Editing Rate-determining step (section)

{{short description|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'''<ref>{{cite journal|url=https://www.researchgate.net/publication/51080585|title= The Rate-Determining Step is Dead. Long Live the Rate-Determining State!|date=June 2011|journal= ChemPhysChem|first1=Sebastian|last1=Kozuch|first2=Jan |last2=Martin|volume=12 |issue=8 |pages=1413–1418 |doi=10.1002/cphc.201100137 |pmid=21523880 }}</ref> or '''r/d step'''<ref>{{Organic Chemistry, Volume 1, 6/E
By Finar}}</ref><ref>{{Aliphatic Organic Chemistry
By Amit Arora}}</ref>) 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 equation]]s is not always easy, and in some cases [[Numerical methods for ordinary differential equations|numerical integration]] may even be required.<ref>Steinfeld J. I., Francisco J. S., Hase W. L. ''Chemical Kinetics and Dynamics'' (2nd ed., Prentice-Hall 1999) ch. 2.</ref> 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 {{NOx|x=2}} 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]].