Allison McCulloch CEM 111.01 Microtheme 6 Hess' Law and the Standard Enthalpies of Formation Hess' Law is as follows: "The overall reaction enthalpy is the sum of the reaction enthalpies of the steps into which the reaction can be divided. Hess' Law incorporates changing the order of a reaction as well as changing the sign from positive to negative or negative to positive on delta H. State functions are important to pay attention to in determining the final answer. Different sequences of reactions are compared to the original reaction (final - initial) and the components of the reaction may have to be multiplied by the stoichiometric coefficient that will make the component even with the original reaction. Take for example the enthalpy of combustion for propane: 3 C (s) + 4H2 (g) -> C3H8 (g) is the original reaction C3H8 (g) + 5O2 (g) -> 3CO2 (g) + 4H2O (l) delta H = -2220.kJ (a) C (s) + O2 (g) -> CO2 (g) delta H =-394 kJ (b) H2 (g) + O2 (g) -> H2O (l) delta H = -286 kJ (c) a, b, and c are the components of the reaction. B must be multiplied by 3 to equal the original equation. The standard enthalpies of formation minus the reactants from the products. Basically, all of the individual enthalpies of formation are added up, after being multiplied by the stoichiometric coefficient. That is the way the total enthalpy of formation is calculated, quite similar to Hess' Law.