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Why do some chemical reactions absorb energy while others release energy?
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chemical reaction
Log burning in a fire. Burning wood is an example of a chemical reaction in which wood in the presence of heat and oxygen is transformed into carbon dioxide, water vapor, and ash.
In a chemical reaction, the absorption or release of energy is determined by the balance between the energy required to break existing chemical bonds and the energy evolved when new bonds are formed.12
- Exothermic reactions: These occur when the energy released during the formation of new, more stable bonds in the products is greater than the energy required to break the bonds in the reactants. This net release of energy often manifests as heat or light.34
- Endothermic reactions: These occur when the energy required to break the reactant bonds is greater than the energy evolved during the formation of product bonds. Consequently, these reactions require a continuous input of energy—such as heat or electricity—to proceed.124
The overall energy change, often measured as the enthalpy of reaction, reflects this difference. While energy balance is fundamental, the favorability of a reaction is also influenced by entropy, which accounts for how energy is distributed within a system and its surroundings.51