An endothermic reaction is a chemical reaction that takes in energy from the surroundings. The opposite of an endothermic reaction is an exothermic reaction. Reversible reactions are where the products can react to remake the original reactants. The energy is usually transferred as heat energy: the reaction absorbs heat. The temperature decrease can sometimes be detected using a thermometer. Some examples of endothermic reactions are:
Endothermic reaction
Sequence of an endothermic reaction
As with exothermic reactions, the process of endothermic reactions also takes place in two steps. First, a certain activation energy must be applied, and then part of this energy is released again. The difference to the exothermic reaction is that this released energy is less than the activation energy and is therefore not sufficient to drive the reaction further. The reaction energy is positive. Therefore, in order not to interrupt the reaction, energy must be continuously supplied from outside during the reaction.
For an endothermic reaction to take place at all (to be exergonic), the reaction must be favored by an increase in entropy and thus have a negative free enthalpy. Endothermic reactions therefore often take place at high temperatures, since at these, according to the Gibbs-Helmholtz equation, the entropy portion of the free enthalpy is greater. This can be seen, for example, in the Boudouard equilibrium, where the endothermic reaction to carbon monoxide takes place at high temperatures.
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For example: If you add water vapor to a coke layer, an endothermic reaction takes place: 
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Industrial chemistry
Endothermic reactions in industrial chemistry are referred to as allothermic. Important examples are allothermal pyrolysis, in which the biomass is split by externally supplied heat, or steam reforming in the production of synthesis gas. In contrast, autothermal reactions are used in industrial terminology.