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Elementary and Complex Reactions

This article discusses elementary and complex reactions and their types. It defines and gives examples of elementary reactions and complex reactions.

A chemical reaction occurs when the old bonds between the reacting substances are broken, and the new bonds between the products are formed. Chemical reactions play an important part in our daily lives and different industries. You can find chemical reactions in your surroundings in processes such as rusting of iron and fermentation of wine. The speed at which the chemical reaction occurs is termed the rate of reaction. It depends on multiple factors, such as pressure, temperature, the nature of reactants, the concentration of reactants, among others. Reactions are of two types—simple or elementary  reactions  and complex reactions. We will discuss both types in detail. 

Elementary reactions

Chemical reactions that take place in only a single step are known as simple reactions or elementary reactions. However, in some simple reactions, multiple side reactions occur along with the main reaction, resulting in product formation. Elementary reactions can be defined as reactions that involve no intermediate steps, thus occurring in one single step. In this type of reaction, the order of the reaction is similar to the coefficient of the reaction.  

Types of elementary reactions 

Generally, there are three types of elementary reactions that take place in different molecules and substances. These are unimolecular, biomolecular, and tetramolecular reactions. 

Unimolecular reaction: This occurs when a reaction consists of only one molecule. This molecule collides with itself to form one or more substances. These reactions are known as first-order reactions because they have only one reactant. Radioactive decay is one of the best examples of unimolecular reactions. 

A → B  

where A = reactant 

B = product 

rate of chemical reaction, r = k [A]

k = reaction rate constant  

Bimolecular reaction: The kind of reaction that occurs when two molecules collide to produce one or more types of products is known as a bimolecular reaction. This type of reaction is known as a second-order reaction. A perfect example of a bimolecular reaction is an organic reaction.  

2A → B 

where 2A = two molecules of reactant 

B = product that is formed 

Rate of chemical reaction, r = k [A]2

k = reaction rate constant 

Tetramolecular reactions: When three molecules collide and produce specific products, it is known as a tetramolecular reaction. These reactions are not common as they require certain conditions to take place. The reacting molecules must be in the proper orientation, and they must have a high energy level to sufficiently collide with each other and form products. This type of reaction is known as a third-order reaction.  


A + A + A → B where Rate = k [A]3 

A + A + B → C where Rate = k [A]2 [B]

 A + B + C → D where Rate = k [A] [B] [C] 

k = reaction rate constant  

Complex reactions

A complex reaction takes place when the reactants are converted into products in multiple steps or more than one step. Some side reactions take place when complex reactions occur. Additionally, a number of steps are involved in the reaction to form the right product.  

Types of complex reactions

Generally, there are three types of complex reactions. These are consecutive or sequential reactions, parallel reactions, and opposing reactions. Here, we have explained each reaction with a complex reaction example given. 

Consecutive or sequential reactions: The reactions in which the reactant forms an intermediate compound and then the intermediate compound is converted into the product in various steps are known as consecutive or sequential reactions. However, the reactants are not converted to the products directly in such a reaction. But it involves an array of steps in which the products are formed.  

A → B ……………  (k1)

B → C ……………. (k2) 

Where A = reactant 

B = intermediate 

C = product 

k1 = first step’s-rate constant 

k2 = second step’s-rate constant 

Parallel reactions: Also known as side reactions, these reactions take place when a reactant reacts in more than a single pathway. In such a reaction, the products formed are two or more. For instance, in such a reaction, a reactant A reacts to form three different products, B, C, and D, through different pathways. All the pathways have different rate constants, k1, k2, and k3. In these three reactions, one reaction is main, whereas others are side or parallel reactions. The main reaction gives maximum yield, whereas other products are formed in lower concentrations. 

Bromination of bromobenzene is an example of a parallel reaction.

Opposing reactions: Also known as reversible reactions, these reactions are known to work in both forward and backward reactions. The reaction mechanism of an opposing reaction looks like A + B  — Kf–  > < –Kr —C + D 

Where A and B are reactants, C and D are products, kf is the rate constant of forward reaction, and kr is the rate constant of a reversible reaction. 

The reaction between CO and NO2 gasses is an example of an opposing reaction. 

Key differences between elementary and complex reactions

Here are some of the key differences between elementary and complex reactions. 

Simple or elementary reactions

Complex reactions

  • A reaction that occurs in one single step is known as a simple reaction. 

  • A reaction that occurs in more than one step is a complex reaction.

  • The order values of the reactions are small. These are 0,1,2,3…

  • The order values of these reactions are large. That is, more than 3. Sometimes these reactions have fractional values too. These are ½, ⅓, 3/2, etc.

  • There are no or few side reactions.

  • Multiple side reactions occur. 

  • Products are directly obtained from the reactants.

  • Products are not formed directly from the reactants. 

  • Reaction rate theory applies to these reactions. 

  • Reaction rate theory does not apply to complex reactions.


For a reaction to take place, it requires suitable conditions. When a reaction gets suitable conditions such as temperature, pressure, and a catalyst, the reaction takes place at a faster rate. Both elementary and complex reactions occur when they get optimum conditions, and the rate at which the chemical reaction takes place is known as the rate of reaction. If we combine both reactions, it can be said that multiple simple or elementary reactions are combined to get complex reactions.