You will notice that these values vary a lot, depending on where you look. It is possible to find an exact value of a bond only if you consider a molecule in particular. For instance, you can look on: CRC Handbook of Chemistry and Physics 85th ed - David R. Lide.

Average Bond energies

(kJ/mol)

Single Bonds

 N-N 161 O-O 139 O-H 470 H-H 435 C-H 414 N-H 389 S-H 339 C-O 352 C-C 348 P-O 419 N-O 222 S-H 339 C-N 293 C-S 260 S-S 214 Si-O 369

Double Bonds

 C=S 477 O=O 402 C=O 700 C=C 615 N=N 418 C=N 615

Triple Bonds

 C ≡ C 812 N ≡ N 946 C ≡ N 890

Energy - Intro

Calculating reaction enthalpy based on bond energies

## Bond energies

Making and breaking chemical bonds is the main subject of chemistry. So, it is logical that bond energies are a crucial issue.

The more energy is involved in a bond, the stronger it is, and, also, the shorter the corresponding bond length is. It makes sense to think that the further apart are the atoms that form a bond, the easier it would be to break that bond.

The tables on the left show average bond energies. It is not an extensive list but it shows the most common bonds, particularly in organic chemistry. The point about these energies being an average is that the exact bond energy depends also on the rest of the molecule. Although some of these bonds represent a whole molecule (like in the case of H-H or N ≡ N), in most cases they have other neighbouring atoms / bonds that will affect their energy. The neighbouring atoms can pull electron density away, for instance, and that affects the bond energy in question. For instance the C=O bond, which is very much commented because it is present in carbon dioxide, can have different values of bonding energy: 728 in the case of ketones, 715 in the case of aldehydes and 686 in formaldehyde. In various books and websites it appears as 800, and that is why I chose 800 for this bond.

It is readily seen that triple bonds are the strongest ones, followed by the double bonds and finally the single bonds.

Notice that the Si-O bond, which is the main constituent of most rocks , is a relatively strong bond (as we would expect). The C-C bond, which forms the backbone of most organic molecules, follows closely. The H-O bond, very common because of its role in water, is also very strong as we expect water to be very stable (oceans cover most of the planet).

The C=O bond is the strongest of the double bonds. If that was not so, it wouldn't be so difficult to get rid of carbon dioxide. Furthermore, this high bond energy is the main reason why combustion is a very exothermic process.

Finally, it is observed that N ≡ N is a very strong bond. That explains why most explosives produce N ≡ N (plus lots of C=O).