Bond Enthalpy Calculator (ΔH from Bond Energies)
List the bond energies broken and formed to estimate the reaction enthalpy.
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Bond Enthalpy Calculator (ΔH from Bond Energies) by TheCalcTools
How the Bond Enthalpy Calculator (ΔH from Bond Energies) works
Breaking bonds takes energy and forming bonds releases it. The reaction enthalpy is roughly the energy of bonds broken minus the energy of bonds formed.
Average bond enthalpies are averages over many molecules, so results are estimates. Values from standard enthalpies of formation are more accurate.
The formula
Worked example
Using the default values shown in the calculator:
| Bonds broken (kJ/mol each, repeat for each bond) | 413, 413, 413, 413, 498, 498 |
|---|---|
| Bonds formed (kJ/mol each) | 799, 799, 463, 463, 463, 463 |
- Methane combustion: broken 4 × 413 + 2 × 498 = 2,648; formed 2 × 799 + 4 × 463 = 3,450.
- ΔH ≈ -802 kJ/mol (Exothermic).
Result:
| Estimated ΔH (kJ/mol) | -802.0 |
|---|---|
| Energy to break bonds (kJ/mol) | 2,648.0 |
| Energy released forming bonds (kJ/mol) | 3,450.0 |
| Reaction type | Exothermic |
Frequently asked questions
Why does my answer differ from the textbook ΔH?
Average bond enthalpies ignore molecular environment and the physical state of water. For methane combustion the estimate is about −818 kJ/mol against a measured −890 kJ/mol (liquid water).
Sources and further reading
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