Endothermic Reactions vs. Exothermic Reactions

Chemical reactions occur everywhere, from our bodies to laboratories. Chemical reactions are classified into two types of the basis on the release or absorption of energy. The chemical reactions in which power is absorbed in heat form is called endothermic reaction, whereas the reactions in which energy is released in the heat form are called exothermic reactions. For example, evaporation of liquid water and the melting of ice cubes are endothermic reactions. In contrast, the formation of ice cubes from the water is an example of an exothermic reaction.

Chemical reactions require a lot of energy that exists in the bonds which hold the molecules together. So, when the reaction occurs between molecules and compounds, which results in the breaking of bonds, release or absorb a massive amount of energy.

On the other hand, the new products which are formed after the reactions require energy, and hence the total energy is calculated by the number of bonds formed and broken. This process in the chemical reaction is termed as a heat of reaction, also known as enthalpy, which is denoted by the sign of ∆H and units are KJ/mol.

Comparison Table

Basis for ComparisonEndothermic ReactionExothermic Reaction
EtymologyGreek word; endo means “inside” and “thermasi” means “to heat.”Greek word; “Exo” means “outside” and “thermasi” means “to heat.”
EnergyAbsorbs energyEvolves energy
Form of energyHeatHeat, electricity, sound or light
EnthalpyEnthalpy is positiveEnthalpy is negative
End products stabilityLess stableStable
ApplicationsChemistry, physics, thermodynamics, etcChemistry, physics, thermodynamics, etc
ExamplePhotosynthesis, melting of ice, breaking of gas molecules, etcCombustion, Formation of ice, burning of coal, etc.

What is Endothermic Reaction?

The word “endothermic reaction” has been derived from the Greek word in which “endo” means “inside” and thermic means “heat.” Thus, the endothermic reaction can be defined as a chemical reaction in which energy is absorbed during the conversion of reactants into products. This occurs because of the dissociation of the bonds between the molecules. The energy is released later on when the new bonds are formed.

In endothermic reactions, products contain more energy than reactants. In these reactions, the heat is taken up from the surrounding environment, so the system in which these reactions are going remains cooler and shows decreased temperature.  The change in heat (enthalpy) during the conversion of reactants to the products becomes higher at the end of the reaction. So, the value of enthalpy or ∆H is always positive.

Endothermic reactions are also called non-spontaneous reactions as they yield products that are higher in energy than the reactants.


Photosynthesis, which occurs daily in the daytime in plants, is an example of an endothermic reaction. In photosynthesis, chlorophyll present in green plants converts the water and carbon dioxide into glucose and oxygen in the presence of sunlight that works as the source of energy.

When a small quantity of ammonium chloride is taken into the test tube and is made to dissolve in the water, the test tube becomes colder. This is also an endothermic reaction in which heat is absorbed from its surround, which is a test tube.

Evaporation or sweating is also an example of an endothermic reaction. Sweating cools a person as water draws heat to change it into gas. Wet skin feels cool in a breeze because the evaporative reaction of the water absorbs heat from the surrounding, which is skin and atmosphere.

Similarly, the conversion of ice into the water through boiling, cooking of an egg, cracking alkanes, and sublimation of carbon dioxide are examples of endothermic reactions.

What is Exothermic Reaction?

The word “exothermic” has been derived from the Greek language in which “Exo” means “outside,” and “thermic” means “heat.” The exothermic reaction is a chemical reaction in which energy is released. So the temperature of the system becomes higher. These reactions are warmer, and sometimes they are seeming dangerous if the reaction is at a higher rate.

In an exothermic reaction, the amount of energy released during the formation of new products or bonds is higher than the total amount of energy needed in the breaking of the reactants or bonds. This is the reason for the heating of the system or reactions as the enthalpy change is lower at the end of the reaction, so ∆H indicates a negative value. The end products of the exothermic reactions are stable.


When laundry detergent is dissolved in the water, there is the production of ample heat that warms up the water. This is an example of an exothermic reaction.

Combustion or burning of coal, a candle, or sugar is also an example of an exothermic reaction.

Formation of ice from water, digestion of food, respiration, the formation of rain, thermite reaction, neutralization reaction, respiration, corrosion of metal, and explosions are other examples of exothermic reactions

Key Differences between Endothermic and Exothermic Reactions

  1. The endothermic reaction requires energy, whereas the exothermic reaction releases energy.
  2. The change in enthalpy is positive in an endothermic reaction, whereas the change in enthalpy is negative in an exothermic reaction.
  3. The temperature decreases with the progression of an endothermic reaction, whereas temperature increases with the progress of the exothermic reaction.
  4. In an endothermic reaction, end products are less stable whereas, in an exothermic reaction, the end products are stable.

Key Similarities between Endothermic and Exothermic Reactions

  • Both endothermic and exothermic reactions form a new product.
  • In both reactions, energy is involved.
  • In both endothermic and exothermic reactions, a chemical change takes place.
  • In both reactions, there is a significant change in temperature.


In conclusion, endothermic and exothermic reactions are two types of chemical reactions that differ from each other based on heat absorption or release. Both are opposite to each other.

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