Summarize:

There are many tables that have information on the average bond energies for a specific bond. These tables can be found online or in a chemistry book. It is important to note that these bond energies are always for molecules in a gaseous state. For our example, you need to find the bond energy for an H-H bond, a Br-Br bond, and an H-Br bond. H-H = 436 kJ/mol; Br-Br = 193 kJ/mol; H-Br = 366 kJ/mol. To calculate bond energy for molecules in a liquid state, you need to also look up the enthalpy change of vaporization for the liquid molecule. This is the amount of energy needed to convert the liquid into a gas. This number is added to the total bond energy. For example: If you were given liquid water, you would need to add the enthalpy change of vaporization of water (+41 kJ) to the equation. In some equations, you may have the same bond broken multiple times. For example, if 4 atoms of hydrogen are in the molecule, then the bond energy of hydrogen must be counted 4 times, or multiplied by 4. In our example, there is only 1 bond of each molecule, so the bond energies are simply multiplied by 1. H-H = 436 x 1 = 436 kJ/mol Br-Br = 193 x 1 = 193 kJ/mol Once you have multiplied the bond energies by the number of the individual bonds, you need to then sum all of the bonds on the reactant side. For our example, the sum of the bonds broken is H-H + Br-Br = 436 + 193 = 629 kJ/mol. Just as you did for the bonds broken on the reactant side, you will multiply the number of bonds formed by its respective bond energy. If you have 4 hydrogen bonds formed, you would need to multiply that bond energy by 4. For our example we have 2 H-Br bonds formed, so the bond energy of H-Br (366 kJ/mol) will be multiplied by 2: 366 x 2 = 732 kJ/mol. Again, like you did with the bonds broken, you will add up all of the bonds formed on the product side. Sometimes you will only have 1 product formed and can skip this step. In our example, there is only 1 product formed, so the energy of the bonds formed is simply the energy of the 2 H-Br bonds or 732 kJ/mol. Once you have summed all of the bond energies for both sides, simply subtract the formed bonds from the broken bonds. Remember the equation: H = H(bonds broken) - H(bonds formed). Plug in the calculated values and subtract. For our example: H = H(bonds broken) - H(bonds formed) = 629 kJ/mol - 732 kJ/mol = -103 kJ/mol. The final step to calculating bond energy is to determine whether the reaction releases energy or consumes energy. An endothermic (one that consumes energy) will have a final bond energy that is positive, while an exothermic reaction (one that releases energy) will have a negative bond energy. In our example, the final bond energy is negative, therefore, the reaction is exothermic.
Look up the bond energies of the bonds in question. Multiply the bond energies by the number of bonds broken. Add up all of the bond energies of the broken bonds. Multiply the bond energies by the number of bonds formed. Add up all of the formed bond energies. Subtract the formed bonds from the broken bonds. Determine whether the entire reaction was endothermic or exothermic.