Hess's Law

Hess's law is a statement of energy conservation applied to reaction enthalpy, and its practical value lies in what it permits: the enthalpy change of a reaction too slow, too violent or too impure to measure directly can be obtained by summing the enthalpies of any set of reactions that add to it. Because enthalpy is a state function, only the initial and final states matter and the route between them is immaterial. In use, reactions may be reversed, in which case the sign of the enthalpy change is inverted, or scaled, in which case the enthalpy is multiplied by the same factor. The standard enthalpies of formation tabulated for thousands of compounds are compiled and applied on exactly this basis.