Hess's Law of Constant Heat Summation

Because $\Delta U$ and $\Delta H$ are both functions of a system's STATE, the heat evolved or absorbed by a given reaction can only depend on its initial and final states -- never on the particular path or sequence of intermediate steps used to get from one to the other. This generalisation is known as Hess's law, stated as: the enthalpy change of a reaction -- whether measured at constant volume or constant pressure -- is the same whether the reaction takes place in a single step or via multiple steps, provided the initial and final states are identical.

Symbolically, if a reaction from A to B can be reached directly (enthalpy change $\Delta H_r$) OR via an indirect route through intermediate states X and Y (enthalpy changes $\Delta H_1$, $\Delta H_2$, $\Delta H_3$ for the three legs of the indirect path), then:

Application of Hess's law. Its main practical value is calculating the enthalpy of reactions that are DIFFICULT to measure directly -- for example, it is very difficult to cleanly measure the heat of combustion of graphite reacting to give pure CO alone, since some of the CO invariably keeps oxidising further to CO$_2$ before you can isolate the measurement. However, the enthalpy of graphite oxidising all the way to CO$_2$, and of CO oxidising to CO$_2$, can each be measured cleanly and separately: $-393.5$ kJ and $-283.5$ kJ respectively. …