An endothermic reaction is a chemical reaction that absorbs thermal energy from its surroundings. Because heat is absorbed, endothermic reactions feel cold. The heat absorbed by the reaction provides the activation energy needed for the reaction to occur. More energy is needed to break chemical bonds than is released reforming them to make new products. The enthalpy change in an endothermic reaction is positive: ΔH > 0.
French chemist Marcellin Berthlot (1827-1907) coined the term “endothermic” from the Greek roots endo– (meaning “within”) and therm (meaning “heat”). The opposite of an endothermic reaction is an exothermic reaction. An exothermic reaction releases heat to the surroundings and feels warm.
Endothermic Reaction Examples
Here is a list of examples of endothermic reactions. Use these to cite examples or get ideas for endothermic reaction demonstrations.
- The reaction of barium hydroxide octahydrate crystals with dry ammonium chloride
- Dissolving ammonium chloride in water
- Cracking alkanes
- The reaction of thionyl chloride (SOCl2) with cobalt(II) sulfate heptahydrate
- Thermal decomposition reactions
- Mixing water and ammonium nitrate
- Mixing water with potassium chloride
- Reacting ethanoic acid with sodium carbonate
- Photosynthesis (chlorophyll is used to react carbon dioxide plus water plus energy to make glucose and oxygen)
- Electrolytic decomposition of sodium chloride (table salt) into sodium hydroxide and hydrogen chloride
Endothermic Process Examples
An endothermic process is a more general term for a heat-absorbing phenomenon. Processes aren’t always easily written as chemical reactions, either because the reactants don’t change their chemical identity (as in phase changes), the chemistry is complex, or the nature of the reactants isn’t known. Here are examples of endothermic processes:
- Melting ice cubes
- Melting solid salts
- Sublimation of dry ice into carbon dioxide gas
- Evaporating liquid water
- Converting frost to water vapor (melting, boiling, evaporation, and sublimation are endothermic processes)
- Making an anhydrous salt from a hydrate
- Forming a cation from an atom in the gas phase
- Nucleosynthesis in stars of elements heavier than nickel
- Nuclear fusion of elements heavier than iron in a supernova
- Splitting a gas molecule
- Separating ion pairs
- Cooking an egg
- Baking bread
Endothermic vs Endergonic
Although the terms “endothermic” and “endergonic” often are used interchangeably, the two terms don’t mean precisely the same thing. Endothermic reactions absorb heat, while endergonic reactions absorb energy. An endothermic reaction is an example of an endergonic reaction. So, not all endergonic reactions are endothermic. For example, an endergonic reaction could absorb sound or light.
- Atkins P.; de Paula J., Keeler, J. (2017). Atkins’ Physical Chemistry (11th ed.). Oxford University Press. ISBN 978-0198769866.
- Crosland, M.P. (1970–1980). “Berthelot, Pierre Eugène Marcelin”. Dictionary of Scientific Biography. 2. New York: Charles Scribner’s Sons. pp. 63–72. ISBN 978-0-684-10114-9.
- Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.