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Robert M. Hanson
Department of Chemistry, St. Olaf College, Northfield, MN 55057

Mechanism-Based Kinetics Simulator allows you to simulate the kinetics of a reaction based on its mechanism. The idea is to write a mechanism and, based on that, follow the course of concentrations or rates of change in concentration of reactants, catalysts, intermediates, and products over time. In addition, you can graph many different relationships related to the reaction either for one species or for the whole set, including: [X], ln[X], 1/[X], or d[X]/dt versus time. The rate of change in a species, d[X]/dt, can also be graphed versus time, [X], or [X]². Questions to consider include: How do the individual concentrations change over the course of the reaction? Which ones rise and which ones fall? Which ones start small, rise, then fall again? Are the individual changes in concentration (d[X]/dt) related? How so? Are these exact, or only approximate correlations? Why would that be? Is there a rate “constant” for the reaction? If so, how would you get its value? You can either select from a predefined mechanism or write your own.

Mechanism-Based Kinetics Simulator has proven to be an effective tool in helping students gain insight into the role of mechanisms in the study of kinetics. The predefined mechanisms and the ability to change the mechanism and/or rate constants on the fly makes for a highly dynamic, interactive classroom experience. One of the more interesting aspects of the simulations is that the rates of change of reactants and products are followed as well, and it is easily demonstrated that our simplistic “rate in equals rate out” notion is flawed when more than one mechanistic step is involved. However, one can easily demonstrate that the assumption of a “steady state” is generally reasonable.

Figure. A twostep mechanism involving a reversible first step has been selected. The steady state involves a relatively small concentration of an intermediate.

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