Multiple reactions and their types

As we know that the reaction takes place in a single step or more than one step. Therefore, when a reaction takes place in a single step or when a single stoichiometric equation (or a single rate equation) represents the entire progression of a reaction, it is called as "Single Reaction" or "Elementary Reaction". Whereas, when a reaction takes place in more than one steps or when more than one stoichiometric equation is required to represent the entire progression of a reaction, it is called as "Multiple Reactions" or "Non-elementary Reactions".

Difference Between Them????          
The main difference between single and multiple reaction is that only a single rate equation is required to describe the kinetic behaviour of a reaction in case of single reactions whereas multiple reaction requires more than one rate equation to describe it's kinetic behaviour.
Desired and Undesired Products??
Since, as we know, multiple reactions are reactions that take place in more than one step, the products formed from these reactions are also more than one. Some of these are desired products and some are undesired one. And our main objective for these chemical reaction is to keep the productivity of the desired product as high as possible and the productivity of the undesired products as low as possible for economic success.

Types Of Reaction
Multiple reactions are subcategories into three basic types namely series reactions, parallel reactions and series-parallel reactions. Let us discuss these reactions one by one;

(A)Series Reactions
These reactions are also known as "Consecutive Reactions". A series reaction is defined as a reaction in which the reactant first converts into an intermediate product and then this intermediate product decomposes into other products. 
In this, the intermediate product is our desired product while the other products are undesired. A series reaction is expressed as;
Here,  
      'B' is the desired product  (intermediate product)  
      'C' is the undesired product.
       k₁ is the rate constant at which 'A' is converted into 'B'
       k₂ is the rate constant at which 'B' decomposes into 'C'.

This series reaction could also be written as;
The rate laws for the above reaction is given as;
       Rate of disappearance of A, 
                                       ➩ { -r = k₁CA } 
       Rate of formation and disappearance of R,
                                       ➩ { -r = k₁CA-k₂CR }
       Rate of formation of S, 
                                       ➩ { +r = k₂CR }

(B) Parallel Reactions
These reactions are also known as "Completing Reactions". A parallel reaction is defined as a reaction in which the reactant is consumed by two different reaction pathways to form different products (or decomposes into two different products). A parallel reaction is expressed as;
Here, 
      'R' is the desired product (intermediate product)
      'S' is the undesired product.
       k₁ is the rate constant at which 'A'  is converted into 'R'  
       k₂ is the rate constant at which 'A' decomposes into 'S'.

And the corresponding rate equations is written as;
Overall disappearance of A, 
                        ➩ { -r = k₁CA+ k₂CA }
Rate of formation of R,
                        ➩ { +r = k₁CA }
Rate of formation of S,
                        ➩ { +r = k₂CA }

(C) Series-Parallel Reactions
When the series and parallel reactions occur simultaneously then these reactions are called as "Series-parallel Reactions".
Denbigh system is an example of series-parallel reactions in which reactant 'A' parallaly gives 'T' and 'R' as two different products. Where the product 'R' is an intermediate and in series 'R' decomposes into product 'S'. Here, R also gives parallaly two distinct products 'S' & 'U' at two different reaction rates constants.

Here, the above reaction behaves like a series or consecutive reaction with respect to 'A' (A➝R➝S➝T) while it behaves like a parallel reaction with respect to 'B'.
The above equation are also a type of series-parallel reaction. Reaction is parallel reaction with respect to 'R' ( as R gives two distinct products A and S) while reaction is a series reaction with respect to 'A' (A➝R➝S).


Hope you have found this article helpful!!
Let me know what you think about MULTIPLE REACTIONS. Feel free to comment if you have any queries.!!

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