NOTES
Nov., 1960 grown by evaporation of PbF2at 1150' were always very thin plates and exhibited the largest ratio of rate in the a-direction to rate in the c-direction. The reason for this change in habit is not known, but it probably arises from a rather subtle change in the structure of the PbF2-ZnO melt. The possibility that a buildup of PbO was responsible in some way for the habit change was eliminated by growing crystals in melts to which PbO had been added. Yo change in growth characteristics was observed. At present, no experimental studies of such melts which could lead to an explanation of the habit change have been made. I n conclusion, large single crystals of ZnO which possess an unusual and convenient habit have been grown from molten lead fluoride. Such crystals already have been used successfully as seeds in hydrothermal crystal growth experiment^.^ The combination of the two methods should lead to growth of the first very large and pure crystals of ZnO. Acknowledgments.-The authors wish to thank W. Hartmann and E. Bloom for the excellent chemical analyses required for this work.
fer were carried out either i n vacuo or in a carbon dioxide-free dry box. The heat content of the capsule was determined by separate experiments over the temperature range investigated. A platinum vs. platinumrhodium thermocouple, previously calibrated against a National Bureau of Standards therniocouple, was used for furnace temperature measurements. The measured heat contents of NazO above 298' K. are given in Table I. The results are based upon a molecular weight of 61.98' for Xa20 and are expressed in defined calories per mole (1 cal. = 4.1840 abs. joules). Corrections for the sodium carbonate impurity were made using the integrated heat capacity data of Popov and Ginzbergs and for sodium peroxide using the data of Chandrasekharaiah, Grimley and M a r g r a ~ e . ~It was necessary to extrapolate the data for Na2C03 and NazO2 to higher temperatures in order to correct the sodium oxide data. Measurements at higher temperatures were prevented by the attack on the gold solder used in fabricating the capsule by either the sodium oxide or the impurities. TABLE I JIEASURED HEATCONTENT OF SODIUM OXIDEABOVE 298.15'' K. (NazO, mol. wt. = 61.98)
THE HIGH TEMPERATURE HEAT CONTENT OF SODIUM OXIDE' BY ROBERT T. GRIMLEY~ AND JOHN L.
T,OK.
MARQRAVE
380.1 477.2 577.2 673.5 776.4
Department of Chemistry, University oJ Wisconsin, Madieon, Wisconsin Received M a y 6 , 1960
A review of available literature indicates that the only high temperature heat content data reported for the alkali oxides are those of Shomate and (>ohen3 for lithium oxide. In addition, Naylor4 has estimated the high temperature heat content of sodium oxide by employing the NeumannKopp rule in conjunction with experimental data for NazTi03, Na2Ti20s,Na2Ti30, and TiOz. Experimental values for the heat content of sodium oxide are presented in this paper. Measurements were made using a copper block drop-type calorimeter which has been described previou~ly.~The sample of sodium oxide was prepared from commercial grade du Pont sodium oxide by purification according to the method of Klemenc, Ofner and Wirth6and X-ray examination of the product showed only the lines of sodium oxide. Analyses were made for NazO, NazOz and h-azCOs and the sample mas found to consist of 96.76% Na20, 2.33% Na2C03 and 0.91% Na20z. The presence of KaOH was ruled out on the basis of indirect analysis. The sample (9.2728 g. in vacu,o) was t'ransferred to x gold capsule, the neck of the capsule squeezed shut and then soldered with gold. All operations of preparation or t,rsns(1) Presented before the 134th Meeting of the American Chemical Society, Chicago, Illinois, September 8, 1958. (2) Department of Physics, University of Chicago, Chicago, Illinois. (3) C . H. Shomate and A . J. Cohen, J . A m . Chem. Soc., 7 7 , 285 (1955). (4) B. F. Naylor, ibid., 67, 2120 (1948). (5) J. L. Margrave and R. T. Grimley, THISJOURNAL,62, 1436 (1958). (6) A. Klemenc, G. Ofner and IT. Kirth. %. anorg. Chem., 265, 221 (1951).
1763
HT
- H298.15,
cal./mole
T, OK.
1418 3191 5257 7190 9276
876.0 980.4 1078.3 1174.6
HT
- Hsoe.ls,
cal./mole
11,447 14,045 16,539 19,409
The high temperature heat content data and the heat capacity may be represented by the equations HT
- H t ~ d =. ~14.497' ~ + 4.94 X
10-W' -4799 cal./mole (29&1170°K.; 1 2 % ) CP = 14.49 9.88 X 1O-T cal./deg./mole
+
A plot of the Shomate functionlo using the low temperature heat capacity data of Furukawa" gave some evidence of a low energy transition in the range 500-800'K. A study of this range using adiabatic methods on a sample of higher purity would appear to be desirable. Destruction of the capsule during measurements at higher temperatures prevented further study by the drop method. From the above equations and a value for S2g8 determined by Furukawal' to be 17.99 e.u., the heat contents, entropies and the free energy functions were calculated at 100' intervals and the results are listed in Table 11. I t is interesting to note that the sodium oxide heat content which Naylor4 estimated by subtracting the a-Ti02 contribution to the XazTiOS heat content agrees remarkably well with the experimental values. The agreement with Naylor's mean values obtained from data on additional compounds is somewhat (7) E. Wichers, J . A m . Chem. Soc., 80,4121 (1958). (8) A I . M. Popov and D. M. Ginrberg, J . Gen. Chem. (U.S.S.R.), 26, 1103 (1956). (9) M. S. Chandrasekharaiah, R. T. Grimley and J. L. Xargrave, Tms JOURNAL,63, 1505 (1959). (10) C. H. Shornate, rbid., 58, 36s (1954). (11) G. E'urukawa, private communication.
NOTES
1764
VOl. 64
poorer, but, in general, does not differ by more than lOql,.
Preparation of Cadmium Chloride.-The anhydrous cadmium Chloride used waa prepared from Baker Chemical Company analytical reagent cadmium chloride containing 2.5 molecules of water. The anhydrous salt was prepared TABLE I1 by recrystallizing CdC12.2.5H20from an aqueous solution, SMOOTHED HEATCONTENTS, ENTROPIES A N D FREEENERGYslightly acidified with hydrochloric acid. These crystals were dehydrated in a tube oven, heated to 150°, through FUNCTIONS FOR SOLIDSODIUJ~ OXIDE which was passed a stream of dry hydrogen chloride. (NazO, mol. wt. = 61.98) The excess hydrogen chloride was removed by means of a current of dry nitrogen. The resulting salt then was Hi,- H n e s . ~ , ST - s ~ s . 1 6 , (FT -THnga"') T ,OK. csl./mole cal./deg./mole cal./deg./mole evacuated until successive samples, taken a t hour intervals, gave a constant pH reading. 400 1 ,789 5.27 18.79 Measurement of pH.-The pH wag measured with a 500 3 ,683 9.49 20.11 Beckman Laboratory Model G pH meter using a Beckman 600 5,676 13.12 21.65 general purpose shielded glaas electrode and a fiber type sealed calomel Beckman electrode. The hydrolysis cell 700 23.23 7,768 16.31 contained openings for two electrodes, a thermometer, a 800 9 ,959 19.27 24.81 conductivity water inlet and a nitrogen inlet and. outlet. 12,247 900 26.34 21.96 The cell containing the wei hed cadmium chlon.de was 14,636 1000 24.48 27.83 flushed with nitrogen gas, before and durlng the time the 17,122 1100 29.27 purified water was added. A magnetic stirring bar agitated 26.85 the solution for 15 minutes before the pH readings were Acknowledgments.-The authors wish to ac- taken. When cadmium chloride is placed in water, these dissociknowledge the support of this research by the United States Navy through the Callery Chemical ation and association reactions may occur
Company under a contract with the Bureau of Aeronautics.
HYDROLYSIS OF C14DMIU&1CHLORIDE AT 25' 1 3 KARL ~ H. GAYERA N D RUDYM. HAAS Department o f Chemtstrv, Wayne Stale University, Delrozt, Mzchigan Received Y a y 0, 1060
The study of the hydrolysis reactions and the evaluation of equilibrium constants for cadmium chloride has been undertaken by other investigators. If a true hydrolysis constant can be obtained for any species of cadmium chloride over a definite concentration range, it can be related t o an over-all reaction. The hydrolysis constants (Kh) and the reactions predicted by others are: at 100" for Kullgren' reported a Kh of 3.3 X the reaction CdClf €320 CdClOH H+; Chaberek, Courtney and Martel12 calculated R Kh of 2.5 X lo-'* at 30" in a 0.1 M potassium chloride medium for the reaction Cd++ H20 Cd(0H) + H+. According t o this last reaction it is assumed that the chloride ion from the potassium chloride has no effect on the hydrolysis reaction of the Cd++ ion. Doubt is cast on this assumption because it is further assumed that the concentration of the Cd++ ion is equal to the total concentration of cadmium chloride used. Our results seem to indicate that the major hydrolysis product at 25" is CtlC1OI.Z.
+
+
+
+
Experimental Purification of Water.-Water was prepared in a manner similar to that described by Gayer and Woontner.3 Ordinary distilled water was distilled in a Pyrex distillation apparatus from an oxidizing solution of sodium hydroxide and potassium permanganate. The resulting water was then redistilled by adding one drop of orthophosphoric acid per liter of water. Last traces of carbon dioxide were removed by boiling the h a 1 distilled water for about 15 minutes. Purification of Nitrogen.-Purified tank nitrogen was passed through several gas washing bottles of dilute sodium hydroxide, sulfuric acid and water. (1) C. Kullgren, Z. physik. Chem. (Leiprig), 86, 466 (1913). (2) 9. Chaberek, Jr.. R. C. Courtney and A. E. Martell, J . Am. Chem. Soc., 74, SO57 (1952). (3) K . H. Gayer and L. Woontner, J . Chum. Ed., 93,296 (1956).
+ +
CdC12 CdCl+ C1C d C l + s Cd++ C1CdClz C1CdCLCdCli 2C1- If CdCla"
+
(1) (2) (3)
+
(4)
In order to obtain the activities of these species, it is necessary to know the dissociation constants of the reactions involved. These are not available as such. As an approximation, however, it was assumed by Harned and Fitzgerald that the first reaction goes to completion, the third and fourth do not occur, and the second is the only one in which an equilibrium is involved.* By means of electromotive force measurements, they calculated the dissociation constant ( K ) for this reaction to be 1.1 X lo-*. Other Kt values given in the literature, assuming only the presence of Cd++, CdC1+ and C1-ions are 0.0106 and 0.0101.6
- aCd"
2--
aCl-
aCdCIC
- CCd* CCl-
.fCdHfCI-
CCdCl+
fCdCIC
Here a, C and f refer to the activity, concentration and activity coefficient, respectively. The various concentrations of CdCl+, C d + + and C1- ions can be evaluated by setting X . = Cd, X M = C1- and M - X = CdCl+, where M IS the molar concentration of the cadmium chloride used. The activity coefficients can be obtained by solving the regular form of the Debye-Huckel equation
+
where Zi is the charge on the ion, a, is its effective diameter and u is the ionic strength of the solution. The constants A and B were taken as 0.509 and 0.330, respectively, and.the effective diameters7 of the Cd++, C1-, CdC1+ and H + ions were taken as 5 X 10-8, 3 X 10-8, 4 x 10-8 and 9 X 10-8 cm., respectively. By the method of successive approximations, the following ionization equation can be solved by successively approximating X, the concentration of the Cd++ion. The activity coefficient of the Cd++ion, fx is given in the regular form of the Debye-Huckel equation
logf, =
-2 . 0 4 d 4 r M
1
+ 1.65 X 10-8-
(4) H. 9. Harned and M. Fitrgerald, J . A m . Chem. SOC.,68, 264 (1936). ( 5 ) H. L. Riley and V. Gallafant, J . Chem. SOC.,514 (1932). 2 6 , 592 (6) E. C. Righellato and C. W. Dsvies, Trana. Faradoar SOC., (1930). (7) J. Kiellsnd, J . Am. Ckem. Soc., 59, 1 6 i 5 (1937).
