To find the heat of reaction, we simply find a change in atrial energy or change in enthalpy of the product. We find the change in entropy or, I'm sorry, enthalpy of the reactants and we subtract this from this and that will give us the heat of reaction. So recall it, enthalpy is a state function and what that basically means that the pathway taken to get to the final product or to get to the final system does not affect the final change in enthalpy. And this jumps directly into something called Hess's Law. Now, Hessa's Law basically states that the steps taken to get from the reactant to the products does not affect the final change in enthalpy, okay? And Hess's Law can be applied to reactions.