New strategies for studies on the kinetics and energetics of the Belousov‐Zhabotinskii reaction

Abstract

Catalysis / Chemical Kinetics / Computer Experiments / Oscillatory Chemical Reactions / Thermal AnalysisThe oscillatory Belousov-Zhabotinskii reaction and its subsystems were studied by z 1700 DTA and UV experiments performed at linearly increased temperature. The starting concentrations of bromate, bromide, catalyst, and malonic acid were varied in order to obtain "concentration codes" of such series. These represent the dependence of any parameters in discrete ranges of the starting concentrations of the different components. Particularly adequate for a model check are the mechanistic codes (= MCC's), which concern the shape index and reaction type index: If these mechanistic coordinates remain constant in such a concentration interval, other data, e.g. activation energies, can be attributed to a rate-determining process. -The results led to a nine-step model which is based on the FKN mechanism and is capable of roughly reproducing the influence of the initial concentrations and heating rate on 11 parameters characteristic of the envelope and of the oscillations of the experimental signals. The MCC's of the experimental and theoretical series are similar, revealing 4-5 rate-determining steps in the different concentration ranges. -The activation energies agree with values from the literature, cited for three steps; these and further resulting activation energies are consistent with our studies with subsystems.

The ‘Theorem of common kinetic runs’ in homogeneous non-isothermal reaction kinetics

“…With view to more complex reactions, where the first test using MCC's was recently published [28], the path of characterizing a reacting system exclusively with respect to its kinetics shouldallow a separation of redundant, nonkinetic data by a concise classification and archivation of those processes which can be studied in real-time, regardless of the method used and the (static) features of the components. Considering the success in also recognizing three-and four-ste p models by assuming superpositions of two two-step models [1], we will probably learn to translate rather complex MCC's to the readily interpretable reaction matrices in the near future.…”

“…The three lines describe the dependences of the reponse parameters S, Mn(init) and Mn(ov) (n = 1 or 2, see later) on Co (as the transmitter parameter), and the rows enumerate those N intervals of In Co, wherein the type of dependence of the response parameter is maintained: increasing(+), decreasing(-) or constant(C). For the 'C'-symbols, a subclassification indicating the corresponding order was introduced [27,28]. Hence, a 'C'-type element of this matrix has a double nature; it indicates a rate-determining step, but also represents the order and, hence, gives hints to the stoichiometry and to the position of this step in the total network.…”

“…The signification of the symbols 'C's '+' and '-' was recently refined [1,27,28]. For the mechanististic coordinates, the 'C' was replaced by order-indicating symbols L(n<l), 1, M (l<n<2), 2 and H (n>2).…”

“…This contains programs for evaluation, graphic representation, and data banks, and was complemented by an improved Gear subroutine [12,36]. The latter was successfully applied to very complex, even oscillatory models [28,37]. The usually bell-shaped derivative signals in practice do not start at absolute zero, but at a temperature which can be precalculated for a one-step process, based on the TCRC, corresponding to ca.…”

Determination of kinetic models by the use of pattern recognition methods

“…A prerequisite for the reliable function of this comparing strategy is the existence of a THEOREM OF COMMON KINETIC RUNS, which means that for a definite model such as 'Mechanistic Concentration Code' ( = MCC; [3]) does not depend on the activation data or the signal parameters of the steps (proportionality coefficients, as reaction enthalpy for DSC or DTA). Recently it could be revealed that the MCC-strings of all two-step models consist of independent main parts (i.e., first and last element = orders of reference step or the other step, respectively, and 'sign' elements, + or-, between) and parts beside which are partially dependent on signal parameters or, less, activation parameters; however, application of the probability theory confirms [4,5] that the deviations of individual total strings from the source strings usually do not exceed 10%.…”