Bond Energy Calculator
The approximate enthalpy of a reaction from the bonds broken and formed — the estimate you can make without a data table of formation enthalpies.
Breaking bonds costs energy; forming them releases it.
How the bond energy calculator works
Breaking bonds costs energy; forming them releases it. So ΔH ≈ Σ(bonds broken) − Σ(bonds formed).
This uses average bond enthalpies, so it is an estimate — typically within 10% for gas-phase reactions and worse where the actual bonds differ from the averages. It is nonetheless the fastest way to see whether a reaction will release energy and roughly how much.
Formula: ΔH ≈ Σ E(broken) − Σ E(formed)
Worked examples
| Inputs | Estimated ΔH | Note |
|---|---|---|
| Methane combustion | -802 kJ/mol | −802 kJ/mol, against a true −890 |
| Bonds of equal strength | 0 kJ/mol | thermoneutral |
| An endothermic case | 400 kJ/mol | +400 kJ/mol |
FAQFrequently asked questions
How does bond energy give a reaction enthalpy?
Breaking bonds costs energy and forming them releases it, so ΔH is the difference between the two totals.
How accurate is it?
Typically within about 10% for gas-phase reactions. It uses averages, so a bond in an unusual environment will deviate.
Why does it disagree with the true value?
Average bond enthalpies are averaged across many molecules. Formation enthalpies are measured for the specific compounds, which is why they are more accurate.
Does it work for reactions in solution?
Poorly. Solvation energies can be large and this method ignores them entirely.
Why is combustion so exothermic?
Because the bonds in carbon dioxide and water are unusually strong — the C=O double bond alone is 799 kJ/mol.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.