Determination of Bond Dissociation Energies by Kinetic Methods

Measurements of rales of reaction may sometimes be used to deduce heats of reaction quite generally, as follows. Consider the general reaction 

Hence if we know experimentally the temperature dependence of ki and 2 can obtain I E. This result is quite independent of any theory of and k and depends on observation only. It does depend, however, on the rate being the same function of concentration throughout the range, and this is seldom experimentally established. If, within the limits of experimental error, we can describe the kinetic results for ki and k2 by the Arrhenius equation, 

Particular assumptions about the kinetics of dissociation reactions, and the activation energy of atom or radical recombination reactions enable the dissociation energy to be deduced from the measurement of ki only. These are discussed in Section 4.2 in the present section we are concerned with the more general case. The typical reaction studied is that of a molecule with a radical or atom to give a radical or atom and another molecule. If the heats of formation of the molecules and one or other of the radicals or atoms are known, the heat of formation of the other atom or radical can be deduced from the heat of reaction, and hence a dissociation energy. For example, considering the reactions 

They followed the work of Bodenstein and Jung who deduced Z)(H2) from a similar study of the reaction between bromine and hydrogen, giving the value jD(H2) =106 kcal. This result differs from the most reliable value by less than 3 kcal. It is, of course, not the most satisfactory determination of D(H2), whereas the present best value of i)(GH3 - H) comes from Kistiakowsky s work, which has received considerable confirmation by other methods. 

Under these circumstances it is probably better to describe Kistia-kowsky s work in detail rather than Bodenstein s, although it must be remembered that the close similarity of the kinetics that Kistiakowsky studied to those of the hydrogen-bromine reaction made less necessary a closely detailed proof of every step. The occurrence of these reactions in the photobromination of methane was shown by measuring the rate of disappearance of bromine photometrically to establish the kinetics. From the plot of [Br2] against time the initial rate of disappearance of bromine was obtained as a function of the initial pressures of the reactants [GH4] and [Br2], and found to fit the equation 

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DETERMINATION OF BOND DISSOCIATION ENERGIES BY KINETIC METHODS... 

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