Bond Energies

Actually, a fairly accurate (give or take 5 to 10%) estimate of the enthalpy change (ΔH) of a reaction can be calculated by using average bond energies. The idea is pretty good example of the first law and how energy is conserved. One can imagine that all the elements as atoms (yes, single separated atoms - we ARE just imagining) are the starting pieces for all compounds and we line them all up on one energy line - the element line. Now make any compound you want. Hopefully you'll want the reactants and the products. Both reactants and products will be much much lower in energy than the elemental atoms. And, each compound is pushed down to those lower energies because of chemical bonds. Each and every bond drops the energy by a fairly specific amount. So it stands to reason, if you add up all the drops for every bond in each compound, you could calculate the "exact" level of lower energy for that specific compound. This would be true on both sides of the reaction arrow. We can't really have the exact amount for every bond out there, but we can have the average amount of each bond type. Using these averages, your reactants will be "about right" and your products will also be "about right".

To do this, you would first START with the reactants - BREAKING all the bonds in them to make all the elemental atoms. You account for every bond broken and sum up the total energy to break them all. This is like tearing apart a nice tinker-toy castle all the way down to nothing but stick and cogs. The second step is re-assembly, which means you will be MAKING all the bonds for the products. Keeping the analogy, you're using the same tinker-toy pieces, but re-assembling into a windmill or anything thing else that is not a castle. Once again, the accounting work is important and you must sum up all the energies of the newly made bonds in the products. The math is easy, just take the difference between the those two sums and you'll have a really good estimate of the change in energy (ΔH) for the reaction.

ΔH = ∑BE_reactants – ∑BE_products

There is a Table of Bond Energies in our Appendix Chapter 10 Section 7

Energy Diagram

methane combustion using bond energies

Values are in appendix and are the following: C-H 413, O=O 498, C=O 799, and H-O 463.

NOTE: You should also realize that you only need to break what you have to in order to make products. Yes, for combustion reactions you have to break every single bond in the hydrocarbon. But in many other reactions, much of the molecule stays intact and you do not need to tear it apart. So remember, break and make only what you have to and you'll save yourself a lot of extra math.