We can estimate the change in enthalpy for a reaction using average bond energies. Energy is required to break a bond, and energy is released when a bond forms. So, we can sum the energy required to break the bonds of our reactants, which is a positive delta H, and subtract the sum of the energy when the bonds of our products are formed, since forming bonds is a negative change in enthalpy. For example, if we have the reaction of methane and chlorine to produce CH3Cl and HCl, we need to break one carbon-hydrogen single bond and one chlorine-chlorine single bond in our reactants, and we'll form one carbon-chlorine bond and one hydrogen-chlorine bond in our products. We could use a table of average bond energies to estimate the energy required to break the bonds of the reactants and the energy released when we form the bonds of our products. Let's practice calculating the change in enthalpy for a reaction. Our first step is to draw the Lewis structures for reactants and products so we can tell what bonds are broken and formed. To calculate the change in enthalpy, we add up the energy required to break the bonds of the reactants and subtract the energy released when the bonds are formed in the products. We need to break one mole of nitrogen-nitrogen triple bonds and three moles of hydrogen-hydrogen single bonds in our reactants. We form two moles of ammonia molecules, and each molecule has three nitrogen-hydrogen single bonds. Plugging in our numbers from the table of bond energies and completing the calculation, we get the change in enthalpy for the reaction is -80 kilojoules.