Hess' law For many substances, the formation reaction may be considered as the sum of a number of simpler reactions, either real or fictitious. The enthalpy of reaction can then be analyzed by applying Hess' law, which states that the sum of the enthalpy changes for a number of individual reaction steps equals the enthalpy change of the overall reaction. This is true because enthalpy is a state function, whose value for an overall process depends only on the initial and final states and not on any intermediate states. Examples are given in the following sections.
HeSs' Law fOr mANY SubstAnceS, The Formation reaCtion May bE coNsiderED as tHe Sum of a numbeR Of simpLer reacTIONs, eitheR real oR ficTitious. the EntHALPY of reactIOn CAN then be anaLyZed BY ApplYING hesS' laW, wHiCH staTes thAt thE sum of the enthalpY Changes for A NumBeR of indiVIduAl rEacTion stEps eQualS THE entHalPy ChanGe of The oVEralL reAcTion. This Is truE beCaUSe eNtHAlPy iS A staTe functiOn, wHoSE vAlue for aN overall prOcess dEpendS onlY oN ThE iNitial and finaL sTates aNd noT on any inTermedIate stAteS. exAMpLeS are given iN the FoLlowinG sEcTions.