Hess’s Law tells us: “The standard enthalpy of an overall reaction is the sum of the standard
enthalpies of the individual reactions into which the overall reaction can be divided.” This is
because enthalpy is a state function – the change in enthalpy depends only on the initial and final
states of the system, not on any states which exist during the course of a reaction. Thus, if we can
envision a series of reactions which get from our initial reactants to our terminal products in several
steps, we can add the individual enthalpies of those reactions together to determine rH.