Endothermic Reactions: Examples And Exothermic Reactions

We explain what endothermic reactions are and examples of endothermic reactions. Also, what are exothermic reactions.

  1. What are endothermic reactions?

It is understood by endothermic reactions to a certain type of chemical reactions (that is: the process of transformation of two or more substances into different ones) that, when they occur, consume caloric energy , that is, in which the products obtained have higher energy levels than initial reagents, since they have taken part of the heat of the environment.

This is summarized in the following formulation: given an enthalpy (H), an endothermic reaction will always have a enthalpy variation (ΔH) greater than zero (ΔH> 0). Recall also that enthalpy is the variable that represents the exchange of energy between a thermodynamic system and its environment.

These types of reactions are commonly used in the chemical ice and cooling industry, since they can be produced in controlled environments to remove heat from the environments or other substances. Subsequently, some of these applications were replaced with the cold generated by electricity (compressors).

  1. Examples of endothermic reactions

endothermic reactions
The ozone layer is formed by converting oxygen atoms into ozone.

Some examples of endothermic reaction are:

Ozone production in the atmosphere

Driven by the sun’s ultraviolet radiation, oxygen atoms (O2) are converted to ozone (O3), absorbing energy from that radiation in the process.

to  hydrolysis of water

To separate the hydrogen (H) and oxygen (O) that make up the water (H2O), it is necessary to add electrical energy in a procedure known as hydrolysis, in which both types of atoms respond to the poles generated by the added electrical current , breaking its molecular union (and consuming energy).


The process of plant nutrition occurs through a series of chemical reactions that break down environmental carbon dioxide (CO2) , in the presence of water and, necessarily, sunlight. This is because said reaction requires an addition of energy to be consumed during the reaction.

Obtaining iron (II) sulfide

To achieve in the laboratory iron (II) sulfide, also called ferrous sulfide (FeS), a first step is required by the formation of hydrogen sulfide (HS) and then combined with the metal , and this reaction requires at all times the addition of heat, in the form of a lighter or an industrial boiler. Such heat is the added energy that the reaction requires in order to occur.

  1. Exothermic reactions

exothermic reactions
Gasoline when burned releases more energy than initially introduced.

The opposite case is represented by exothermic reactions, that is, those that occur when they release a certain amount of energy to the environment , in the form of heat. In these cases, logically, the enthalpy variation will be less than zero (ΔH> 0) since the products have less energy than the initial reagents, since some of said chemical energy has been released into the environment in the form of heat.

Examples of this type of reactions are all forms of combustion and oxidation , such as gasoline or fossil fuels, which when burned in the presence of oxygen release an amount of energy much higher than initially introduced (the spark the motor). The same occurs in the phase changes of the matter from the gaseous state to the liquid, or from the latter to the solid.

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