Average Bond Energies
Average bond energies (kJ/mol) and average bond lengths (Å) for single, double, and triple bonds. Estimate a reaction enthalpy from bond energies: ΔH ≈ Σ D(bonds broken) − Σ D(bonds formed).
Estimating ΔH°rxn = Σ (bonds broken) − Σ (bonds formed) when formation data is not handy.
Average values over many compounds, gas phase.
These are averages, so the estimate is approximate; formation enthalpies give a more exact answer.
Average Bond Energies and Bond Lengths
Estimate a reaction enthalpy as ΔH ≈ Σ(bonds broken) − Σ(bonds formed) using these averaged values.
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| H–H | Single | 436 | 0.74 |
| C–S | Single | 260 | 1.81 |
| F–Cl | Single | 255 | — |
| H–C | Single | 415 | 1.09 |
| C–Cl | Single | 330 | 1.77 |
| F–Br | Single | 235 | — |
| H–N | Single | 390 | 1.04 |
| C–Br | Single | 275 | 1.94 |
| Si–Si | Single | 230 | 2.35 |
| H–O | Single | 464 | 0.96 |
| C–I | Single | 240 | 2.13 |
| Si–P | Single | 215 | — |
| H–F | Single | 569 | 0.92 |
| N–N | Single | 160 | 1.45 |
| Si–S | Single | 225 | — |
| H–Si | Single | 395 | 1.48 |
| N=N | Double | 418 | — |
| Si–Cl | Single | 359 | 2.03 |
| H–P | Single | 320 | 1.42 |
| N≡N | Triple | 946 | 1.10 |
| Si–Br | Single | 290 | — |
| H–S | Single | 340 | 1.34 |
| N–O | Single | 200 | — |
| Si–I | Single | 215 | — |
| H–Cl | Single | 432 | 1.27 |
| N–F | Single | 270 | — |
| P–P | Single | 215 | 2.21 |
| H–Br | Single | 370 | 1.41 |
| N–P | Single | 210 | — |
| P–S | Single | 230 | — |
| H–I | Single | 295 | — |
| N–Cl | Single | 200 | 1.75 |
| P–Cl | Single | 330 | 2.03 |
| C–C | Single | 345 | 1.54 |
| N–Br | Single | 245 | — |
| P–Br | Single | 270 | — |
| C=C | Double | 611 | 1.33 |
| O–O | Single | 140 | 1.48 |
| P–I | Single | 215 | — |
| C≡C | Triple | 837 | 1.20 |
| O=O | Double | 498 | 1.21 |
| S–S | Single | 215 | 2.05 |
| C–N | Single | 290 | 1.47 |
| O–F | Single | 160 | — |
| S–Cl | Single | 250 | 2.07 |
| C=N | Double | 615 | 1.38 |
| O–Si | Single | 370 | 1.66 |
| S–Br | Single | 215 | — |
| C≡N | Triple | 891 | 1.16 |
| O–P | Single | 350 | 1.63 |
| Cl–Cl | Single | 243 | 1.99 |
| C–O | Single | 350 | 1.43 |
| O–Cl | Single | 205 | — |
| Cl–Br | Single | 220 | — |
| C=O | Double | 741 | 1.20 |
| O–I | Single | 200 | — |
| Cl–I | Single | 210 | — |
| C≡O | Triple | 1080 | 1.13 |
| F–F | Single | 160 | 1.42 |
| Br–Br | Single | 190 | 2.29 |
| C–F | Single | 439 | 1.35 |
| F–Si | Single | 540 | 1.56 |
| Br–I | Single | 180 | — |
| C–Si | Single | 360 | 1.86 |
| F–P | Single | 489 | 1.57 |
| I–I | Single | 150 | 2.66 |
| C–P | Single | 265 | 1.87 |
| F–S | Single | 285 | 1.56 |
Important Notes
- These are AVERAGE values over many compounds, so bond-energy estimates of ΔH are approximations; enthalpies of formation give more accurate values when available.
- ΔH ≈ Σ D(bonds broken) − Σ D(bonds formed). Breaking bonds costs energy (positive); forming bonds releases it.
- For the same pair of atoms, more bonds = stronger and shorter: C–C 345 kJ/mol (1.54 Å) → C=C 611 (1.33 Å) → C≡C 837 (1.20 Å).
- Bond lengths (Å) are average values; a dash means the length source does not tabulate that bond (mostly interhalogen pairs and some Si/N/P/O combinations).
- This table is a general reference; averaged bond energies vary somewhat from one compound to another.
Verification references: OpenStax Chemistry 2e, §7.5 Strengths of Ionic and Covalent Bonds (Tables 7.2 and 7.3); CRC Handbook of Chemistry and Physics, 2007, and Olmsted & Williams, Chemistry, 5th ed. (2011), via Chempendix.