Difference Between Elementary Step and Rate Determining Step

The key difference between elementary step and rate determining step is that elementary steps are chemical reactions where reactants react to give either a final product or an intermediate whereas rate determining step is the slowest reaction step of a multistep process.

The terms elementary step and rate determining step are used in discussing the reaction rate of chemical reactions, especially when there are two or more steps before forming the final product. Each and every step of this multistep process is called an elementary step. These steps have different rates. The rate determining step of the process has the slowest rate.

CONTENTS

1. Overview and Key Difference
2. What is an Elementary Step 
3. What is a Rate Determining Step
4. Side by Side Comparison – Elementary Step vs Rate Determining Step in Tabular Form
5. Summary

What is an Elementary Step?

Elementary steps are the single steps included in a multistep process. An elementary step is a chemical reaction where one or two reactants react to give either a final product or an intermediate product. It is a single reaction step, and it has a single transition state. Generally, if there are no intermediate products produced during a particular chemical reaction, this reaction is called an elementary reaction. Moreover, a mechanism for a chemical reaction is a collection of elementary steps. Therefore, an elementary reaction describes the single moment during a reaction in which molecules break and/or form new bonds.

The sum of all the balanced elementary steps gives the overall reaction. The elementary steps of a multistep process have different reaction rates; e.g. some elementary steps occur fast while other steps are very slow. Therefore, the rate determining step or the slowest step of the reaction is also a type of elementary reaction.

Elementary reactions can be categorized based on their molecularity. Here, the number of molecules involved in the reaction is used to give the molecularity of the elementary step. E.g. if there is a single reactant, it is unimolecular, and if there are two reactants, it is bimolecular. Unimolecular and bimolecular are the most common types of elementary reactions. Termolecular (three reactants) reactions are rare since collision of three molecules at the same time is rare.

What is a Rate Determining Step?

The rate determining step is the slowest step of a multistep reaction process. It is a single step among a series of steps. However, some reactions have only one chemical reaction (not a series of reactions); thus, these reactions are always the rate determining reaction. The reaction having the slowest rate is taken as the rate determining reaction because it limits the rate of the reaction.

An example is given below.

NO2 + NO2 → NO + NO3 (slow step, rate-determining)

NO3 + CO → NO2 + CO2 (fast step)

What is the Difference Between Elementary Step and Rate Determining Step?

The key difference between elementary step and rate determining step is that elementary steps are chemical reactions where reactants react to give either a final product or an intermediate whereas rate determining step is the slowest reaction step of a multistep process. Therefore, an elementary step can be either fast or slow, while a rate determining step is always the slowest step.

Below infographic summarizes the difference between elementary step and rate determining step.

Summary – Elementary Step vs Rate Determining Step

The terms elementary step and rate determining step are used in discussing the reaction rate of chemical reactions when there are two or more steps occurring before forming the final product. The key difference between elementary step and rate determining step is that elementary steps are chemical reactions where reactants react to give either a final product or an intermediate whereas rate determining step is the slowest reaction step of a multistep process.

Reference:

1. “Elementary Steps.” Chemistry LibreTexts, Libretexts, 19 May 2020, Available here.
2. “Boundless Chemistry.” Lumen, Available here.
3. “Elementary Reaction.” Wikipedia, Wikimedia Foundation, 5 Sept. 2019, Available here.

Image Courtesy:

1. “Reaction Co-ordinate Diagrams for reactions with 0, 1, 2 intermediates” By AimNature – Own work (CC BY-SA 3.0) via Commons Wikimedia