Four H atoms and one C atom are attracted to each others electrons. When all the atoms come together and form mole CH₄ 1652 kJ is released. What holds the H atoms in place, the attraction of the C electrons to H, and the H's electrons to C; we say that there is a bond holding the C and the H atoms together. Since there are four bonds each bond contributes a certain amount of stability.

In this manner we can estimate the bond energy for all sorts of different compounds.

Does this mean that all C-H bonds are "worth" 413 kJ of energy?

No, looking at a few simple molecules will show that this is not true.

When discussing bond strength we usually refer to bond dissociation energies. The book uses the letter D for bond dissociation energies; I typically use all three letters BDE (other books also use BDE). DH_D = DH_BDE

Is DH_rxn for the following DH_BDE; i.e., does the reaction written below describe the process of sepearting two atoms?

NO! Putting KBr in water does not separate the two atoms; it separates the two ions.

The reaction which describes the BDE for KBr is

Back to our original question, are all C-H bonds worth 413 kJ/mol? The reaction which describes the DH_BDE for CH₄ is

These are NOT reactions which describe the BDE for methane:

I wrote these examples because they seem to be popular mistakes.

Why are the bond energies different for the same bond? Because they are not the same bond.

The carbon to which the hydron is attached is different in each molecule. A carbon with 3 Cl's on it is different than a C with 3 H's bonded to it.

So it would seem reasonable to say that an H would bond differently to a C that has an H on it as compared to a C that has an F on it.

Now that we know what bond energies are, we can use them to calculate the DH's for reactions. Bond Energies can be used to estimate the amount of energy released or absorbed in a reaction.

1 mole of H-H bonds are broken which requires 432 kJ, and 1/2 mol O=O bonds are broken 495, and 2 mol O-H bonds are formed 2 x 467.

So, DH = -2 x 467 + 432 + 1/2(495) = -255 kJ/mol

You may have noticed that...

DH_rxn = DH_BDEreactant - DH_BDEproduct

energy is added to break bonds in the reactants

energy is released as the bonds form

So, it is not as simple as a bond is a bond. Bonds are effected by atoms that are not directly participating in the bond under discussion. Which means that the model of an isolated bond between two atoms really is not the whole story; additionally, when using BDE's to predict DH of a reaction the model (the bond in the table) should resemble bond in the molecule as closely as possible.