Some Aspects of the Theory of Unimolecular Gas Reactions

• Published in: Proceedings of the Royal Society of London. Series A, Mathematical and physical sciences Mathematical and physical sciences, 1959-02, Vol.250 (1260), p.121-131
• Main Authors: Gill, E. K., Laidler, K. J.
• Format: Article
• Language: English
• Subjects: Chlorides; Degrees of freedom; Hydrogen; Kinetics; Molecules; Nitrogen; Nitrous oxide; Ozone; Peroxides; Vibration
• ISSN: 1364-5021; 0080-4630; 1471-2946; 2053-9169
• DOI: 10.1098/rspa.1959.0055

An examination is made of the experimental data for the low-pressure second-order uni­molecular decompositions of ozone, nitrous oxide, hydrogen peroxide, nitrogen pentoxide, ethane (into two methyl radicals), cyclopropane and ethyl chloride. The rates are considered in the light of the theories of energization due to Hinshelwood, Kassel, and Rice & Rams-perger (H. K. R. R.) on the one hand, and to N. B. Slater on the other. For nitrous oxide, hydrogen peroxide and ethane Slater’s rates of energization are too low by a significant factor, whereas the H. K. R. R. theories can give a satisfactory interpretation. For ozone, nitrogen pentoxide, cyclopropane and ethyl chloride the Slater rates of energization appear to be of the correct order of magnitude, while the H. K. R. R. rates are too high unless one employs fewer degrees of freedom than are actually in the molecule. These results are explained on the hypothesis that Slater’s theory is correct as far as the breakdown of the energized molecule A* is concerned, but is not always correct with regard to the rate of formation of A*. If flow of energy between the normal modes can take place a molecule A', energized in the H. K. R. R. sense but not having the right distribution of energy to be energized in Slater’s sense, may become an A*. Reasons are advanced for believing that in small molecules the flow of energy will tend to be more rapid than in large ones. A steady-state treatment of the overall process is presented.