endothermic reaction

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What are exothermic and endothermic reactions?

endothermic reaction

chemical reaction

Also known as: endoergic reaction

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June 26, 2026

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An endothermic reaction is a chemical reaction where energy is absorbed from the surroundings. This is the opposite of an exothermic reaction, which releases energy. In any chemical reaction, bonds between atoms in the reactants must be broken, which requires energy. New bonds are then formed in the products, which releases energy. If more energy is absorbed to break bonds than is released when new bonds are formed, the reaction is endothermic.1

An example of an endothermic reaction is the decomposition of limestone (calcium carbonate) into lime (calcium oxide) and carbon dioxide. This process requires significant heat input to occur. Another example is the electrolysis of water, which uses electrical energy to break water down into hydrogen and oxygen.1

The heat of reaction, or enthalpy of reaction ((\Delta H)), quantifies the heat absorbed or released. A positive (\Delta H) indicates an endothermic reaction, while a negative (\Delta H) signifies an exothermic reaction.2

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chemical reaction
Energy considerations
The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved.
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The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved.
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chemistry of industrial polymers
Industrial polymerization methods
…on the other hand, is endothermic—that is, the reaction requires an input of heat from an external source. In these cases the reactor must supply heat in order to maintain a practical reaction rate.
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…on the other hand, is endothermic—that is, the reaction requires an input of heat from an external source. In these cases the reactor must supply heat in order to maintain a practical reaction rate.
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heat of reaction
…reaction is said to be endothermic; if negative, exothermic.
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…reaction is said to be endothermic; if negative, exothermic.
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metallurgy
Pyrometallurgy
…are either exothermic (energy-releasing) or endothermic (energy-absorbing). One example of an exothermic reaction is the oxidation of iron sulfide (FeS) to form iron oxide (FeO) and sulfur dioxide (SO₂) gas:
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…are either exothermic (energy-releasing) or endothermic (energy-absorbing). One example of an exothermic reaction is the oxidation of iron sulfide (FeS) to form iron oxide (FeO) and sulfur dioxide (SO₂) gas:
Read More