Hess's Law

Hess's law states that the total enthalpy change of a chemical reaction is the same regardless of whether the reaction takes place in a single step or through a series of intermediate steps, provided the initial and final conditions are identical. Russian chemist Germain Hess published the law in 1840 based on his own calorimetric experiments, establishing one of the earliest quantitative statements of what would later be understood as a consequence of the conservation of energy, since enthalpy is a state function that depends only on a system's initial and final states rather than the path taken between them. The law allows the enthalpy change of a reaction that is difficult or impossible to measure directly to be calculated instead by adding together the enthalpy changes of a series of other reactions that sum, algebraically, to the reaction of interest. Hess's law is applied throughout thermochemistry, including in the construction of standard enthalpy of formation tables, from which the enthalpy change of virtually any reaction can be calculated by combining the tabulated formation enthalpies of its reactants and products.