Bond Dissociation Energy Calculator
Calculate reaction enthalpy from bond energies using sum of bonds broken minus bonds formed.
Estimating Enthalpy from Bond Energies
Every reaction breaks bonds and forms new ones. Breaking always costs energy; forming always releases it. The net enthalpy is the difference, and the sign convention follows directly.
Note this is the opposite order from the formation-enthalpy formula, which is products minus reactants. Here it is reactants minus products, because breaking bonds is the energy input.
| Bond | Energy (kJ/mol) | Bond | Energy (kJ/mol) |
|---|---|---|---|
| H–H | 436 | C–H | 413 |
| O=O | 498 | C–C | 348 |
| C=O (in CO2) | 799 | C=C | 614 |
| O–H | 463 | C≡C | 839 |
| N≡N | 945 | Cl–Cl | 243 |
Why This Method Is Only Approximate
Tabulated bond energies are averages across many different molecules. The C–H bond in methane is not identical to the one in toluene, yet both are represented by 413 kJ/mol. Errors of 20–40 kJ/mol are typical, and the method assumes all species are gaseous — no allowance is made for the enthalpy of vaporisation or resonance stabilisation.
Where accuracy matters, standard enthalpies of formation give better results because they are measured for specific compounds rather than averaged across a class.
Worked Examples
Common Mistakes
It is bonds broken minus bonds formed — the reverse of the formation-enthalpy convention. Getting it backwards flips the sign.
Methane has four C–H bonds, not one. Multiply each bond energy by how many of that bond appear in the balanced equation.
Bond energies apply to gas-phase species. Reactions involving condensed phases need additional enthalpy terms for phase changes.
Averaged bond energies typically give results within 20–40 kJ/mol of the true value. That is useful for estimation, not for precise work.
Frequently Asked Questions
Formula Explorer connections
Interpretation: This relationship tracks energy transfer, state-function change or the balance between enthalpy and entropy in a chemical process. Assumption: Keep energy units compatible, use kelvin for absolute temperature, and match standard states and reaction stoichiometry. Thermodynamic favorability does not determine reaction speed.