Bond Energy Calculator
This bond energy calculator estimates the enthalpy change of a reaction from bond energies. Breaking bonds takes energy in, and forming bonds gives energy out, so the reaction enthalpy is the total energy of the bonds broken in the reactants minus the total energy of the bonds formed in the products. Enter both totals and the calculator returns the estimated enthalpy and whether the reaction is exothermic or endothermic. Because tabulated bond energies are averages, this gives an estimate rather than an exact value, but it is a quick and useful check. The formula, a worked example and the assumptions are below.
The formula
Reaction enthalpy is approximately the sum of the bond energies of bonds broken minus the sum of the bond energies of bonds formed: delta H = (bonds broken) minus (bonds formed). A negative value is exothermic.
Worked example
If breaking the reactant bonds needs 1500 kJ/mol and forming the product bonds releases 1800 kJ/mol, delta H = 1500 minus 1800 = minus 300 kJ/mol, exothermic. Enter 1500 and 1800 to confirm.
Frequently asked questions
How do bond energies give reaction enthalpy?
Add the energies of all bonds broken, subtract the energies of all bonds formed. Breaking costs energy, forming releases it.
Why is it only an estimate?
Tabulated bond energies are averages over many molecules, so the result is approximate, not exact.
What sign means exothermic?
A negative delta H. More energy is released forming bonds than is used breaking them.
Who this calculator is for
This calculator is for chemistry students, teachers and lab workers.
What this calculator assumes
- You enter values in the units shown.
- Standard conditions unless stated.
- Results are rounded for display.
Formula and sources
Related calculators
- Molarity Calculator: concentration in mol/L.
- pH Calculator: acidity from H+ concentration.
- Ideal Gas Law Calculator: PV = nRT.
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