- Rate-determining step
The rate-determining step (RDS) is a
chemistryterm for the slowest step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in. Similarly, the rate of reactiondepends on the rate of the slowest step.
For example, the reaction + → + can be thought of as occurring in two elementary steps:
# + → + "(slow step)"
# + → + "(fast step)"
As the second step consumes the produced in the slow first step, it is limited by the rate of the first step. For this reason, the rate-determining step is reflected in the
rate equationof a reaction.
Another example of a rate-determining step is the formation of a carbo-cation from a
haloalkaneduring the SN1 reaction of tertiary haloalkanes with sodium hydroxide.
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 interact and form the product. This case is referred to as
diffusion controland generally occurs where the formation of product from the activated complex is very rapid and thus the supply of reactants and their interaction is rate determining.
The concept of the rate-determining step is very important to the optimization and understanding of many chemical processes such as
reaction coordinate, the transition statewith the highest energy is the rate-determining step of the reaction.
*Zumdahl, Steven S. Chemical Principles. Ed. Boston, Houghton Mifflin Company: 2005, pp 727-728.
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