# Thermochemistry: What is the difference between the calculation of bond enthalpy and Hess's Law

I know that Hess's law is an indirect way to find the heat of reaction using the known heat of reaction of 2+ thermochemical equations, thus the formula is delta heat of reaction = enthalpy of formation (product)- enthalpy of formation(reactant). But then, bond enthalpy also finds the change in heat of reaction with the formula: change in heat of reaction = bonds broken (reactant) – bonds formed (product). If it's negative, it's exothermic, and if positive, it's endothermic.

I am confused at the relationship between these 2 concepts since they are rather opposing each other.

Consider as reference state all your reactants and products in their atomic states, that is, with all bonds broken. Forming a bond releases energy, so the reactants and products all have a negative enthalpy $$\Delta H = -\sum\Delta H_\text{bonds}$$ with respect to this reference state. The enthalpy of reaction remains \begin{align*}\Delta H_\text{rxn}&=\Delta H_\text{prod}-\Delta H_\text{reac}\\ &= -\sum\Delta H_\text{bonds,prod}-\left(-\sum\Delta H_\text{bonds,reac}\right)\\ &= \sum\Delta H_\text{bonds,reac}-\sum\Delta H_\text{bonds,prod},\end{align*}