Bond Energies and the Heat of Reaction, cont.

Using a table of bond energy, we can determine how much energy is associated with each of these two processes.

Breaking bonds

In the reaction 2 C₂H₆ + 7 O₂ --> 4 CO₂ + 6 H₂O we broke 2 (C-C) + 12(C-H) + 7(O=O).

Using the table, we can assign numbers to each of those bonds and determine the amount of energy required to break all of these bonds. (Be sure to notice that the O=O bond is NOT the same as 2 O-O bonds!)

Making bonds

In the same reaction, we made 8(C=O) + 12(O-H).

Using the same table, we can assign numbers to each of those bonds and determine the amount of energy released when those bonds are created. (Again, notice that the C=O bond is NOT the same as 2 C-O bonds.)

Putting it together 

Every reaction takes in some heat and gives off some heat.

That idea is worth repeating: Every reaction takes in some heat and gives off some heat.

If the reaction takes in more heat than it gives off, it is endothermic.

If the reaction gives off more heat than it takes in, it is exothermic.

We can determine this by looking at the relative size of the two numbers calculated above. In this case, the bonds formed give off more energy than was required to break the bonds, so this reaction is exothermic.

However, we can be more precise. We can determine how much extra energy is given out. Remember that exothermic processes have negative energy values, while endothermic processes have positive energy values.

Applying that idea here leads to the following math problem.

adding a negative is the same as subtraction, so

This idea can be generalized to: ΔH_rxn = bonds broken - bonds made (the minus in the equation takes care of the negative sign on the exothermic process for you.