Endothermic means to absorb heat. Its etymology stems from the Greek suffix –thermic, meaning “to heat”, and the Greek prefix endo-, meaning “inside”. It refers to a transformation in which a system receives heat from the surroundings: Q > 0. When the transformation occurs at constant pressure: ∆H > 0; and constant volume: ∆U > 0. If the surroundings do not supply heat, an endothermic transformation leads to a drop in the temperature of the system.
In chemistry an endothermic reaction is a chemical reaction in which the products have more energy than the reactants. A net input of energy, usually in the form of heat, is required. Endothermic reactions absorb heat from their environment, and contrast with exothermic reactions, in which heat is released.
Although the process of bond breaking amongst reactants in a chemical process requires an initial input of energy (the activation energy), in the case of an endothermic reaction, the energy released when bonds are formed to create reactants is less than that required to break the bonds in the products; bonding electrons in the products are therefore at a higher energy than the reactants. Heat energy from the material surrounding the reactants is usually what breaks their bonds, so as heat energy is transferred from the surroundings to the reactants, the surroundings get colder. This is often summarized in a chemical equation as follows:
Reactants + Energy → Products
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