Average bond enthalpies
The average enthalpy needed to break one mole of a given bond in gas-phase molecules. They let you estimate reaction enthalpies for gas reactions when formation data are missing.
| H–H | single | 436 |
|---|---|---|
| C–H | single | 413 |
| N–H | single | 391 |
| O–H | single | 463 |
| S–H | single | 339 |
| Si–H | single | 323 |
| H–F | single | 567 |
| H–Cl | single | 431 |
| H–Br | single | 366 |
| H–I | single | 299 |
| C–C | single | 348 |
| C=C | double | 614 |
| C≡C | triple | 839 |
| C–N | single | 293 |
| C=N | double | 615 |
| C≡N | triple | 891 |
| C–O | single | 358 |
| C=O | double | 799 |
| C≡O | triple | 1072 |
| C–F | single | 485 |
| C–Cl | single | 328 |
| C–Br | single | 276 |
| C–I | single | 240 |
| C–S | single | 259 |
| N–N | single | 163 |
| N=N | double | 418 |
| N≡N | triple | 941 |
| N–O | single | 201 |
| N=O | double | 607 |
| O–O | single | 146 |
| O=O | double | 495 |
| F–F | single | 155 |
| Cl–Cl | single | 242 |
| Br–Br | single | 193 |
| I–I | single | 151 |
| S–S | single | 266 |
| Si–Si | single | 226 |
| Si–C | single | 301 |
| Si–O | single | 368 |
These are averages over many compounds and apply to gases only; the same bond in a particular molecule can differ by 5–10 %. For diatomic molecules a more exact value follows from the formation enthalpies of the gaseous atoms (see the standard thermodynamic data table).