Types of Chemical Reactions

There are thousands of chemical reactions. But most of them can be categorized into a few basic types. The knowledge of the type of a chemical reaction helps us to understand the nature of the reaction. The reaction type also gives us a basic idea about reactants and products involved in the reaction.

Here are different types of chemical reactions. Some reactions might fit into multiple categories; we have no hard classification.

Combination reaction

A combination reaction, aka synthesis reaction, is a reaction in which two or more reactants combine to form a single product. The reaction can be generalized as:

A + B --> AB

The final product contains the elements of all reactants. There are no byproducts.

Hydrogen gas reacts with chlorine gas to hydrogen chloride is a combination reaction.

H2(g) + Cl2(g) --> 2HCl(g)

Most metals react to form oxides when exposed to oxygen. The oxidation of metal is usually exothermic. For example, sodium metal turns to sodium oxide when it comes in contact with oxygen.

4Na + O2(g) --> 2Na2O(g)

Decomposition reaction

The decomposition reaction is the reverse of the combination reaction. In a decomposition reaction, a single compound decomposes to form two or more simpler products. It is known as the analysis reaction since the simpler products can be analyzed to find the properties of the original compound.

Normally, we require the input of energy to split the parent compound. The energy can be in the form of light, heat, or electricity.

The generalize form is AB --> A + B

The electrolysis of water is an example of a decomposition reaction.

2H2O(l) --> 2H2(g) + O2(g)

The final products do not necessarily have to be single-element compounds. For example, calcium carbonate decomposes to form calcium oxide and carbon dioxide gas.

CaCO3(s) --> CaO(s) + CO2(g)

Single-displacement reaction

The single-displacement reaction, also known as substitution reaction, is a reaction in which a chemical species displaces another chemical species in the compound.

The generalized form of this type is as follows:

A + BC --> B + AC

In the above reaction, A displaces B from BC to form AC. The chemical species that displaces and the chemical species that is displaced have similar characteristics. In the previous equation, A and B must be similar to each other.

Consider the following example of a single displacement reaction.

Zn(s) + CuSO4(aq) --> Cu(s) + ZnSO4(aq)

In the above chemical reaction, Zinc replaces copper in copper sulphate. Both zinc and copper are metals.

Another thing to note in a single-displacement reaction is the species that displaces is more reactive than the species that is displaced. So Zn is more reactive than Cu.

Double-displacement reaction

In a double displacement reaction, the two ions of compounds displace each other to form two new compounds. The generalized form of the double-displacement reaction is as follows:

AB + CD --> AC + BD

The ion A displaces C in CD to form AC and BD.

Usually, a double-displacement reaction takes place in the aqueous solution.

This reaction type is also called the metathesis reaction.

When potassium iodide is mixed with the aqueous solution of lead nitrate, the potassium nitrate is produced, and the solid lead iodide gets precipitated.

2KI(aq) + Pb(NO3)2(aq) --> 2KNO3(aq) + PbI2(s)

Combustion

The combustion reaction involves oxygen gas as one of the reactants. The second reactant is usually fuel, like hydrogen or hydrocarbon.

The combustion reactions are exothermic; light and heat are liberated as the reaction proceeds to completion.

Common products of the combustion reaction are carbon dioxide and water vapor.

Consider the combustion of hexane (C6H14):

C6H14 + 15.5O2 --> 6CO2 + 7H2O + heat