Enthalpy Since enthalpy is the heat content of a system under constant pressure, the change ΔH is negative in an exothermic reaction, and positive in an endothermic reaction. The standard reaction enthalpy, ΔH0, is the enthalpy change between 1 mol of products and reactants in the standard state (105 Pa and 298.15 K). The standard enthalpy of formation, ΔHf0, of a compound is the standard reaction enthalpy for the formation of the compound from its constituent elements. Since enthalpy is a state function, the standard reaction enthalpy is calculated by defining the standard enthalpy of formation of simple substances to be zero.
Namely,