Elementary Or Non-Elementary Reactions

Chemical reaction simply define as a reaction in which a substance (or a compound) is converted into a new one with different property. In a reaction, the substance (or a compound) which is converted into new substance (or changes their state) is called 'Reactant' and a newly formed substance is called 'Product'. We see many chemical changes around us in our daily life. 

A chemical reaction is usually written by placing an arrow symbol between the reactants and the products. 
For Example
If A and B are two reactants in a chemical reaction while C and D are two products, then this reaction can be represented as;
                  A  +   B       ➝      C  +  D
              (Reactants)          (Products)

Some of these chemical reactions occur in one step while in others the products are formed in a series of steps. On this basis chemical reactions are of two types: Elementary and Non-elementary.........
In this article, we will learn about elementary and non-elementary reactions with the help of examples.

Types Of Reactions 
Generally, a chemical reaction rarely proceed in a single step that's why based on the steps involved in a reaction, these reactions are two types namely;
........"Reaction in which the rate equation corresponds to a stoichiometric equation are called elementary reactions otherwise it is called as non-elementary reactions".......

ELEMENTARY REACTIONS
......"An elementary reaction is define as a reaction in which the products are formed by the original reactants in a single elementary step".......
It has a single transition state (no intermediates formed).
Reactant ➝ Transition State ➝ Product 
(No formation of intermediate)

In this type of reaction, the observed rates law and the experimental rates law, both are identical. 
........"Reaction in which the rate equation corresponds to a stoichiometric equation are called elementary reactions"........
Therefore, the rates of these reactions can be obtained directly from the balanced chemical equation.
For Example
  • C₂H₅OH + CH₃-COOH ➝ CH₃COO-C₂H₅ + H₂O
  • 2NO + O₂ ➝ 2NO₂   
  • Radioactive decay in which particles are
     emitted from an atom.

NON-ELEMENTRY REACTION/ COMPLEX REACTION
......."A reaction in which the products are formed from the reactants through a series of elementary steps, is called as Non-elementry reaction"........
In this type of reaction, the observed rate and the experimental rates are not identical. It involves the formation of intermediates with transition states. (An intermediate is formed between the two transition states) 
Reactants ➝ Transition State ➝ Intermediate ➝Transition State ➝ ...... ➝ Products
The intermediate formed during the reaction is not present in the overall balanced chemical equation of that reaction.

The more intermediates there are in the reaction, the more steps the reaction will take. Generally, the reaction will be completed in one more step than the number of intermediates present in the reaction. For example, if two intermediate molecules are present in the reaction, then the reaction will be completed in three elementary steps.
In the above figure, three intermediate molecules are formed. Therefore, the reaction will be completed in four elementry steps.
The rates of these reactions cannot be determined directly from a balanced chemical equation. Rather, it is obtained from the slowest step of the reaction which is also the rate determining step of the reaction.
For Example
              2HBr + O₂ ➝ H₂O₂ + Br₂
The above reaction is proceed in three different elementary step-
    Elementary Step 1- H-Br + O₂ ➝ HO₂Br
    Elementary Step 2- HO₂Br + H-Br ➝ 2HOBr
    Elementary Step 3- 2HOBr ➝ H₂O₂ + Br₂

........"A comparation of the stoichiometric equation with the experimental expression can suggest whether the reaction is elementary or not. That means if both are identical then the reaction is elementary otherwise non-elementary".........

Difference Between Elementary And Non-Elementary Reactions
  • Elementary reactions occur in a single steps while non-elementary reactions occur in more than one steps or multiple steps.
  • Elementary reactions have only one transition state while non-elementary reactions have more than one transition states.
  • Intermediate can not be detected in elementary reactions while non-elementary reactions have one or more than one intermediates.
  • Therefore elementary reactions are simple while non-elementary reactions are complex in nature.
  • In elementary reactions, reaction's order equals to the stoichiometric coefficients whereas in non-elementary reactions, reactions order may or may not be equal to the stoichiometric coefficients.
FAQs
What Are Elementry Reactions????
An elementary reaction is a single step reaction with a single transition state and no intermediates.
What Are Complex Reactions????
Complex selections are reactions that occur in a series of elementary steps with intermediate and multiple transition states. For any reaction to be completed in more than one step, the reaction must contain at least one intermediate molecule and two transition states.
How do you find out whether a reaction is elementary or not ???
A comparation of the stoichiometric equation with the experimental expression can suggest whether the reaction is elementary or not. This means that if both are equal then the reaction will be primary otherwise non-primary.


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