Bond Energy Calculator
Estimate ΔH from bonds broken and bonds formed.
Work out Bond Energy. Estimate ΔH from bonds broken and bonds formed. Free, with no account and nothing to download.
Bond enthalpies in kJ/mol, comma separated. List every bond, so a double bond counted twice means two entries.
H–Cl is 431, O–H is 463, C–H 413, C–C 347, C=O 799.
Bond enthalpies are averages across different molecules, so this is an estimate. Enthalpies of formation give the exact value.
Enthalpy change
-183 kJ/mol
Exothermic — releases heat
Breaking bonds always costs energy and forming them always releases it — there is no such thing as an exothermic bond break. A reaction is exothermic when the bonds in the products are collectively stronger than the ones in the reactants. Tabulated bond enthalpies are averages across many molecules, so this gives an estimate, typically within 10–20 kJ/mol of the measured value. For an accurate figure use standard enthalpies of formation instead. The method also only works for gas-phase reactions, because it ignores the energy of condensing or dissolving anything.
How the Bond Energy Calculator works
List the bond enthalpies of everything broken and everything formed and this returns the enthalpy change, along with whether the reaction is exothermic or endothermic. Bond enthalpies are averages, so this is an estimate — typically within 10 to 20 kJ/mol of the measured value.
Also known as: bond enthalpy calculator · enthalpy change from bond energies · exothermic or endothermic calculator · delta h from bond enthalpies
Frequently asked questions
How do I calculate enthalpy change from bond energies?
ΔH = bonds broken − bonds formed. Breaking costs energy and forming releases it, so a reaction is exothermic when the products' bonds are collectively stronger.
Why does my answer differ from the accepted value?
Because tabulated bond enthalpies are averaged across many different molecules. A C–H bond in methane is not identical to one in ethanol. For an exact value use standard enthalpies of formation.
Does this work for reactions in solution?
No. The method covers gas-phase reactions only, because it takes no account of the energy of condensing, dissolving or solvating anything.
Can breaking a bond ever release energy?
No, never. Bond breaking is always endothermic. When people describe ATP hydrolysis as an energy-releasing bond break, the energy actually comes from the stronger bonds formed afterwards.
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