Abstract
IN a recent communication by Gozzo and Patrick1 the heat of formation of difluoromethylene was estimated at ⩽ −43 kcal/mole from an investigation of the kinetics of pyrolysis of chlorodifluoromethane. We completed a similar investigation of this pyrolysis in the range 530°–700° C some time ago and obtained slightly different results (to be submitted for publication elsewhere). We believe that the main reaction steps are as follows: ( The inhibiting effect of HCl (that is, reaction 2) was confirmed by copyrolysis of CF2HCl and HCl. Further, pyrolysis of a mixture of CF2HCl with HBr led to CF2HBr as the main product. Reaction 4 was inferred from the reversibility of the overall reaction: The function p(C2F4)p2(HCl)/p2(CF2HCl) has the form of an equilibrium constant, and in static system investigations at 600° C this function was found to increase with time and attain a maximum value, after which it fell slowly because of the formation of side products. It was possible to extrapolate the curve and make an estimate of what the final value of the function would have been had there been no side reactions. A value of 8.7 × 10−2 atm. was obtained. Published thermodynamic data2,3 lead to a value of 30.1 cal/mole deg for ΔS° for reaction (5) and hence to a value of 30.5 kcal/mole for ΔH°, at 600° C, which corresponds to ΔH° = 29.1 kcal/mole at 25° C. If the heat of formation of tetrafluoroethylene is taken as − 151.5 kcal/mole, this leads to a value of − 112.3 kcal/ mole for the heat of formation of Chlorodifluoromethane at 25° C, which is close to several recent estimates1,4–6.
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References
Gozzo, F., and Patrick, C. R., Nature, 202, 80 (1964).
Weissman, Herman B., Meister, Arnold G., and Cleveland, Forest F., J. Chem. Phys., 29, 72 (1958).
Mann, D. E., Acquista, Nicolo, and Plyler, Earle K., J. Res. Nat. Bur. Stand., 52, 67 (1954).
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Maslov, P. G., and Maslov, Yu. P., Chem. Abs., 53, 1910 h (1959).
Skuratov, S. M., and Kolesov, V. P., Zh. fiz. Khim., 35, 1156 (1961).
Park, J. D., Benning, A. F., Downing, F. B., Laucius, J. F., and McHarness, R. C., Indust. Eng. Chem., 39, 354 (1947).
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EDWARDS, J., SMALL, P. Pyrolysis of Chlorodifluoromethane and the Heat of Formation of Chlorodifluoromethane and Difluoromethylene. Nature 202, 1329 (1964). https://doi.org/10.1038/2021329a0
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DOI: https://doi.org/10.1038/2021329a0
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