The enthalpy a reaction experiences, DH, is the amount of heat that is absorbed or released by a reaction at constant pressure. This number is a conditional function. This means that no matter how the products are created by the reactants the enthalpy of the reaction will remain constant. Consider the figure below. In accordance with Hess’ Law, as an effect of enthalpy shift as a function of the state the enthalpy variation for process 1 equals the total of the enthalpy change from processes 2 and 3.