Data reference—Bond energies

Average bond energies estimate the enthalpy needed to break bonds in gaseous molecules; exact values apply to a specified molecule and bond environment. For a reaction, ΔH ≈ Σ(bonds broken) − Σ(bonds formed).

Draw or count the bonds on each side, keep the sign convention, and remember that average values are approximations because bond strength depends on molecular environment.

Breaking one H–H and one Cl–Cl bond then forming two H–Cl bonds gives ΔH from broken minus formed energies; the result is approximate when table averages are used.

Do not reverse the subtraction or treat an average bond energy as an exact value for every molecule.