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[[Image:Hess cycle.svg|thumb|A representation of Hess's law (where H represents enthalpy)]]
'''Hess's law of constant heat summation''', also known simply as '''Hess' law''', is a relationship in [[physical chemistry]] named after [[Germain Henri Hess|Germain Hess]
Hess's law is now understood as an expression of the fact that the [[Standard enthalpy of reaction|enthalpy of a chemical process]] is independent of the path taken from the initial to the final state (i.e. enthalpy is a [[state function]]). According to the [[first law of thermodynamics]], the enthalpy change in a system due to a reaction at [[Isobaric process|constant pressure]] is equal to the heat absorbed (or the negative of the heat released), which can be determined by [[calorimetry]] for many reactions. The values are usually stated for reactions with the same initial and final temperatures and pressures (while conditions are allowed to vary during the course of the reactions). Hess's law can be used to determine the overall energy required for a chemical reaction that can be divided into synthetic steps that are individually easier to characterize. This affords the compilation of [[standard enthalpy of formation|standard enthalpies of formation]], which may be used to predict the enthalpy change in complex syntheses.
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