Химикалық реакцияның ең баяу кезеңі

Slowest step of a chemical reaction
In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate determining step (RDS or RD step or r/d step) or rate limiting step. For a given reaction mechanism, the prediction of the corresponding rate equation (for comparison with the experimental rate law) is often simplified by using this approximation of the rate determining step. In principle, the time evolution of the reactant and product concentrations can be determined from the set of simultaneous rate equations for the individual steps of the mechanism, one for each step. However, the analytical solution of these differential equations is not always easy, and in some cases numerical integration may even be required. The hypothesis of a single rate determining step can greatly simplify the mathematics. In the simplest case the initial step is the slowest, and the overall rate is just the rate of the first step. Also, the rate equations for mechanisms with a single rate determining step are usually in a simple mathematical form, whose relation to the mechanism and choice of rate determining step is clear. The correct rate determining step can be identified by predicting the rate law for each possible choice and comparing the different predictions with the experimental law, as for the example of and CO below. The concept of the rate determining step is very important to the optimization and understanding of many chemical processes such as catalysis and combustion.

As an example, consider the gas phase reaction + CO → NO + If this reaction occurred in a single step, its reaction rate (r) would be proportional to the rate of collisions between and CO molecules: r = k[][CO], where k is the reaction rate constant, and square brackets indicate a molar concentration. Another typical example is the Zel'dovich mechanism.

Another example is the unimolecular nucleophilic substitution (SN1) reaction in organic chemistry, where it is the first, rate determining step that is unimolecular. A specific case is the basic hydrolysis of tert butyl bromide by aqueous sodium hydroxide. The mechanism has two steps (where R denotes the tert butyl radical ):
Formation of a carbocation R−Br → + Nucleophilic attack by hydroxide ion + → ROH. This reaction is found to be first order with r = k[R−Br], which indicates that the first step is slow and determines the rate. The second step with OH− is much faster, so the overall rate is independent of the concentration of OH−. In contrast, the alkaline hydrolysis of methyl bromide (CH3Br) is a bimolecular nucleophilic substitution (SN2) reaction in a single bimolecular step. Its rate law is second order: r = k[R−Br][].

A useful rule in the determination of mechanism is that the concentration factors in the rate law indicate the composition and charge of the activated complex or transition state. For the –CO reaction above, the rate depends on []2, so that the activated complex has composition , with 2 entering the reaction before the transition state, and CO reacting after the transition state. A multistep example is the reaction between oxalic acid and chlorine in aqueous solution: + → 2 + 2 + 2
Also, for reaction steps that are not first order, concentration terms must be considered in choosing the rate determining step.

Not all reactions have a single rate determining step. In particular, the rate of a chain reaction is usually not controlled by any single step.

In the previous examples the rate determining step was one of the sequential chemical reactions leading to a product. The rate determining step can also be the transport of reactants to where they can interact and form the product. This case is referred to as diffusion control and, in general, occurs when the formation of product from the activated complex is very rapid and thus the provision of the supply of reactants is rate determining.