What is an Endothermic Reaction?

There are many chemical reactions which involve the exchange of energy between the molecules and compounds. The bonds which hold all the molecules together contains a tremendous amount of energy. A large amount of energy is released as soon as the bonds broke. It means that it takes a great distribution of energy to to form bonds. The total energy of a chemical reaction is calculated by the number of bonds that are broken and formed.

An endothermic reaction takes place when a great amount of energy is needed to break the existing bonds in the reactants. It means that an endothermic reaction needs or takes in energy to continue. This energy can be provided in different forms but is basically in the form of heat. These reactions often draw heat from their environment to take place. Electrolysis is an example of an endothermic reaction; you can do this by just dissolving salt or sugar into the water when you are in the kitchen.

Endothermic Reactions Contains More Energy: Endothermic reactions absorbs energy and stores them in the form of chemical bonds. In this types of a reaction product of the reaction comprises of more energy compared to that of reactants at the initial stage of the reaction. It is mainly due to stored energy.

Endothermic Reactions Cool Their Surroundings: As mentioned earlier endothermic reaction absorbs heat energy into themselves. As a result, heat leaves the surroundings, and there is a drop in temperature. For instance, the cold pack that we make use to relief pain works based on endothermic reaction. That is the cold pack absorbs heat from the skin and pack feels cold as it absorbs energy from the surroundings that have been placed.  

Examples of Endothermic Reaction

1.   Photosynthesis

This process includes chlorophyll in green plants which converts carbon dioxide and water into oxygen and glucose. Here, the energy is provided by the Sunlight energy. It is an important reaction for the plant’s life.

Endothermic Reaction

2. In a test tube, when a small quantity of ammonium chloride (NH4Cl) is dissolved in water, the tube becomes colder. Heat is absorbed from the surroundings during the chemical reaction.

                                      NH4Cl(s) + H2O(l) →→ NH4Cl(aq) – heat

3. When crystals of sodium thiosulphate (Na2S2O3.5H2O), also known as hypo gets dissolved in water, a cooling effect takes place.

                                   Na2S2O3.5H2O + H2O →→ Ns2SO3(aq) – heat


Practise This Question

Value of gas constant R is –