How can we calculate the heat of combustion of ethanol from these data? Notice that the combustion reaction can be obtained by the following combination of formation Reactions 1 through 4:

combustion reaction = (products) - (reactants)
combustion = 2(R3) + 3(R4) - (R1) - 3(R2)

From the additivity principle, it follows that the heat of combustion of ethanol is found from the corresponding combination of heats of formation:

heat of reaction = 2(-94.1) + 3(-68.3) - (-66.4) - 3(0)   =  -326.7 kcal

Notice that we can manipulate these heats of formation as if they were absolute enthalpies of molecules, rather than enthalpies of formation of the molecules from elements. Everything pertaining to the elements from which they came cancels out on both sides of the equation.