Bond Enthalpy Calculator
Estimate a reaction’s enthalpy change from the bonds it breaks and makes. Type a reaction from the built-in molecule library or pick bonds and counts directly; you get ΔH ≈ Σ(bonds broken) − Σ(bonds formed), a per-bond table, an energy-level diagram and the ΔH° from enthalpies of formation to compare with.
How to Use
- Choose From molecules to type a reaction, or Bonds directly to pick the bonds yourself.
- For molecules, type the reaction with = or -> between the sides, such as CH4 + 2O2 -> CO2 + 2H2O, using the molecules listed under the box. If the coefficients don’t balance, it is balanced for you.
- For bonds, choose up to four kinds of bond broken and four formed, with how many of each. The list covers the 68 bonds in OpenStax Table 7.2.
- Read ΔH, the energy to break the old bonds and the energy released by the new ones. A negative ΔH is exothermic.
- Compare with the ΔH° from enthalpies of formation, and see every bond in the Show Work table.
H2 O2 N2 F2 Cl2 Br2 I2 HF HCl HBr HI H2O H2O2 NH3 CH4 CO CO2 HCN CH3Cl CH3OH C2H2 C2H4 C2H6Worked Example
Burning methane, CH₄ + 2O₂ → CO₂ + 2H₂O. Bonds broken: 4 C–H at 415 and 2 O=O at 498, so 1,660 + 996 = 2,656 kJ. Bonds formed: 2 C=O at 741 and 4 O–H at 464, so 1,482 + 1,856 = 3,338 kJ. ΔH ≈ 2,656 − 3,338 = −682 kJ per mole of methane. Enthalpies of formation of the gases give −802.55 kJ, so the average bond energies underestimate this one by 15%.
H₂ + Cl₂ → 2HCl. Broken: H–H 436 + Cl–Cl 243 = 679 kJ. Formed: 2 × H–Cl 432 = 864 kJ. ΔH ≈ 679 − 864 = −185 kJ, within 0.21% of the −184.61 kJ from enthalpies of formation, because diatomic molecules have exactly the bonds the table describes.
The common mistake: formed minus broken. Subtracting the other way gives 3,338 − 2,656 = +682 kJ for methane, an endothermic fuel. Bond breaking costs energy, so the broken bonds carry the plus sign: ΔH ≈ Σ(broken) − Σ(formed) = −682 kJ. (With enthalpies of formation it is the other way round: products minus reactants.)
Show Work
Formulas
Bond Energies and Hess’s Law
Germain Henri Hess, working in St Petersburg, showed in 1840 that the heat of a reaction is the same however many steps it is split into. That is why a reaction can be imagined as two steps, breaking every bond in the reactants to make gaseous atoms and then forming the products’ bonds, and why the result can be checked against enthalpies of formation.
Linus Pauling used bond energies in 1932 to define electronegativity: a bond between unlike atoms is stronger than the average of the two like bonds, and the extra energy grows with the difference in electronegativity. His 1939 book The Nature of the Chemical Bond made tables of average bond energies a standard tool for estimating reaction enthalpies.
The bond energies here are from OpenStax Chemistry 2e, Table 7.2 “Bond Energies (kJ/mol)”, and the enthalpies of formation of the gases from its Appendix G, “Standard Thermodynamic Properties for Selected Substances” (both CC BY 4.0). Other tables give slightly different averages for some bonds.
About This Tool
This calculator estimates ΔH from average bond energies. Type a reaction made of the 23 library molecules (from H₂ and O₂ to methanol, ethane, ethene and ethyne) and it balances it if needed, counts every bond broken and formed from each molecule’s structure, and compares the estimate with the ΔH° from enthalpies of formation of the gases. Or pick any of the 68 bonds in the table with counts of your own. The energy-level diagram shows the climb to separate atoms and the drop to the products.
Average bond energies ignore each molecule’s surroundings, so treat the result as an estimate; it is closest for small molecules like HCl and furthest for molecules such as CO₂ whose bonds are unusually strong. Everything runs in your browser; nothing is sent anywhere.
Related tools: Lewis Structure & VSEPR Shape Calculator, Gibbs Free Energy Calculator, and Chemical Equation Balancer.
Frequently Asked Questions
How do you calculate ΔH from bond enthalpies?
Add up the energies of the bonds broken, add up the energies of the bonds formed, and subtract: ΔH ≈ Σ(broken) − Σ(formed). For H₂ + Cl₂ → 2HCl, 436 + 243 − 2 × 432 = −185 kJ, against −184.61 kJ from enthalpies of formation.
Why is the bond-enthalpy answer different from the real ΔH?
Tabulated bond energies are averages over many molecules, and a bond’s strength depends on its neighbours. For burning methane the estimate is −682 kJ, while enthalpies of formation give −802.55 kJ, 15% more. Most of the gap is CO₂: its C=O bonds are much stronger than the 741 kJ/mol average, about 801 kJ/mol each if the real value is put back into the sum.
Why is it broken minus formed and not the other way round?
Breaking a bond always takes energy in (positive) and forming one always gives energy out (negative). For methane, 2,656 kJ goes in and 3,338 kJ comes out, so ΔH = 2,656 − 3,338 = −682 kJ. Swapping them gives +682 kJ, which would wrongly make burning methane endothermic.
Does it matter if water is a liquid?
Yes. Bond energies describe gases, so the estimate is for gaseous products. With liquid water, ΔH° for burning methane is −890.57 kJ instead of −802.55 kJ; the extra 88.02 kJ is the heat released when 2 mol of steam condenses.
What is a bond enthalpy?
The energy needed to break one mole of a bond in the gas phase, giving separate atoms. H–H is 436 kJ/mol. Multiple bonds are stronger but not two or three times as strong: C–C is 345, C=C 611 and C≡C 837 kJ/mol in OpenStax Table 7.2.
How do I use the Bond Enthalpy Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Fuel energy
The estimate for methane, 682 kJ/mol, is 42.5 kJ per gram; enthalpies of formation give 50.0 kJ/g with water as steam.
Hydrogenation
Adding H₂ to ethene breaks a C=C and an H–H and forms a C–C and two C–H: ΔH ≈ −128 kJ against −136.4 kJ from enthalpies of formation.
Ammonia synthesis
Breaking N≡N (946 kJ) and three H–H, then forming six N–H bonds, gives ΔH ≈ −86 kJ; enthalpies of formation give −91.8 kJ.
Fluorine chemistry
H₂ + F₂ → 2HF releases about 542 kJ because F–F is weak (160 kJ/mol) and H–F is strong (569 kJ/mol).
Reactions with no tables
When a compound has no measured ΔfH°, bond energies still give a first estimate, such as −218 kJ for 2H₂O₂ → 2H₂O + O₂ (−211.04 kJ from enthalpies of formation).
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