Bond Enthalpy Reaction Calculator
Chemical reactions break bonds in the reactants and form new bonds in the products. Breaking bonds requires energy while forming them releases energy, so a reaction's approximate enthalpy change is the total energy of bonds broken minus the total energy of bonds formed. This calculator takes those two totals and returns the estimated reaction enthalpy, plus a clear exothermic or endothermic classification. Bond enthalpy values are average, empirical figures, so enter the summed totals yourself; the arithmetic here is exact.
Bond enthalpy formula
delta H(reaction) = sum(bonds broken) - sum(bonds formed)
broken = energy absorbed (positive)
formed = energy released
negative delta H means exothermic, positive means endothermic
For methane combustion, breaking the C-H and O=O bonds costs about 2,648 kJ/mol while forming the C=O and O-H bonds releases about 3,466 kJ/mol, giving an estimated enthalpy near -818 kJ/mol.
Bond energy facts
- Breaking a chemical bond always requires an input of energy.
- Forming a bond always releases energy of the same magnitude as breaking it.
- The method assumes all species are in the gas phase.
- Average bond enthalpies make this an approximation, not an exact value.
- Stronger product bonds than reactant bonds give an exothermic reaction.
Bond enthalpy: frequently asked questions
How do bond enthalpies give a reaction enthalpy?
Breaking bonds absorbs energy and forming bonds releases it. The approximate reaction enthalpy equals the total energy of all bonds broken in the reactants minus the total energy of all bonds formed in the products. A negative result is exothermic and a positive result is endothermic.
Why is this an estimate rather than an exact value?
Bond enthalpy tables list average values measured across many molecules, so a specific bond in a specific molecule may differ. The method also assumes all reactants and products are gaseous. For exact figures, use standard enthalpies of formation in a Hess law calculation instead.
What is the sign convention?
Bonds broken cost energy (positive contribution) and bonds formed release energy (negative contribution). Because the formula subtracts bonds formed, a reaction that forms stronger bonds than it breaks gives a negative, exothermic enthalpy change.
Do I enter individual bonds or totals?
Enter the summed bond enthalpy of all bonds broken in the reactants and the summed bond enthalpy of all bonds formed in the products. Add up each bond energy times the number of those bonds before entering the totals.
Where do average bond enthalpy values come from?
Average bond enthalpies are empirical, tabulated from spectroscopic and thermochemical measurements. Because the figures vary by source and are not derivable from first principles, you supply the totals yourself; the calculator performs only the deterministic subtraction.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.