Third Report of the Committee on Photochemistry Reprint and Circular Series of the National Research Council (1938) / Chapter Skim
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Specific Reaction Rates in Photoprocesses
Specific Reaction Rates in Photoprocesses
Pages 75-84
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From page 75... ...
K ROLLEFSON Department of Chemistry, University of California, Berkeley, California Received May 25, 1988 One task involving photochemical chain reactions which has been undertaken by many investigators is that of evaluating the specific reaction rates (or rate constants)
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From page 76... ...
Many studies with atomic hydrogen have shown that the recombination occurs at every collision during which a third body is present to remove some of the energy (2, 10, 32~. Similarly, a comparison of the thermal and photochemical rates of formation of hydrogen bromide has led to the conclusion that bromine atoms recombine by a
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From page 77... ...
The principal difficulties in the way of securing exact values for the rate constant of such a chain-terminating reaction are the determination of the accommodation coefficient and the rate of approach to the wall. The latter rate is complicated by the fact that, usually, the heat of reaction sets up convection currents which make it
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From page 78... ...
They are probably not large, but even a small activation energy would introduce a rather large uncertainty into the specific reaction rate. Until further data are available, we must conclude that the constants for reactions between two radicals are not sufficiently well known to be used in calculating rates of photochemical chain reactions.
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From page 79... ...
The rate of the reaction Cl + H2 > HCl + H was measured directly by Rodebush and Klingelhoefer (24) , who used a known concentration of.chlorine atoms produced by an electric discharge in chlorine gas.
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From page 80... ...
have presented data which indicate that only one collision in six thousand of chlorine atoms with a silver chloride surface leads to the formation of molecules. This efl ect does not seem to be due to a heat of activation but rather is an accommodation coefficient analogous to the steric factor for bimolecular reactions.
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From page 81... ...
Thus, instead of having the overall rate constant expressed in terms of the constants for initial and final steps and one step of the chain (k2 in the case of the formation of hydrogen chlorides we find that the constants of two or more steps of the chain appear in the rate equation. Such constants are indeterminate from rate measurements alone, and up to the present time no one has determined the constants for any system of this type from experimental data.2 The second method for evaluating the constants of steps in a chain, the use of competitive reactions, has not been used very extensively as yet.
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From page 82... ...
Naturally this method is limited to those systems in which the chlorine atoms react at a rate comparable with that with hydrogen. Thus, the observation that in mixtures of ethylene, hydrogen, and chlorine the halogen adds to the ethylene with no appreciable formation of hydrogen chloride tells us that the first step in the addition reaction is very fast but does not permit an exact calculation of its specific rate.
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From page 83... ...
AICKIN AND BAYLISS: TranS. FaradaY SOC.
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