Endothermic reaction

In chemistry, endothermic reactions are those in which energy must be supplied. The standard enthalpy difference Δ {\displaystyle \Delta H^{\circ }}is positive. The enthalpy H is the sum of the Internal Energy of a system and the product of pressure and volume. It is the heat content of a system at constant pressure. If Δ denotes \Delta Hthe difference between the enthalpies of the final ( H_{2}) and initial substances ( H_{1}), i.e. the absorbed energy, the following applies to endothermic reactions: {\displaystyle \Delta H=H_{2}-H_{1}>0}.

An endothermic reaction is therefore a reaction in which energy, for example in the form of heat, is absorbed from the environment. It is the opposite of an exothermic reaction. An example of an endothermic reaction is photosynthesis, which takes place in plants.

The melting of ice or evaporation of water is also endothermic, but it is not a chemical reaction, but a physical change in the state of aggregation.

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.

Enthalpieprofil einer endothermen Reaktion

Legend:
 {\displaystyle \!\ H:={\text{Enthalpie}}}
 \!\ \Delta ^{{\ddagger }}H:={\text{Aktivierungsenthalpie}}
 \!\ \Delta _{{\mathrm {R}}}H:={\text{Reaktionsenthalpie}}

left: Initial state of reactants: stablemiddle
: Transition state of activated complex: unstableright
: Final state of products: metastable

For example: If you add water vapor to a coke layer, an endothermic reaction takes place: {\displaystyle \mathrm {C+H_{2}O\ {\xrightarrow {\triangle }}\ CO+H_{2}} }.

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.


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