About the Bond Energy Calculator
This bond energy calculator estimates the enthalpy change of a gas-phase reaction from average bond enthalpies. List the bonds broken in the reactants and the bonds formed in the products — for example “4 C-H, 2 O=O” and “2 C=O, 4 O-H” for burning methane — and it adds up each side, subtracts, and tells you whether the reaction releases or absorbs heat.
Bond energies for 68 common single, double and triple bonds between H, C, N, O, F, Si, P, S, Cl, Br and I are built in from Table 7.2 of OpenStax Chemistry 2e. Type single bonds with a hyphen, double bonds with =, and triple bonds with # or ≡; atom order does not matter. If a bond is missing, or you want a specific value from your textbook, append it after a colon (e.g. “2 C=O:799” for the C=O bonds in CO₂).
Because the values are averages over many molecules, the result is an estimate: for methane combustion it gives about −682 kJ/mol, while standard enthalpies of formation give −802 kJ/mol (with water as a gas). It is ideal for GCSE, A-level and general chemistry questions, and for quick sense-checks of whether a reaction is exothermic.
With the default inputs, the estimated δh (per mole of reaction) is -682 kJ/mol. Change any value above to recalculate instantly.
How to use the bond energy calculator
- 1Write the balanced equation and draw the structures to count each type of bond.
- 2Enter the bonds broken in the reactants, e.g. "4 C-H, 2 O=O".
- 3Enter the bonds formed in the products, e.g. "2 C=O, 4 O-H".
- 4Read ΔH: negative means exothermic, positive means endothermic.
- 5Optionally add a custom value after a colon for any bond not in the table.
Formula and method
Breaking a chemical bond always requires energy and forming one always releases energy. Treating a reaction as breaking every bond in the reactants and then forming every bond in the products, the enthalpy change is approximately the total bond energy of the reactants minus that of the products. A negative ΔH means stronger bonds are formed than broken, so the reaction is exothermic.
Each D is the average bond energy for that bond type in the gas phase, taken from OpenStax Chemistry 2e Table 7.2. Real bond strengths vary with the molecule (C=O in CO₂ is about 799 kJ/mol versus 741 kJ/mol on average), and the method ignores phase changes, so results are estimates.
- ΔH
- Enthalpy change of reaction (kJ per mole of reaction as written)
- D
- Average bond energy (kJ/mol)
Worked examples
Combustion of methane
CH₄ + 2O₂ → CO₂ + 2H₂O. Breaking 4 C–H (4 × 415) and 2 O=O (2 × 498) takes 2,656 kJ; forming 2 C=O (2 × 741) and 4 O–H (4 × 464) releases 3,338 kJ. ΔH ≈ 2,656 − 3,338 = −682 kJ/mol.
Hydrogen + chlorine → hydrogen chloride
H₂ + Cl₂ → 2HCl. Bonds broken: 436 + 243 = 679 kJ. Bonds formed: 2 × 432 = 864 kJ. ΔH = 679 − 864 = −185 kJ, matching the OpenStax worked example.
Haber process for 2 mol of reaction
N₂ + 3H₂ → 2NH₃. Breaking N≡N (946) and 3 H–H (1,308) costs 2,254 kJ; forming 6 N–H releases 6 × 390 = 2,340 kJ. ΔH ≈ −86 kJ per mole of reaction, or −172 kJ for 2 mol.
Frequently asked questions
How do you calculate enthalpy change from bond energies?+
Add up the energies of all bonds broken in the reactants, add up the energies of all bonds formed in the products, and subtract: ΔH = bonds broken − bonds formed. A negative answer means the reaction is exothermic.
Why is the bond energy answer different from the textbook ΔH?+
Bond energies are averages across many compounds, and they apply to gases. Values from standard enthalpies of formation are specific to each substance and phase, so they are more accurate. For CH₄ + 2O₂ → CO₂ + 2H₂O(g), bond energies give about −682 kJ/mol versus −802 kJ/mol from enthalpies of formation.
Is bond breaking endothermic or exothermic?+
Bond breaking is always endothermic (energy is absorbed) and bond forming is always exothermic (energy is released). Whether the whole reaction is exothermic depends on which total is larger.
What is the bond energy of C=O?+
The average C=O bond energy is about 741 kJ/mol (OpenStax Table 7.2), but the C=O bonds in carbon dioxide are stronger, about 799 kJ/mol. Use the “:799” override if your course uses that value.
Do I need to count every bond?+
You can skip bonds that appear unchanged on both sides, since they cancel. Counting everything is safer, though, and gives the same answer.