The Purpose of Kinetic Measurements

The manner in which the rates of reactant consumption and of formation of each individual product varies with reactant concentrations and temperature affords information that is useful in two ways first, as providing the basis for the reactor design if the reaction is to be operated on a significant scale, and second, and more to the immediate point, to give a framework within which a reaction mechanism can be formulated. Whatever the practical difficulties of obtaining this information, and they can be considerable with microporous catalysts and those undergoing rapid deactivation, it is essential for mechanistic analysis. It cannot be stressed to strongly that no formulation of a reaction mechanism can be accepted as plausible until shown to be consistent with experimentally determined kinetics. The corollary is however equally important the mechanism cannot be deduced from kinetic measurements alone, because many different mechanisms can lead to the same kinetic expression. 

Kinetic analysis therefore proceeds into two convergent directions. Experimental results are often first fitted to an empirical rate expression in which rate is proportional to the product of the pressure P of each component raised to the power of its order of reaction thus for the process 

The second approach starts with an idea of possible mechanism, leading to a theoretical kinetic equation formulated in terms of concenhations of adsorbed reactants and intermediate species use of the steady-state principle then leads to an expression for the rate of product formation. Concentrations of adsorbed reactants are related to the gas-phase pressures by adsorption equations of the Langmuir type, in a way to be developed shortly the final equation, the form of which depends on the location of the slowest step, is then compared to the Power Rate Law expression, which if a possibly correct mechanism has been selected, will be an approximation to it. A further test is to try to fit the results to the theoretical equation by adjusting the variable parameters, mainly the adsorption coefficients (see below). If this does not work another mechanism has to be tried. 

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Spectra similar to those on the catalyst surface could be observed in solution of DPE in weak acids such as acetic acid-sulfuric acid or in benzene-trichloroacetic acid mixtures. For the purpose of kinetic measurements the reaction was studied in a ternary solvent composed of chloroacetic, acetic and sulfuric acids. The reaction vessel consisted of two compartments separated by a break-off seal. Solutions containing the olefin in acetic-chloroacetic acid and solutions containing the active acid reagent were separately degassed at - 195°C and sealed off. After equilibration at 25°C the reaction was initiated by vigorous mixing and the rates were monitored spectrophotometrically. Data from a typical experiment are presented in Fig. 40. Initially the concentrations of the carbonium ion and of the 6050 A species increased simultaneously after several hours both approached their maximum intensity but while the former remained constant or increased very slowly, the latter decreased in concentration until after about 10 days... 

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