Bond Energy Calculator
Estimate the enthalpy change of a reaction by comparing the energy needed to break bonds in the reactants with the energy released when bonds form in the products. Uses the formula: delta-H = sum of bonds broken minus sum of bonds formed.
Bond energy (also called bond dissociation energy) is the energy required to break one mole of a particular bond in the gas phase. By adding up all bonds broken and subtracting all bonds formed, you get an estimate of the reaction enthalpy.
The Formula
ΔH = Σ(bond energies of bonds broken) - Σ(bond energies of bonds formed)
Breaking bonds requires energy (endothermic). Forming bonds releases energy (exothermic). If more energy is released than consumed, the reaction is exothermic overall.
Worked Example: Combustion of Methane
CH4 + 2O2 -> CO2 + 2H2O
Bonds broken (reactants):
- 4 C-H bonds: 4 x 413 = 1652 kJ
- 2 O=O bonds: 2 x 498 = 996 kJ
- Total: 2648 kJ
Bonds formed (products):
- 2 C=O bonds (in CO2): 2 x 799 = 1598 kJ
- 4 O-H bonds (in 2 H2O): 4 x 463 = 1852 kJ
- Total: 3450 kJ
ΔH = 2648 - 3450 = -802 kJ/mol (exothermic)
The accepted value is -890 kJ/mol. Bond energy estimates are approximate because they use average values, not exact values for the specific molecule.
Common Bond Energies (kJ/mol)
| Bond | Energy | Bond | Energy |
|---|---|---|---|
| C-H | 413 | O-H | 463 |
| C-C | 347 | O=O | 498 |
| C=C | 614 | C-O | 358 |
| C-N | 305 | C=O | 799 |
| N-H | 391 | H-H | 436 |
Why Estimates Differ from Actual Values
Bond energies are averages across many molecules. The actual C-H bond energy in methane (439 kJ/mol) differs from the average C-H value (413 kJ/mol) because the molecular environment affects bond strength. Use Hess's Law with formation enthalpies for more precise results.