Bond Energy Reaction Enthalpy Calculator
Estimate the enthalpy change of a reaction from the bond enthalpies of bonds broken in the reactants and bonds formed in the products.
Negative ΔH means the reaction is exothermic (releases heat).
- 1
Total energy to break bonds
436 + 243 = 679 - 2
Total energy released forming bonds
431 + 431 = 862 - 3
Reaction enthalpy ΔH = bonds broken − bonds formed
679 − 862 = -183
How does this calculator work?
Reaction enthalpy from bond energies equals the total enthalpy of bonds broken in the reactants minus the total enthalpy of bonds formed in the products, in kJ/mol. A negative result is exothermic (heat released); a positive result is endothermic (heat absorbed). It is an estimate using average gas-phase bond enthalpies.
Formula
How this is calculated
Breaking a chemical bond requires energy (endothermic), while forming a bond releases energy (exothermic). The bond-enthalpy method approximates a reaction's enthalpy change by summing the energy needed to break every bond in the reactants and subtracting the energy released when every bond in the products forms. Enter one bond enthalpy per value (kJ/mol), separated by commas or new lines, in each list.
The calculator sums the "bonds broken" list to get the total energy absorbed, sums the "bonds formed" list to get the total energy released, then computes ΔH = broken total − formed total. A negative ΔH means more energy is released forming new bonds than is needed to break old ones, so the reaction is exothermic; a positive ΔH is endothermic.
This is an estimate because tabulated bond enthalpies are averages across many molecules rather than exact values for a specific compound, and the method assumes all reactants and products are gases. It ignores phase changes and intermolecular forces, so it works best for gas-phase reactions and as a quick approximation rather than a precise calorimetric measurement.
Frequently asked questions
Breaking bonds costs energy and forming bonds releases it. Net enthalpy is energy in minus energy out, so ΔH = (energy to break reactant bonds) − (energy released forming product bonds).
Standard bond enthalpies are averages over many molecules and assume gas-phase species. Real reactions involve specific molecular environments and phase changes, so the bond-energy estimate differs slightly from measured ΔH.
A negative ΔH indicates an exothermic reaction that releases heat to the surroundings, while a positive ΔH indicates an endothermic reaction that absorbs heat.
Also known as
TG we-Calculate Editorial Team. (2026). Bond Energy Reaction Enthalpy Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/bond-energy-calculator
TG we-Calculate Editorial Team. "Bond Energy Reaction Enthalpy Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/bond-energy-calculator.
TG we-Calculate Editorial Team, "Bond Energy Reaction Enthalpy Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/bond-energy-calculator
@misc{wecalculate_bond_energy_calculator, title = {Bond Energy Reaction Enthalpy Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/bond-energy-calculator}}, year = {2026}, note = {TG we-Calculate} }
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