Editing Standard enthalpy of reaction (section)

== Variation with temperature or pressure ==
The variation of the enthalpy of reaction with temperature is given by [[Gustav_Kirchhoff#Kirchhoff's_law_of_thermochemistry|Kirchhoff's Law of Thermochemistry]], which states that the temperature derivative of ΔH for a chemical reaction is given by the difference in [[heat capacity]] (at constant pressure) between products and reactants:

:<math>\left(\frac{\partial \Delta H}{\partial T}\right)_p = \Delta C_p</math>.

Integration of this equation permits the evaluation of the heat of reaction at one temperature from measurements at another temperature.<ref>[[Keith J. Laidler|Laidler K.J.]] and Meiser J.H., "Physical Chemistry" (Benjamin/Cummings 1982), p.62</ref><ref>[[Peter Atkins|Atkins P.]] and de Paula J., "Atkins' Physical Chemistry" (8th edn, W.H. Freeman 2006), p.56</ref>

:<math>\Delta H^\circ \! \left( T \right) = \Delta H^\circ \! \left( T^\circ \right) + \int_{T^\circ}^{T} \Delta C_P^\circ \, \mathrm{d} T</math>

Pressure variation effects and corrections due to mixing are generally minimal unless a reaction involves non-ideal gases and/or solutes, or is carried out at extremely high pressures. The [[enthalpy of mixing]] for a solution of ideal gases is exactly zero; the same is true for a reaction where the reactants and products are pure, unmixed components. Contributions to reaction enthalpies due to concentration variations for solutes in solution generally must be experimentally determined on a case by case basis, but would be exactly zero for [[ideal solution|ideal solutions]] since no change in the solution's average intermolecular forces as a function of concentration is possible in an ideal solution.