Feb., 19G1
?;oms
tion remains unchanged even after the reaction rate has dropped to 1/10 of its initial value. Therefore under the conditions of these experiments the rate of supply of H atoms from the gas is considerably greater than their rate of depletion due to reaction with the propylene in the film. The results plotted in Fig. 3 in the Arrhenius form yield an activation energy of 1500 cal./mole showing no appreciable isotope effect when D instead of H is the reactant. The data in Fig. 2 show that a change of diluent from propane to Freon-12 has no effect upon the reaction rate. It is unlikely that strong interactions between diluent and reactant exist and the activation energy obtained approximat)es that for free molecules in the gas. Darwent and Robert's value of 5 kcal./mole was determined from measurements at only two temperatures and depended upon their postulated mechanism for the H2Sphot~lysis.~Their result is well outside of any reasonable limits of error in our determination provided the H atom concentration in the film does not change significantly in the 77 to 90°K. temperature range of these experiments. From our measured El, the 300°K. rate constant, and a 5.5 k. collision diameter for H C3H6, a steric factor of 3 X lop3is computed. Evans and SzwarcIogive a value of for propylene addition reactions on theoretical grounds. The value of 0.5 given by Darwent and Roberts appears to be too large. The results in Fig. 3 show that kl(H) = 18 exp( - 1500/RT) set.-'. The maximum gas phase concentration of H atoms, calculated from the hydrogen pressure and temperature of the tungsten surface, is 4 X lO-'O moles/cc. It is of interest to note that if (H) in the film is about a factor of 10 lower than this maximum gas phase value, kl = 10l2 exp(-l500/RT) and 1011 is obtained for the rate constant at 300°K.
+
A
o Method
I I
0
"
0
$1
I II
m
.\
I
20
,
,
40
1
,
60
80
100
% RbCI. Fig. 1.-Comparison of the Pm3m disappearance temperature (curve A) with the click temperature (curve B). Three different cooling procedures were used in the determination of curve B.
pure cesium chloride, which readily undergo the Fm3m-Pm3m transition upon cooling, do not exhibit the click phenomenon and at room temperature these crystals are somewhat rubber-like and quite difficult to grind. However cesium chloride (IO) M. G . Evans and M. Szwarc, Trans. F a m d a y Sac., 46, 940 containing as little as one mole per cent. of rubidium (1949). distinctly shows the click phenomenon! It has been shown' recently that a complete series of solid solutions of rubidium chloride and high REACTIONS BETWEEN DRY INORGANIC temperature (Fm3m) cesium chloride is stable SALTS. XI. A STUDY OF T H E Fm3m --t over certain temperature ranges. In the high Pm3m TRANSITION I N CESIUM CHLORIDE- rubidium chloride end of the series, the solid solutions are stable at room temperature but at lower RUBIDIUM CHLORIDE MIXTURES rubidium chloride percentages a partial unmixing of BY LYMAN J. WOODWITH GERVASIOJ. RICONALLA AND the solid solution occurs as the temperature is JOSEPH D. LAPOSA lowered. Department of Chemistry, Saint Louis University, Saint Louis, Missouri In the previous paper' it was shown that the solid Received J u l y $9, 1960 solution of rubidium chloride and high temperature In the course of a recent study of solid solutions cesium chloride, which has Fm3m symmetry, is of rubidium chloride and high temperature cesium stable in the region above curve A of Fig. 1. Upon chloride' a curious phenomenon was observed when cooling from above curve A to room temperature it working with crystals which were formed by freez- was found that some of the high temperature cesium ing low rubidium chloride (under 35 mole %) melts. chloride undergoes the Fm3m + Pm3m transition When these crystals, which were translucent, were and separates out of the solid solution as pure low cooled to some 300 to 400"below the freezing point, temperature cesium chloride. The work reported they changed quite suddenly to a white opaque in this note shows that the unmixing of solid solumass and the change was accompanied by an evolu- tions in the 5 to 35 mole % rubidium chloride range tion of heat and an audible click. The opaque white does not begin at curve A but rather that there is a mass was quite brittle and easily reduced to a fine long delay before unmixing can be detected. When powder by slight grinding. By contrast crystals of the solid solution crystals are prepared directly from the melt, the delayed unmixing temperatures (1) L. J. Wood, Chsa. Sweeney, S.J. and Sr. M. Therese Derbes, (click temperatures) are represented by curve B. J . A m . Chem. SOC.,81, 6148 (1958).
KOTES
37s
T'ol. 65
TABLE I X-RAY( H I G ITEMPERATURE ~ CAMERA) RESULTS FOR VARIOUS HEATTREAThIEPL'TS OF TIIE 90: 10 CsCI-RhC1 Film no.
La-31
PrehEat,b
C
398
Subsequent treatment
Exposed to X-rays a t oncec
X-Ray lines after 4 hr. Exposure to Mo rays Pm3m Fm3m
Remarks
Above completion temp. of Pm3mFm3m previously found a t 395" La-40 392 Evposed t o X-rays a t onceC Weak Strong Below completion temp. of Pm3mFm3m La-41 392 Cooled to 1%'-exposed to X-rays at oncecjld Medium Strong With Pm3m initially present, the amt. of this phase became greater upon cooling La-30 423 Cooled to 68°-exposed to X-rays a t once Strong Medium Strong Pm3m lines-low reappearstrong ance temp.-delayed unmixing of Fm3m solid soln. indicated This mixture was melted, cooled to room temperature and reduced to a fine powder by grinding. The fine powder was screened through a 200 mesh sieve and placed in a fine capillary tube for X-ray analysis. After each X-ray exposure, the sample was allowed to cool to room temperature (this wa8 necessary for the removal of the film from the camera). This temperature was: approached from below and maintained constant (&0.75O) by careful hand control for 30 minutes before X-ray exposure. Temperature maintained constant (10.75') during X-ray exposure by means of thyratron control. This temperature was approached from above and care was taken not to drop below this selected temperature a t any time.
Rapid cooling of melts containing one to five mole per cent. of rubidium chloride exhibited the click phenomenon but slow cooling (cooling over a period of from three to five hours) of these mixtures brought about gradual unmixing of the solid solution and no sudden evolution of heat corresponding to the click phenomenon was observed.2 The high temperature solid solutions were readily formed again by reheating from below the click temperature (curve B). Upon cooling these reformed solid solutions however, the click phenomenon never was observed although the high temperature X-ray camera showed definitely that the phenomenon of delayed unmixing again occurred. It can now be concluded that the audible sound observed simultaneously with the click phenomenon 1i3 caused by the sudden shattering of the relatively large solid solution crystals obtained directly from .the melt. The white mass resulting from the delayed unmixing of these large crystals was found to be made up of extremely fine crystals3 of Pm3m cesium chloride and residual Fm3m solid solution and the extreme brittleness of the white mass (described above) obviously is due to this mixture of fine crystals. Heating these small crystals to a temperature above curve A forms only small crystals of the Fm3m solid solution which crystals do not produce an audible click as they unmix upon cooling. Working with a 9O:lO mixture of these small crystals it wa,sfound possible to delay greatly the beginning of uinmixing by rapid cooling in a capillary tube. Th.e 90: 10 mixture was first heated in (2) I n 1910, S. Zernczuzny and F. Rambach reported (2.anorg. Chem., 6 5 , 418 (1910)) freezing curves for small additions of either potassium chloride or rubidium. chloride to cesium chloride in rough approximate agreement with the first part of curve A rather than curve B. This report would seem t o indicate that rapid cooling might be required t o cause a delayed Fm3m-Pm3m transition as represented by curve B. (3) A small amount of a n 85: 15 molar mixture of cesium chloride and rubidium chloride w & s melted a n d drawn into a capillary sample tube where i t was allowed to freeze and then cool through the click temperature range t o room temperature. Even though the sample was not ground and screened as is usually done, satisfactory X-ray lines were nevertheless obtained which showed clearly t h a t the unmixing of the d i d solution associated with the click phenomenon produced a mixture of very fine powders.
None
>\IIXTCREn
Strong
the high temperature X-ray camera t o a few degrees above curve A (423"-Table I) until re-formation of Fm3m solid solution was complete after which the sample was cooled to a series of successively lower and lower temperatures (with reheating to 423" between each test) until the appearance of the low temperature (Pm3m) form of cesium chloride was first observed a t 68" (La-30). This long delay in unmixing was avoided by beginning the cooling from just below curve .A at which point the mixture still contained a trace of t>he low temperature cesium chloride (La-41).
HEATS OF FORMSTIOIV OF a-PHASE SILVER-INDIUM -4LLOYS' BY RAYMOND L. ORR A Y D RALPHHULTGRES Department of Mznernl Technologzi, Unzaersit?~of Californza, Berkeleu, Cali/omza Recezued Julu 29, 1960
Heats of formation of the Ag-rich terminal CYsolid solutions of Cd, In, Sn and Sb in Ag a t 723" K. have been measured by Kleppa.2-i The trend in the experimental values showed the heats of formation to become less exothermic with increasing atomic number of the solute except for In, which forms a more exothermic solution than any of the others. This could be interpreted as indicating an anomalously high affinity between Ag and In. However, in other criteria of bond strength such as the effect of solutes on the lattice constant of silver, In falls in a normal sequence between Cd and Sn. It therefore seemed worthwhile to independently measure heats of formation of solid solutions of In in Ag. This has been done as reported in this paper. The relationship between heats of formation and bond strengths also is discussed. (1) This work sponsored by Office of Ordnance Research. U. 9. Army. (2) 0. J. Kleppa, J . Am. Chem. Soc., 7 6 , 6028 (1034). (3) 0.J. Kleppa, Acta M e t . , 3, 255 (1955). (4) 0.J. Kleppa, THIS JOURX.AL, 60, 846 (1956).
