Difference between endothermic and exothermic reaction

Surely more than once someone has heard these terms during a chemistry class or maybe just wondering what is the difference between these two types of reaction. In this article we want to explain the differences between endothermic and exothermic reaction.

To understand the differences between these terms it is important to know that a chemical reaction is a process that allows one or more substances, called reagents, to be transformed into another resulting substances that will have different properties, called products. In the same way, we must know that enthalpy is a magnitude of thermodynamics which is expressed by the letter H and its variation expresses the amount of energy that is released or absorbed in a reaction.

Exothermic Reaction

The word “exo- has greek origin and means “out of, in this case we associate it as -out-. Therefore, it can be considered an exothermic reaction to any chemical reaction that will be able to dislodge or release energy in other words an exothermic reaction is the one which releases heat. In general, the exothermic reactions exist particularly in oxidation reactions. An oxidation reaction is one where a transfer of electrons between the reagents, where one of the reagents gives electrons and the other reagent. The exothermic reactions are negative enthalpy differences, due to release energy.

Among the exothermic reactions, we can highlight the changes in the state of matter as:

  • Condensation: the change of gaseous state to liquid state.
  • Solidification: the change from liquid to solid state.

Also, we can mention as an example of exothermic reaction to combustion, where there is an element that is burned, called fuel and the element that allows the oxidation called oxidizer. This reaction deprecates caloric energy and light energy. Equally, sometimes, it is necessary to use an activation energy that is the one that allows the reaction to occur.

Some examples of exothermic reactions are:

  • Burn coal, wood, fuel, plastic, among others.
  • The digestion of living beings.
  • The oxidation of metals.

Endothermic reaction

The word -endo- has Greek origin and means -inde-, in this case we associate it as -inward. Therefore, an endothermic reaction can be considered to any chemical reaction that is capable of absorbing energy. Consequently, in the endothermic reactions, the enthalpy of the products is greater (positive) than that of the reactants, ie the exothermic reactions will present positive enthalpy differences due to the energy they absorb.

Some examples of endothermic reactions may be photosynthesis of plants, reaction of iron with sulfur, reaction of water with potassium iodide, change of state from solid state water to liquid state (because ice absorbs Energy in the form of heat and that is why it changes state – it melts), among others.

Differences between exothermic reaction and endothermic reaction.


Exothermic reaction Endothermic reaction.
  •  An exothermic reaction releases energy into the environment.
  • An exothermic reaction has a negative enthalpy difference.
  • The final products are stable in exothermic reactions.
  • Examples of exothermic reactions are:

Burn coal, wood, fuel, plastic, among others.

The digestion of living beings.

The oxidation of metals.

  •  An endothermic reaction absorbs the energy of its surroundings.
  • An endothermic reaction has a positive enthalpy difference.
  • End-products in endothermic reaction are less stable.
  • Examples of endothermic reactions are:

Photosynthesis of plants

Reaction of iron with sulfur

Cooking an egg

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