Rate-determining step

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The rate-determining step (RDS) is a chemistry term 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 reaction depends on the rate of the slowest step.

For example, the reaction NO_2(g) + CO_(g) → NO_(g) + CO_2(g) can be thought of as occurring in two elementary steps:

1. NO₂ + NO₂ → NO + NO₃ (slow step)
2. NO₃ + CO → NO₂ + CO₂ (fast step)

As the second step consumes the NO₃ 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 equation of a reaction.

Another example of a rate-determining step is the formation of a carbo-cation from a haloalkane during the second-order nucleophilic substitution reaction of tertiary haloalkanes with sodium hydroxide.

The concept of the rate-determining step is very important to the optimization and understanding of many chemical processes such as catalysis and combustion.

In a reaction coordinate, the transition state with the highest energy is the rate-determining step of the reaction.

References

• Zumdahl, Steven S. Chemical Principles. 5^th Ed. Boston, Houghton Mifflin Company: 2005, pp 727-728.

See also

• Product-determining step