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reaction enthalpyZoomA-Z

Subject - Physical Chemistry, Thermodynamics

The differential reaction enthalpy ΔrH is a partial differential quotient of the total differential of the state function H=H(p,T,ξ). To demonstrate, this differential quotient is represented as a sum of the partial molar enthalpies of educts and products.

ΔrH=(Hξ)p,T=iνiHpart,iΔr=ξ=differential operatorξ=turnover variablep=pressureT=absolute temperatureνi=stoichiometric numberHpart,i=partial molar enthalpies of substance i

The sum of partial molar enthalpies (partial molar size) can be written as a difference because the stoichiometric numbers of products are counted as positive while those of the educts enter the equation as negatives.


In case of an ideal reaction mixture, partial molar enthalpies can be substituted with molar enthalpies.

The integral reaction enthalpy ΔH is obtained by integration of the differential reaction enthalpies.

ΔH=ξ1ξ2ΔrHdξ=H(ξ2)H(ξ1)Δ=symbol for difference

Frequently, the integral reaction enthalpy ΔH  is described as the difference of molar enthalpies Hm,i  of the starting materials (products) and end products (educts) of a chemical reaction.


Molar instead of partial values can be used when assuming ideal conditions of the reaction mixture.

The integral reaction enthalpy is determined by measuring the heat of reaction of a reaction. Endothermic reactions show a positive enthalpy while reactions with negative enthalpy are called exothermic reactions.

If the pressure during the reaction remains constant, the integral reaction enthalpy equals the heat of reaction Qp being given or taken up by the reaction.