Endothermic versus exothermic reactions


A chemical reaction is a process where the basic structure of a substance changes and the substance becomes a new substance with one or more different properties. The chemical reaction will occur in the generation or absorption of heat. Because of this reaction, the molecular structure of the substance changes, and new substances are produced (1) & (2). The following article mainly defines endothermic versus exothermic reactions.

There are two types of chemical reactions based on heat exchange

  1. Endothermic reaction: Heat is absorbed
  2. Exothermic reaction: Heat is released

The following discussion mainly describes the endothermic and exothermic reactions and what the difference between them is.

Endothermic reaction

A chemical reaction in which heat is absorbed is called an endothermic reaction. The matter produced as a result of an endothermic reaction is called endothermic matter (3)

Examples of an endothermic reaction

1. The reaction of nitrogen and oxygen produces nitric oxide gas and absorbs heat. So this is an example of endothermic chemical change and NO is endothermic matter.

N₂      +       O₂      =      2NO   –   Heat (43000 kcal)

Nitrogen     oxygen       nitric oxide

2. When ammonium nitrate (NH₄NO₃) or ammonium chloride (NH₄CL) is dissolved in water, heat is absorbed and the solution cools. This is a physical endothermic reaction.

Exothermic reaction

The chemical reaction that produces heat, is called an exothermic reaction. The matter produced by this chemical reaction is known as exothermic matter (3).

Examples of exothermic reactions

1. When water is mixed with lime (calcium oxide), water is absorbed and a lot of heat is generated. As a result, calcium hydroxide (Ca(OH)₂) and heat are generated. This method is called the slaking of lime. It is an exothermic chemical reaction.

CaO     +        H₂O    =   Ca(OH)₂    +   Heat (15,500 kcal)

Calcium oxide    water    calcium hydroxide

2. When sulfuric acid (H₂SO₄) is mixed with water, a lot of heat is generated and the solution is hot. It is an exothermic physical reaction.

Difference between endothermic reaction and exothermic reaction

Based on the above information some of the key differences between the endothermic reaction and exothermic reaction. (1) & (2)


Endothermic reaction

Exothermic reaction

1. Heat absorb/ generate An endothermic reaction is a chemical reaction, where heat is absorbed. An exothermic reaction is a chemical reaction, where heat is generated.
2. Energy In the process of endothermic, the energy is absorbed. In the process of exothermic, the energy is released.
3. Amount of energy In the process of endothermic, the products contain more energy. In an exothermic reaction, the product contains less energy.
4. Temperature In this reaction, the temperature will decrease during the process. In this reaction, the temperature will increase during the process.
5. Potential energy As energy is absorbed in this process, so the total potential energy is increased in the endothermic reaction. Energy is released in this process, so the total potential energy is decreased in the exothermic reaction.
6. Nature of the surrounding area In this reaction, the surrounding area is cold because heat absorbs and the temperature drops the surrounding area. In this reaction, the surrounding area is hot because heat release and temperature increase the surrounding area.
7. Product nature In the endothermic reaction products are less stable In the exothermic reaction products are stable.

Similarities between endothermic reaction and exothermic reaction

  • In the endothermic and exothermic reactions, the molecules of matter are changed.
  • Both reactions change the molecules of matter to form new substances.
  • In both reactions (endothermic & exothermic), heat is involved.
  • Temperature changes are very important in both reactions. (1) & (3)

Is melting endothermic or exothermic?

Melting is a type of endothermic reaction. In this process, heat is given to the substance and due to this external heat bond breaks and the matter converts from one state to another.

Is freezing endothermic or exothermic?

Freezing is an exothermic process. It is the transition from a liquid state to a solid state. During the process of freezing energy in the form of heat is emitted.

Is sublimation endothermic or exothermic?

Yes, sublimation is an endothermic process. Sublimation is a process of transition of a solid directly into a gas.

Is photosynthesis endothermic or exothermic?

Photosynthesis is a process of making food and releasing oxygen to the plants. And yes energy in the form of light is required for this process of a chemical reaction.

Written By: Manisha Bharati

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