THE JOURNAL OF
PHYSICAL CHEMISTRY
Registered in U.S. Patent Oflce @ Copyright, 1968, by the American Chemical Society
VOLUME 72, XUMBER 6 JUNE 15, 1968
The Explosive Decomposition of Chlorine Dioxide1 by Edward T. McHale and Guenther von Elbe Kinetics and Combustion Group, Atlantic Research Corporation, A Dinision of T h e Susquehanna Corporation, Alexandria, Virginia 2Z251.4 (Receiced February 6 , 1068)
The explosive decomposition of chlorine dioxide has been studied over the pressure and temperature ranges 0.2-40 torr and 54-134’. The reaction is characterized by long induction periods which were measured as a function of temperature, pressure, vessel size, irradiation, added gases, and other chlorine oxides. The explosion is of the degenerate chain-branching type, and the intermediate responsible for the delayed reaction has been identified as C120s. The experimental results together with reliable enthalpy data and other studies on chlorine oxides allow a mechanism t o be written. It is shown that the observed characteristics of the explosive decomposition of C102 are fully consistent with the reaction-kinetic equations developed on the basis of the mechanism.
A. Introduction The know11 stable oxides of ch’-rine are chlorine monoxide, C1,O ; chlorine dioxide, ClOz ; chlorine hexoxide, ClzOe (which exists as c103 in the vapor phase) ; and chlorine Ineptoxide, Cl20,. I n addition, a fifth compound of empirical formula C101.5, probably the sesquioxide, C1203,has been discovered in the course of this study., The free-radical species C10 is known to exist3 and the peroxy radical, C100, probably also e ~ i s t s . ~ - jThis class of compounds has been the subject of considerable research over the past decades, and many publications have appeared dealing with their reactions and slow and explosive decomposition. The striking feature of all the work on the kinetics of these compounds is that no mechanisms have been proposed which account for the thermal decomposition of the stable compounds. The principal reason for this, in the case of the two simplest of the stable oxides, C1,O and ClOz, is Iths?futhe explosive decompositions are of a degenerate chain-branching type. Delayed explosions such as these are due to the buildup of an intermediate, which gives rise to a branched-chain reaction. The intermediates have not been identified, and hence it \vas not possible to postulate a decomposition mechanism, even though accurate rate data were available. Another iinpediment to understanding what elementary reactions have an important role in chlorine
oxides kinetics was an erroneous value for the heat of formation of the C10 radical. Thus an extensive discussion of such reactions by Szabo6is no longer relevant, since he based his arguments entirely on a value of 37 kcal/mol, whereas more recent spectroscopic data have established AHf’2~8for C10 as 24.2 kcal/mol.’ We report here a study of the explosive decomposition of CIOz in the range 54-134’ and 0.2-40 torr. Experiments were performed on the “induction time to explosion” as a function of pressure and temperature, the effect of vessel size and added gases, and the effect of addition of other chlorine oxides. The intermediate responsible for degenerate chain branching has been identified as C1203. The results of the present study together with those of Schumacher and Stiegers on the (1) This work was supported by the Air Force Office of Scientific Research, Propulsion Division, under Contract No. AF 49(638)-1645. (2) E. T . McHale and G. von Elbe, J. Amer. Chem. SOC.,89, 2795 (1967). (3) G. Porter and F. J. Wright, Discussions Faraday SOC., 14, 23 (1953). (4) S. W. Benson and J. H. Buss, J . Chem. Phys., 27, 1383 (1957). (5) S. W.Benson and K. H. Anderson, ibid., 31, 1082 (1959). (6) 2. G. Szabo, J . Chem. SOC.,1356 (1950). (7) “JANAF Thermochemical Tables,” The Dow Chemical Co., .Midland, Mich., 1964. ( 8 ) Ha-J. Schumacher and G. Stieger, 2. Phys. Chem., B7, 363 (1930).
1849
EDWARD T. MCHALEAND GUENTHER VOX ELBE
1850 thermal decomposition and the present reliable enthalpy data on the chlorine oxides allow us to write a mechanism which can successfully account for the behavior of CIOz over a wide range of conditions.
B. Experimental Section Chemicals. Chlorine dioxide was synthesized as needed by mixing KC103 with moist oxalic acid and warming to about 90". 2KC103
+ 2HzCz04
1
