Exothermic and Endothermic Reactions [Definition, Examples, and Differences]

Exothermic Reactions

Exothermic reactions are reactions or processes that emit energy, usually in the form of heat or light. In an exothermic reaction, energy is released since the overall energy of the products is less than the overall energy of the reactants.

For this reason, the change in enthalpy, ΔH, for an exothermic reaction will constantly be negative. In the presence of water, a strong acid will dissociate quickly and release heat, so it is an exothermic reaction.


Numerous chain reactions emit or release energy in the form of heat, light, or sound. Exothermic reactions may take place spontaneously and lead to higher randomness or entropy (ΔS > 0) of the system. In the laboratory, exothermic reactions produce heat or may even be explosive.

Endothermic Reactions

Endothermic reactions are reactions that need external energy, typically in the form of heat, for the reaction to proceed. Considering that endothermic reactions attract heat from their environments, they tend to cause their environments to cool off.

They are also normally non-spontaneous, given that endothermic reactions yield products that are greater in energy than the reactants. As such, the change in enthalpy for an endothermic reaction is constantly positive. In order to melt the ice cube, heat is needed, so the procedure is endothermic.

Further Reading:  Basic Concepts of Chemistry


Endothermic reactions can not take place spontaneously. Work needs to be done in order to get these reactions to take place.

Reaction progression
Exothermic reaction

In an exothermic reaction, the overall energy of the products is less than the total energy of the reactants. For that reason, the change in enthalpy is negative, and heat is released to the surroundings.

Endothermic reaction

In an endothermic reaction, the products are higher in energy than the reactants. For that reason, the change in enthalpy is positive, and heat is taken in from the environments by the reaction.

Examples of Endothermic and Exothermic Processes
Endothermic Reactions

Photosynthesis is an example of an endothermic chain reaction. In this process, plants utilize the energy from the sun to transform CO2 and water into glucose and oxygen. This reaction requires 15MJ of energy (sunshine) for every single kg of glucose that is produced:

sunlight + 6CO2(g) + WATER(l) = C6H12O6(aq) + 6O2(g)

Other examples of endothermic processes include:

  • Dissolving ammonium chloride in water.
  • Breaking alkanes.
  • Nucleosynthesis of elements heavier than a nickel in stars.
  • Evaporating liquid water.
  • Melting ice.
Exothermic Reactions

An example of an exothermic reaction is the mixture of salt and chlorine to yield table salt. This reaction produces 411 kJ of energy for each mole of salt that is produced:

Na(s) + 0.5 Cl2(s) = NaCl(s)

Other examples of exothermic procedures consist of:

  • The thermite reaction.
  • A neutralization reaction (e.g., mixing an acid and a base to form a salt and water).
  • Many polymerization reactions.
  • Combustion of a fuel.
  • Nuclear fission.
Further Reading:  Basic concepts of Chemistry - Part 2
Endergonic and Exergonic Reactions

Endothermic and exothermic reactions refer to the absorption or release of heat. There are other kinds of energy that may be produced or soaked up by a chain reaction. Examples include light and noise. In general, reactions involving energy might be categorized as endergonic or exergonic.

An endothermic reaction is an example of an endergonic reaction. An exothermic reaction is an example of an exergonic reaction.

Difference Between Endothermic and Exothermic Reactions

Endothermic Reaction                                    Exothermic Reaction
A reaction in which the system absorbs energy from its surrounding in the form of heat. A reaction in which energy is released from the system in the form of heat.
The energy is absorbed from the surrounding environment into the reaction. The energy is released from the system to its surrounding environment.
Energy remains in the form of heat. Energy is emitted as heat, electricity, light, or sound.
Melting ice, evaporation, cooking, gas molecules, photosynthesis is a few examples. Rusting iron, settling, chemical bonds, surges, nuclear fission are a few examples.