Bond Enthalpy Calculator

Work out bond enthalpy instantly with clear inputs, formula shown and shareable results.

About Bond Enthalpy Calculator

Breaking bonds costs energy and forming bonds releases it, so a reaction's enthalpy estimated from bond enthalpies is energy in minus energy out: bonds broken minus bonds formed. When the products' bonds are collectively stronger the ratio exceeds one and the reaction is exothermic. Bond enthalpies are averages across many molecules, so this route is an estimate rather than a measurement.

Formulas

Bond enthalpy route

dH = sum(bonds broken) - sum(bonds formed)

Strength ratio

ratio = energy formed / energy broken

Worked example

With the default values below, Bond Enthalpy Calculator returns Reaction Enthalpy From Bond Energies of -46.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total energy of bonds brokenminimum 0 kJ/mol: 2,648 kJ/mol
Total energy of bonds formedminimum 0 kJ/mol: 2,694 kJ/mol

Results

Reaction Enthalpy From Bond Energies: -46.00 kJ/mol
Thermal Character: Exothermic - stronger bonds formed
Formed-To-Broken Energy Ratio: 1.0174
Same Value In kcal/mol: -10.994

Step by step

1Values used
2Bond enthalpy route
3Strength ratio
4Reaction Enthalpy From Bond Energies= -46.00 kJ/mol
5Thermal Character= Exothermic - stronger bonds formed
6Formed-To-Broken Energy Ratio= 1.0174
7Same Value In kcal/mol= -10.994

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does this disagree with tabulated reaction enthalpies?

Bond enthalpies are averaged over many compounds, so a specific molecule's bonds differ by a few tens of kJ/mol. Formation enthalpies are compound-specific and more accurate.

Does this method work for reactions in solution?

Poorly. Bond enthalpies are defined for gas-phase species, and solvation energies in solution are often larger than the bond differences.