What is an elementary step in a reaction?

Short Answer

An elementary step in a reaction is a single, simple stage in a larger reaction mechanism. It represents one molecular event where reactant particles directly collide or interact to form products or intermediates. Unlike the overall reaction, which shows only the final change, an elementary step shows exactly what happens at that moment.

Each elementary step has its own speed, molecularity, and pathway. Many chemical reactions occur through several elementary steps arranged in sequence. The slowest step among them controls the overall reaction rate and is known as the rate-determining step.

Detailed Explanation :

Elementary Step in a Reaction

An elementary step is the simplest and smallest unit of a chemical reaction. It describes exactly what happens at the molecular level during a single event in the reaction process. While the overall reaction equation shows only reactants and final products, it does not show the detailed moves. The elementary step fills this gap by explaining how molecules interact in each stage. In many reactions, the transformation from reactants to products does not occur in one jump but through a series of these small steps.

Each elementary step corresponds to one collision or one molecular change. These steps may form intermediates, break bonds, form new bonds, or rearrange atoms. Together, all elementary steps make up the reaction mechanism, which is the complete sequence explaining how a reaction actually occurs.

  1. Characteristics of an Elementary Step

Elementary steps have unique features that distinguish them from overall reactions:

  1. a) Represents a Single Molecular Event

It shows one small stage in the reaction, such as one collision, one bond break, or one rearrangement.

  1. b) Immediate and Direct

The reactants in an elementary step directly produce products or intermediates without further subdivisions.

  1. c) Has Definite Molecularity

Molecularity refers to the number of particles involved in that step.

For example:

  • Unimolecular step: involves one molecule
  • Bimolecular step: involves two molecules
  • Termolecular step: involves three molecules (rare)
  1. d) Has Its Own Rate Law

The rate law for an elementary step can be written directly from its molecularity, unlike overall reactions.

  1. e) May Produce Intermediates

Many steps form species that do not appear in the overall equation.

  1. Examples of Elementary Steps

Elementary steps occur in many reactions. Here are simple examples:

Example 1: Ozone Formation

Overall reaction:
O₂ + O → O₃
This can be one elementary step if the reaction occurs in a single collision.

Example 2: Reaction of NO and O₂

Overall:
2NO + O₂ → 2NO₂

Possible elementary steps:

  1. NO + O₂ → NO₃ (intermediate)
  2. NO₃ + NO → 2NO₂

Each line represents one elementary step.

Example 3: Decomposition Reaction

N₂O₅ → NO₂ + NO₃
This single decomposition is an unimolecular elementary step.

  1. Molecularity of Elementary Steps

Molecularity tells how many particles participate in an elementary step:

  1. a) Unimolecular

Only one molecule reacts.
Example: A → B + C
Common in decomposition reactions.

  1. b) Bimolecular

Two molecules collide.
Example: A + B → AB
Most common type of elementary step.

  1. c) Termolecular

Three particles collide at the same time.
Example: A + B + C → ABC
Very rare because simultaneous collisions are unlikely.

  1. Elementary Steps vs Overall Reaction

It is important to understand the difference:

Overall Reaction

  • Shows only the starting reactants and final products
  • Does not provide details of each stage
  • Does not show intermediates or bond-breaking sequence

Elementary Step

  • Shows a single stage of the reaction
  • Includes intermediates
  • Helps explain mechanism and rate

For example:
2NO₂ → 2NO + O₂ (overall)
But the mechanism may have more than one elementary step.

  1. Role of Elementary Steps in Reaction Mechanisms

A reaction mechanism is made up of multiple elementary steps arranged in order. These steps:

  1. a) Explain the Pathway

Show how reactants transform into products.

  1. b) Identify Intermediates

Reveals temporary species formed during the reaction.

  1. c) Determine the Rate-Determining Step

The slowest elementary step controls the overall reaction speed.

  1. d) Help Derive Rate Laws

Only elementary steps have rate laws based on molecularity.

  1. e) Allow Better Control

Understanding steps helps chemists design catalysts and improve efficiency.

  1. Why Elementary Steps Matter in Chemistry

Elementary steps are important because:

  1. a) They Provide a Realistic Picture

Chemistry does not happen in one big leap but through small events.

  1. b) They Help Predict Reaction Behaviour

Knowing steps helps predict how a reaction responds to temperature, pressure, or catalysts.

  1. c) They Help in Designing Catalysts

Catalysts work by creating easier elementary steps with lower activation energy.

  1. d) They Improve Industrial Processes

Understanding steps helps industries optimize reactions for speed and yield.

  1. e) They Enhance Learning of Chemical Kinetics

Students and scientists can understand reaction rates more clearly.

Conclusion

An elementary step is a single, simple stage in a chemical reaction that describes a direct molecular event such as collision, bond breaking, or bond formation. Multiple elementary steps together form the reaction mechanism. Each step has its own molecularity, rate law, and intermediates. Understanding elementary steps helps chemists explain reaction pathways, calculate reaction rates, and design better chemical processes. Thus, elementary steps are the foundation of reaction mechanisms and chemical kinetics.