Hess's Law

[KarenaKaing_1D]

In Lavelle's course reader, he states enthalpy changes are additive, which is Hess's Law. What exactly does that mean? And what are state functions?

[Albert Agabekyan 1A]

I believe you can view "additive" as being another way of stating that the overall state enthalpy of chemical equations are found through summations, such as taking the enthalpy of the reactants and the enthalpy of the product in order to figure out the Standard Enthalpy Change of Formation.

State functions are functions in which the actual value is independent from the path that was completed in order to reach that quantity.

[AlisonWong_1D]

It also means that you can add the enthalpies of two different chemical reactions to get the net enthalpy of both reactions together.

For example, to form nitrogen oxide, there are two steps.
N2 + O2 -> 2 NO (enthalpy = 180 kJ)
2 NO + O2 -> 2 NO2 (enthalpy = -112 kJ)

Since we know the enthalpies for each of those reactions, we know the enthalpy for both of them together (180 + -112 = 68 kJ).
So the enthalpy of reaction for the formation of nitrogen oxide is 68 kJ.

This is an application of Hess's Law. It basically says that since the path the reaction takes doesn't matter, you can just add the results (the amount of heat given off at the end - that is, enthalpy).
(And the path doesn't matter because enthalpy is a state function, as Albert has defined).