NOTES
Feb., 1961
375
It is noted that for initial ICN and KI concentrations less than 0.01 M , K1 is essentially independent of reactant concentration, indicating that the assumption that pH = -log UH+ and application of the I- activity corrections are justified in these circumstances. The average value and standard deviation for K1 in runs 1-11 is 0.94 i 0.05. There is a slight upward trend in the separate averages of the groups of runs at different reactant concentration in this range but the experimental error is such that the trend is not significant. ICN I12CN(3) The acceptable constancy of K1 by our method of calculation a t low concentrations of reactants would ICN + CYI(Ch')2(4) appear to rule out any significant contribution to and the dissociation constant1z of HCN, K s = the equilibrium by the so-called "anomalous I + 6.1 X at 25". containing species" mentioned earlier. The large From stoichiometry, it follows that13 disparity between Kovach's corrected value5 for K1 and the present one is not cause for concern, (1-1 = (KI)o - [(L)li (121, 2(13-) (LC?U'-)l since Kovach's conductivity method3was undoubtand edly subject to considerable error. The values for (I(CN)z-) + (CN-) + (HCS)h + (HCN), = K z have been calculated on the assumption that (I2)h (12)~ (Id-) 71- Y I ~ - , and they are seen to be subject only to random error in the low-concentration runs. The whence it is seen that mean and standard deviation of K z for runs 1-11 is (0.71 f 0.04) X lo3,in excellent agreement with other determinations of this c ~ n s t a n t . ' ~The ~~~ limited data a t ICY and K I concentrations in excess of 0.01 171 suggest that both K1 and Ks beand come greater but more experiments would be neces[(IZ)h D1a (I3-)] sary to determine the exact nature of this effect. (HCN), = Financial support of this work through Contract 11 DHCN &&(ICN)/(H+) Ks(H+)I KO. AT(30-1)-1578 between the U. S. Atomic EnI n practice, the denominator of the expression for ergy Commission and the University of Buffalo is (HCN) reduces to unity, since all the other terms gratefully acknowledged. are negligible under the experimental ~0nditions.l~ (16) A. D.Awtrey and R. E. Connick, J . A m . Chem. Soc., 73, 1842
mean and standard deviation of 35.21 f 0.13 at 25". Dr2 is also independent of total (Iz), as expected on comparison with the carbon tetrachloride/water distribution of i ~ d i n e . ~ Table I11 summarizes the data on the equilibrium constants K1 and Kz. The method of calculation makes use of the distribution coefficients reported above, the known equilibrium constants5 Ks = 1.17 and K4 = 2.5 at 25" for the aqueous reactions
+
Jr
+
+
+
+
+
+
(3x3 +
+
-
+
(1951).
TABLE I11 EQUILIBRIA IN AQUEOUS ICN Run no.
Initial (ICN) = (KI), M
pH
Total ionic strength'
+ KI 71-b
(17) L. I. Katzin and E. Gilbert, ibzd., 77, 5814 (1955). AT
Kio
25" Kz X 10-a
1 0.003 5.79 0.042 0.85 0.85 0.64 4.96 ,012 2 ,003 .90 .89 .70 3 ,003 4.98 .010 .91 .93 .71 4 .003 5.03 .010 .91 .93 .69 5 .006 5.94 ,045 .85 .96 .67 6 ,006 5.06 ,015 .89 .87 .73 7 ,006 5.13 .013 .90 .93 .79 8 .006 5.18 .013 .90 .99 .71 9 ,009 5.96 ,045 .85 1.00 .72 10 ,009 5.25 ,017 .89 0.98 .73 11 ,009 5.36 .016 .89 1.00 .68 12 .03 5.53 .037 .86 1.08 .79 13 .06 5.62 .067 .83 1.73 .95 Includes ionic reactants and phosphate buffer. * Interpolated from data of Latimer.*E Corrected for activity of
I -.
(12) K. P.$ng, J . Chem. SOC.,3822 (1959). (13) It is assumed that the concentration of ionic species in heptane is negligible. (14) I n a typical run (No. 3), 2.63 ml. of 0.00783 N thiosulfate was needed t o titrate 50 ml. of heptane phase: absorbancy of 11 in heptane phase was 0.4235 and of 11- in water phase was 0.743; aH+ was 1.05 X 10-6. Whence, [(Id (ICN)], = 2.06 X 10-4 M , (Idh = 4.75 X 10-6 M , and ( h - ) = 2.81 X 10-8 M , leading t o K e x p = 0.845 a n d RI = 0.93. (15) W. &I. Latimer, "The Oxidation States of the Elements a n d Their Potentials in Aqueous Solutions," 2nd Ed., Prmtice-Hall, Inc., New York, N. T.,1952 !I. 3-53>
+
THE ACTIVATION Eh'ERGY FOR HYDROGEK ST0,V ADDITION TO PROPYLENE BY MILTOXD. SCHEER AND RALPH KLEIN Natzonal Bureau of Standards, Washmgton, D. C. Received July 25, 1960
The kinetics of the addition of hydrogen atoms to propylene to form isopropyl has been studied in both the gas'-j and condensed Melville and Robb2 measured a gaseous H atom concentration by the molybdenum oxide technique and determined the rate constant to be 1.3 X 1011cc./mole sec. a t 291°K. They calculated an activation energy of 2.6 kcal., assuming a steric factor of lo-?. Callear and Robb4measured H atom concentration with a sensitive platinum resistance thermometer and found a value of 4.8 X 10" for the room temperature rate constant. Back5 measured the com(1) B. 9. Rabinovitch, S G. D a h and C. A. Winklei-, Can. J . Res , B21, 251 (1943). (2) H.W. Melville and J. C. Robb, PTOC. Roy SOC.(London), A196, 494 (1949); A202, 181 (1951). (3) B. deB. Daruent and R. Roberts, Disc. Faraday SOC.,14, 55 (1953). (4) A. Callear and J. C. Robb, Trans. Faraday Soc.. 61, 635 (1955). (5) R. A. Back, Can. J . Chem, 87, 1834 (1959). (6) R Klein and M. D. Scheer, THIEJOCRNAL62, loll (1958). (7) >I, D. Rcheer and R. Klein, ibid., 63, 1517 (1939),
376
Vol. G5
NOTES
measured the rate of this reaction as a function of temperature in dilute propylene films below 100°K. and find a considerably lower activation energy. Experimental A schematic diagram of the apparatus used is shown in Fig. 1 . Dilute (10:l) mixtures of propylene (in either propane or Freon-12) were made from gases containing less than 0.1 mole % impurity. The flat surface (1)is initially immersed in refrigerant and valve (6) quickly opened to the reactant sample volume (8) containing 8 micromole of propylene and 80 micromoles of dihient. In this way a film about cm. thick is condensed on the cold surface. The entire reaction vessel is then immersed in the refrigerant. Twenty micromoles of hydrogen is admitted through a heated palladium thimble. The tungsten ribbon is heated electrically to a temperature of 1800'K. and maintained constant. After an arbitrary time, the hydrogen is pumped away, the products warmed up, collected and analyzed for residual propylene by gas phase chromatography. Different mixtures of liquid oxygen and nitrogen were used as refrigerant baths. Temperatures were measured with a calibrated copper-constantan thermocouple.
Results and Discussion Unlike butene-1, 3-methylbutene-1 or isobutene, propylene does not produce H D with D a t o r n ~ . ~ J The reactions which occur below 90°K. in propylene films are9 W
ki + CH&H=CH* + CHVeHCH3 kz 2CH36HCH3 +CH&HlCI13 + CH,CH=CH? H
I
S
I
-0.8
1
-1.0 0
k3 CH3CHCHz 2CH3CHCH3 -+ I CH3CHCHs
2400 3600 4800 6000 Time (sec.). Fig. 2.--.Rnte of propylene depletion by H and D atoms in thin, di1ut.e films a t 77.3"K.
-2.6
1200
The ratio k2/k3is about 9, independent of temperature from 77 to 90°K. and concentration up to one hundred-fold dilution of the ~ r o p y l e n e . ~Assuming a steady-state concentration for R. the following expression for the propylene concentration as a function of time ( t ) results
where (CaH&is the propylene concentration at t = 0 and (H) is the hydrogen atom concentration in the soIid film. It has been demonstrated6previously \ & -2.8 brd that the reaction rate in the film is proportional to the gaseous H atom concentration generated by -3.0 the dissociation of Hz on a hot tungsten surface. Further if the films are sufficiently thin, and dilute I -3.2 in olefin, the hydrogen atoms diffuse with sufficient rapidity so that their concentration is essentially -3.4 1 _I--constant through the film.9 1.10 1.111 1.14 1.16 1.1s 1.20 1.22 1.21 1.26 1.28 1.30 Figure 2 shows how equation 1 is followed a t 102/T. 77.3"K. when films of propylene, diluted with Fig. 3.--4rrhenius plot for the reaction 13 CH&II=CHg either propane or Freon-12, react with either D or + CH~CHCHI. H atoms. The 30% difference in rate between the H and D atom reactions, seen in Fig. 2, can probpetition for E[ atoms by propylene and propane a t ably be ascribed to a difference between (D) and ambient temperatures, and found a value near 10" (H) in the film. Similar measurements were made for the addition reaction. Danvent and Roberts3 a t different film temperatures up to the boiling measured the rate of Hz production from the pho- point of liquid oxygen. I n all cases, the reaction tolysis of H2Ei in the presence and absence of pro- rate was proportional to the propylene concentrapylene a t 298 and 478°K. They postulated a tion up to 90% conversion. This strict first-order mechanism for this process and derived an activa- behavior demonstrates that the H atom concentration energy of 5.0 kcal./mole with a steric factor of (8) A. N. Ponornorov and V. L. Talrose, DokZadg Acad. Sci. US. 0.5 for the addition reaction. Their rate constant S.R., 130, 120 (1960). was 1.6 X 1011 a t 300"K., in agreement with the (9) R. Klein, M. D. Scheer and J. G.Waller, T H r q JOURNAL, 64, 1247 values obtained by other investigators. We have (1960). h
\
"T
v
-
+
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).
