H. F. Cordes and S. R. Smith
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compatible with the observed activation energies. The l Z g + level at 37 kcal above the ground state would be produced by reactions having heats of reaction greater than the observed activation energies. For both the unimolecular and the bimolecular initiation reactions, the most reasonable activated complex has Czu symmetry, and the symmetry allowed 0 2 product is If crossing-occurs between the two energy surfaces, then the low preexponential factor may be an indication of a low transfer coefficient. Under these circumstances, the preexponential factors would not be expected to be greatly affected by the nature of the medium. Since this is what is observed, one must conclude that the drastic rotational restriction previously proposed for the activated complex is in error, and the initiation reaction may well be unimolecular, or perhaps bimolecular with less restriction than had been assumed. In looking at Table I, ref 1, there appears to be a trend of increasing preexponential factor with decreasing cation size. This trend is probably not significant as the LiC104 mixtures with the other alkali perchlorates have rates the same as those for the pure material. This is in contrast to the conclusion drawn in ref 5. Several attempts were made to find an inert diluent for LiC104, as such a diluent would allow a direct test of the order of the initiation reaction. No inert diluent was found.
Supplementary Material Available. The graphs pertinent to the data will appear following these pages in the microfilm edition of this volume of the journal. Photo-
copies of the supplementary material from this paper only or microfiche (105 x 148 mm, 2 4 reduction, ~ negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-74-773. References and Notes ( I ) H. F. Cordes and S. R . Smith, J. Phys. Chem., 72, 2189 (1968). (2) H. F. Cordes, J. Phys. Chem.,'72, 2185 (1968). (3) M. M. Markowitz and D. A. Boryta, J. Phys. Chem., 65, 1419 (1961 ). (4) M. M. Markowitz and D. A. Boryta, J. Phys. Chem., 66,358 (1962). (5) F. Solymosi, Acta Chim. (Budapest), 57, 11 (1968). (6) H. F. Cordes and S.R. Smith, J. Phys. Chem., 78, 776 (1974). (7) E. A. Burns,AnaL Chem., 32, 1800 (1960). (8) F. Solymosi, Z.Phys. Chem. (Frankfurtam Main), 57, 1 (1968). (9) I. M. Koltoff and R. Belcher. "Volumetric Analysis, Vol. Ill, Interscience, New York, N. Y., 1957. (10) Throughout this manuscript X i will denote the anion mole fraction of the ClOi-1- anion. X ( 0 p ) is the moles of 02 produced per mole of clod- present' initially. X(Ag+) is the-cation mole fraction of Ag+. A superscript zero will denote a value at time t = 0. (11) M. M. Markowitz, D. A. Boryta, and H. Stewart, J. Phys. Chem., 68, 2282 (1964). (12) All heats of reaction are taken from 13. The values for the oxychloro anions are taken to be those for the infinitely dilute aqueous solutions. The values for the Na+ salts (solid) give nearly the same relative values, except that there is no value for NaOCI. (It is assumed that the differences between the heats of reaction do 'not change on going to the LiClOl system.) The quantities A and are positive quantities that may be needed for excitation of the products to conserve spin, and/or symmetry. (13) F. D. Rossini, National Bureau of Standards Circular KO. 500, U. S. Government Printing Office, Washington, D. C., 1952; Supplement, June 1965. (14) W. C. Hamilton, "Statistics in Physical Science," Ronald Press, New York, N. Y., 1964, Section 3-6.
Thermal Decomposition of Lithium Perchlorate. 11. The Chloride Catalysis H. F. Cordes* and S. R. Smith Chemistry Division, Research Department, Naval Weapons Center, China Lake, California 93555 (Received September 4, 1973) Publication costs assisted by the Naval Weapons Center
The decomposition of liquid lithium perchlorate is catalyzed by the chloride produced by the. reaction. The catalysis is first order in C1- . Catalytic rate constants obtained for very small degrees of conversion agree with those for high degree of conversion. The results are compared with other work and mechanisms are discussed.
Introduction The authors have reported on the initiation rate of the lithium perchlorate decomposition.l Markowitz and Boryta2.3 found that after initiation, the reaction was autocatalytic and was affected by Cl-. Solymosi4 has reported similar results. These results were obtained at high degrees of conversion of the perchlorate. The present work presents data a t low percentage conversion and demonstrates the order of the catalysis by the C1-, The amounts of C103- produced are also presented. The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
Experimental Section The procedure used to measure the rates of oxygen evolution using the mass spectrometer have been described previously.1 Those runs made by measuring the pressure of evolved gases in a closed volume used a previously reported system.5 In addition several decompositions were made in which a known weight (-0.5 g) of LiC104 was placed in a quartz tube which was then inserted into a Pyrex tube. The sample was heated to about 260", degassed, evacuated, cooled to room temperature, and
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sealed. The samples were then heated for known times at constant temperature, quenched, and analyzed for c103and C1-. Materials. Most of the chemicals were the same as used previously.1 In addition, reagent grade LiCl from Baker and Adamson was used. Analytical Procedures. The c103- was analyzed by a benzidine method.6 The chloride was analyzed potentiometrically. Results Gaseous Products. The gaseous products were the same as for inhibited AgC104l except that small amounts of Cl2 were present in the evolving gases. The amount of Cl2 was about one-third the amount of Cl02. For two decompositions made in sealed systems the ratio of moles gas evolved to moles LiC104 decomposed were 1.88 and 1.98. The gas when analyzed on the mass spectrometer was pure 0 2 . Any Cl02 and Cl2 were apparently lost in transfer. Analysis of Residues. Qualitative tests showed less than 10-4 anion mole fraction for Cl02- and C10- a t about 20% decomposition.7 The following relationship between c103- and C1- was found when X1 < 0.02*
x,= I + sx,
*
The least-squares parameters at 402" are I = (1.63 0.42) X and S = 0.142 0.004. The errors listed are one standard deviation. A graph is available in microfilm (see paragraph a t end of paper regarding supplementary material). Data were also gathered at 360.0, 381.8, and 420.0".The slopes of the lines are given in Table I under the column labeled dX4/dX1. The values are all least-squares parameters. Values of X04 and XO1 were obtained from samples that had been inserted in the furnace and removed after 12.5 min so as to correspond to zero time for the mass spectrometer data. The C1- produced was negligible and the C103- agreed with the intercepts of the X4 us. X1 lines on the Xq axis. This C103- is most likely produced along with the C02 during warm up.
*
Oxygen Evolution Rates LiC104 Alone. At constant temperature the rate of 0 2 evolution from a melt of LiC104 alone was observed to rise exponentially with time. dX(Oz)/dt = R exp Kt. A typical plot is in microfilm. The extent of decomposition was never more than 1 or 2%. In some runs, particularly at higher temperatures, the curves were observed to flatten at long time. This is believed to be due to distillation of LiCl out of the mixture a t the low pressures (
