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En­thalpy is a state and cal­cu­la­tion vari­able in ther­mo­dy­nam­ics. En­thalpy (H) stands for the amount of heat giv­en off or ab­sorbed by exother­mic or en­dother­mic re­ac­tions. In the past, this quan­ti­ty was al­so called the unit of heat. Its unit of mea­sure­ment is kilo­joules (kJ). En­thalpy al­so de­scribes the mea­sure of the en­er­gy of a ther­mo­dy­nam­ic sys­tem. It is cal­cu­lat­ed as the sum of the in­ter­nal en­er­gy (U) of a sys­tem and its prod­uct of pres­sure ( p ) and vol­ume (V).

Exothermic and endothermic reactions

Both re­ac­tions are ba­si­cal­ly ac­com­pa­nied by a turnover of energy.

If ther­mal en­er­gy is re­leased in the form of heat dur­ing a re­ac­tion, we speak of an exother­mic (ex = out­ward, therm = heat) re­ac­tion. Ex­am­ples from every­day life are the burn­ing of wood in a stove and the burn­ing of gasoline/diesel in an engine.

If ther­mal en­er­gy is added to a re­ac­tion in the form of heat, we speak of an en­dother­mic (en­do = in­ward, therm = heat) re­ac­tion. An ex­am­ple from every­day life is sher­bet pow­der in wa­ter. When the sher­bet pow­der (cit­ric acid + sodi­um hy­dro­gen car­bon­ate) dis­solves in the wa­ter, the en­er­gy from the wa­ter is need­ed. This re­ac­tion al­so makes the wa­ter a lit­tle colder.


The en­tropy (S) in­flu­ences in which di­rec­tion a re­ac­tion pro­ceeds. A re­ac­tion likes to go from an or­dered state to a dis­or­dered state. This is al­so the rea­son why the burned ash (-> dis­or­dered state) will not form a log (-> or­dered state). Al­so the dis­solved ef­fer­ves­cent tablet (-> dis­or­dered state) will not as­sem­ble to a sol­id tablet (-> or­dered state).

There­fore the state­ment is true: The more pos­si­bil­i­ties there are, which a state can take, the more like­ly they meet. 

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