ions will be involved to give the unsaturated hydrocarbon and hydrogen. This result was confirmed with mass spectral analysis. I I I . 1-Propanol on Xodiuin Hydroxide Treated Chronzia-Aluinina. Figure 3 shows the result of adsorption of propanol on sodium hydroxide treated chromia-alumina at different temperatures. The spect,ra are similar to those of the propanol adsorbed on sodium hydroxide-treated y-alumina except, (i) the hydrogen-bonded hydroxyl region is less intense and, (ii) the band structure below 1300 cm-l is more pronounced, This suggests an increase in the number of nlkoxide species formed on the surface. It may be mentioned that the alkoxide species may form equally well on chromium ions. The infrared spectrum of the gaseous product is unchanged from that of propanol up to 300". At 400", the presence of unsaturated bonds, especially C5Hs, and carbon monoxide is indicated and confirmed by mass spectral analysis, along with 2% hydrogen, The decrease in the percentage of hydrogen found compared with that from sodium hydroxide-treated y-alumina may be attributed to chemisorption of hydrogen on chromium ions, which did not desorb when pumped at room temperature. Burwell, et al.,?4 and Van Reijen, et al.,ls have shown that surface chromium ions, on catalyst reduction, are in divalent state, Cr2+, surrounded by oxygen neighbors, and are responsible for the dehydrogenation character of such a catalyst. So, in effect, by adding chromia, one increases the number of surface sites available for dehydrogenation reaction. The decarboxylation reacThe Journal of Physical Chemistry
The above results show that, under the conditions studied, four types of surface species are mainly formed: (i) hydrogen-bonded propanol (I) strongly physically adsorbed to the high-frequency surface hydroxyl groups; (ii) the aluminum propoxide (11) chemisorbed on the electron-abstracting Lewis-acid aluminum ion, Ala+; (iii) the carboxylate with the bridge structure (111) in which a hydroxyl group is involved; and (iv) the conjugated hydrocarbon species. These observations concur with the results reported by earlier workers, though no single prior study has reported all of these species. The appearance of and extent of formation of each species varies with temperature and the composition of the catalyst. The formation of the fourth species with the conjugated olefinic structure, formed only on sodium hydroxide-doped catalyst, is tentatively explained through a carbonium ion and carbanion ion mechanism. The formation of water during the dehydration is amply confirmed by both mass spectra and infrared analysis. In the latter, the hydrogen-bonded hydroxyl region increases in intensity and the 1650-~m-~ band due to H-0-H bending appears, both of which disappeared upon pumping at room temperature. In the case of hydrogen, the onIy evidence obtained for its presence is the mass spectral analysis. The stoichiometric amounts of hydrogen suggested by the proposed mechanism were not quantitatively established. Very recently, Shimizu and Gesser,le while studying the esr spectra of hydrogen adsorbed on porous glass, observed signals due to hydrogen on the surface, and have suggested that the hydrogen atom becomes attached to the aluminum atom of the porous glass. This may be one (14) R. L. Burwell, A. B. Littlewood, M. Cardew, G. Pass, and C. T. H. Stoddart, J. Amer. Chem. Soc., 82, 6272 (1960). (15) L. L. Van Reijen, W. M. H. Sachtler, P. Cossee, and D. N. Brouwer, Third Congress on Catalysis, North Holland Publishing Go., Amsterdam, 1965,p 829. (16) M. Shimieu and H. D . Gesser, private discussion.
723
ELECTRICAL CONDUCTAWES OF AQUEOUS SODIUM IODIDE of the reasons accounting for the variation in the amount of hydrogen observed in the products via mass spectral analysis of products desorbed from sodium hydroxidedoped and chromium oxide-doped catalysts. Chromium ions themselves may also present a strong adsorption site for hydrogen. Thus it appears that the main reaction of propanol on y-alumina is that of dehydration, involving primarily one type, the least acidic, of surface hydroxyl groups. Once these high-frequency surface sites have been removed by doping with sodium hydroxide, the hydrogenation reaction involving the Ala+ becomes dominant. The addition of chromium oxide to the catalyst creates more sites for dehydrogenation.
Adsorption of propanol on chromia-alumina catalyst, without sodium hydroxide treatment, gave similar results to those obtained on pure y-alumina. This suggested that for dehydrogenation to occur, the activity of the dehydrating sites must be first suppressed. Further studies using the intermediates observed in propanol dehydrogenation3 and the effect of preadsorbed hydrogen, water, and carbon monoxide will be reported later.
Acknowledgment. This work was supported by funds provided by the National Research Council of Canada and by the University of Alberta.
Electrical Conductances of Aqueous Sodium Iodide and the Comparative Thermodynamic Behavior of Aqueous Sodium Halide Solutions to 800" and 4000 Bars1 by Lawrence A. Dunn2 and William L. Marshall Reactor Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
67850
(Received September 9, 1968)
From electrical conductance measurements on dilute (0.001-0.10 m) aqueous solutions, the ionization behavior of NaI was studied in the temperature range 0-800" and a t pressures to 4000 bars. Both the conventional ( K ) and complete (KO) ionization constants were calculated for comparison with published values for NaC1 and NaRr. As expected, XaI ionized to a greater extent than either NaBr or NaCl, the order being directly proportional to the anion size. The net change ( k ) in waters of solvation on ionization decreased from 10.2 for NaCl to 9.7 for NaI. For the temperature range 400-800", the vaii't Hoff isochore yielded standard thermodynamic functions for the complete equilibrium, NaX(so1vated) kHzO e Na+(solvated) X-(solvated). The AH" obtained, approximately constant with temperature, mas essentially the same as found for NaBr and NaCl. With the calculated values of AGO and an average value for AH" of -7.0 kcal mol-l, standard entropy changes of -88.3, -86.2, and -82.9 cal mol-I deg-I for the complete reactions were obtained for NaC1, KaBr, and NaI, respectively. The negative values of AS" show that order is increased by additional solvation on ionization of the electrolyte. Thus, a t high temperatures, inclusion of the solvent as a reactant provides a simple description whereby each equilibrium can be described by only three values ( k , AH", and AS") which are essentially independent both of temperature and pressure. The closeness in values of k and AS" for the three halides suggests that averaged values from these salts may be used to estimate the equilibrium properties of most 1-1 salts between 400 and 800".
+
Introduction The properties of aqueous electrolyte solutions at high temperatures and pressures, particularly in the supercritical region, are amenable to investigation by conductance techniques. This method has been applied in several recent publications from this laboratory to the study of the behavior of dilute aqueous solutions of KHS04,3NaCl,4NaBr,5and HBra to 800" and to pressures of 4000 bars. The results from these studies have indicated that, at high temperatures and pressures, aqueous electrolyte solutions exhibit a simplified be-
+
havior not evident a t lower temperatures. It was found that the isothermal limiting equivalent conductances of these salts in the temperature range 100-800" (1) Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corp. Presented before the Division of Physical Chemistry a t the 156th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 8-13, 1968. (2) Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pa. 15213. (3) A. S. Quist and W. L. Marshall, J . Phys. Chem., 70, 3714 (1966). (4) A. S. Quist and W.L. Marshall, ibid.,72,684 (1968). (5) A. 9.Quist and W. L. Marshall, ibid., 72, 2100 (1968). (6) A. S. Quist and W.L. Marshall, ibid., 72, 1545 (1968). Volume 73, Number 9 March 1969
