Reaction Kinetics and the Catalytic Cycle

We can measure the rates of the overall catalyzed reaction as a function of variables such as catalyst concentration, substrate concentration, temperature, solvent (for a liquid phase reaction), gas flow rate (over a heterogeneous catalyst) and so on. Such a kinetic analysis gives a useful guide, but it must then be interpreted in terms of a chemically meaningful model. 

In a catalytic cycle where the reaction passes through a number of steps there is the concept of the rate controlling step, just as in any other reaction. It is defined as the step which, if it is speeded up or slowed down by changing its rate constant (as opposed to the concentration terms), has the greatest relative effect on the overall reaction rate. Under those conditions it is the key step in the reaction, and if we can speed up that step we speed up the overall process, or if 

The rates of the overall reactions can be related to the rate law expressions of the individual steps by using the steady state approximation. However simple kinetic data alone may not distinguish a mechanism where, for example, a metal and an olefin form a small amount of complex at equilibrium that then goes on to react, from one in which the initial complex undergoes dissociation of a ligand and then reacts with the olefin. As a reaction scheme becomes more complex such steady state approximations become more complicated, but numerical methods are now available which can simulate these even for complex mixtures of reactants. 

In an industrial context the reaction kinetics are often studied anyway not only for insight into a catalyst mechanism, but to determine, for example, the required reactor size and to predict how the reaction is to be controlled. 

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