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 properly arrive at the right "exact" level of lower energy. And, this would be true on both sides of the reaction arrow. Your reactants will be "about right" and your products will also be "about right". Now START with the reactants, BREAK all the bonds in them to make all the elemental atoms, and then finally MAKE all the bonds for the products. The math is easy, just take the difference between the those two values and you'll have the change in energy (ΔH) for the reaction.

ΔH = ∑BEreactants – ∑BEproducts

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.



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