Understanding Endothermic and Exothermic Reactions

Endothermic vs Exothermic

Sun shines through green leaves as a plant performs photosynthesis, an example of an endothermic reaction

Frank Krahmer/Getty Images

Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ΔH < 0). In the lab, exothermic reactions produce heat or may even be explosive.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH).

Examples of Endothermic and Exothermic Processes

Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. This reaction requires 15MJ of energy (sunlight) for every kilogram of glucose that is produced:

sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)

Other examples of endothermic processes include:

  • Dissolving ammonium chloride in water
  • Cracking alkanes
  • Nucleosynthesis of elements heavier than nickel in stars
  • Evaporating liquid water
  • Melting ice

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

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

Other examples of exothermic processes include:

  • The thermite reaction
  • A neutralization reaction (e.g., mixing an acid and a base to form a salt and water)
  • Most polymerization reactions
  • Combustion of a fuel
  • Respiration
  • Nuclear fission
  • Corrosion of metal (an oxidation reaction)
  • Dissolving an acid in water

Demonstrations You Can Perform

Many exothermic and endothermic reactions involve toxic chemicals, extreme heat or cold, or messy disposal methods. An example of a quick exothermic reaction is dissolving powdered laundry detergent in your hand with a bit of water. An example of an easy endothermic reaction is dissolving potassium chloride (sold as a salt substitute) in your hand with water.

These endothermic and exothermic demonstrations are safe and easy:

Endothermic vs Exothermic Comparison

Here's a quick summary of the differences between endothermic and exothermic reactions:

Endothermic Exothermic
heat is absorbed (feels cold) heat is released (feels warm)
energy must be added for reaction to occur reaction occurs spontaneously
disorder decreases (ΔS < 0) entropy increases (ΔS > 0)
increase in enthalpy (+ΔH) decrease in enthalpy (-ΔH)

Endergonic and Exergonic Reactions

Endothermic and exothermic reactions refer to the absorption or release of heat. There are other types of energy which may be produced or absorbed by a chemical reaction. Examples include light and sound. In general, reactions involving energy may be classified as endergonic or exergonic, An endothermic reaction is an example of an endergonic reaction. An exothermic reaction is an example of an exergonic reaction.

Key Facts

  • Endothermic and exothermic reactions are chemical reactions that absorb and release heat, respectively.
  • A good example of an endothermic reaction is photosynthesis. Combustion is an example of an exothermic reaction.
  • The categorization of a reaction as endo- or exothermic depends on the net heat transfer. In any given reaction, heat is both absorbed and released. For example, energy must be input into a combustion reaction to start it (lighting a fire with a match), but then more heat is released than was required.


Resources and Further Reading

  • Qian, Y.‐Z., et al. “Diverse Supernova Sources for the r‐Process.” The Astrophysical Journal, vol. 494, no. 1, 10 Feb. 1998, pp. 285-296, doi:10.1086/305198.
  • Yin, Xi, et al. “Self-Heating Approach to the Fast Production of Uniform Metal Nanostructures.” Chemistry of Nanomaterials for Energy, Biology and More, vol. 2, no. 1, 26 Aug. 2015, pp. 37-41, doi:10.1002/cnma.201500123.