Bond Energy and the Heat of Reaction

To understand how bond energies relate to the heat of reaction it will help to look at a specific example.

2 C₂H₆ + 7 O₂ --> 4 CO₂ + 6 H₂O

Imagine that you had built models of the compounds on the left (2 models of C₂H₆ and 7 models of O₂). You would then have all of the pieces needed to build models of the products - that's what it means for the reaction to be balanced.

In order to make the products, you would first need to take apart the reactant models. Nature has the same requirement. In order to make the CO₂ and H₂O of the products, nature must first take apart the reactants.

In other words, we must first break the bonds in the reactants, then we can make the bonds in the products.

The best way to see that is to draw the Lewis Dot Structures for the reaction.

For this reaction to occur we need to break all of the bonds on the left side of the reaction. Specifically, we need to break
  2 (C-C) + 12(C-H) + 7(O=O)

To complete the reaction we then need to make all of the bonds on the right side of the reaction. Specifically, we need to make 8(C=O) + 12(O-H)

To turn this into useful information, we need to go a step further.