Bond Enthalpy Formula
Estimate reaction enthalpy from bond energies
Formula
ΔHrxn = Σ(bonds broken) - Σ(bonds formed)
Breaking bonds requires energy (+); forming bonds releases energy (-).
Common Bond Energies (kJ/mol)
C-H: 413
C-C: 348
C=C: 614
O-H: 463
C=O: 799
O=O: 498
N-H: 391
N≡N: 945
H-H: 436
Example: CH₄ + 2O₂ → CO₂ + 2H₂O
Bonds broken: 4 C-H (4×413) + 2 O=O (2×498) = 1652 + 996 = 2648 kJ
Bonds formed: 2 C=O (2×799) + 4 O-H (4×463) = 1598 + 1852 = 3450 kJ
ΔHrxn = 2648 - 3450 = -802 kJ/mol
Answer: ΔH ≈ -802 kJ/mol (exothermic)
Notes
- Bond energies are average values; actual values vary by molecular environment.
- More accurate than bond energies: use standard enthalpies of formation.
- Useful for estimating ΔH when thermodynamic data unavailable.