Bond Energy Reaction Enthalpy Calculator
Estimate a reaction’s enthalpy change from bond energies: energy to break reactant bonds minus energy released forming product bonds.
How it works
ΔH ≈ Σ(bond energies broken) − Σ(bond energies formed)
Breaking bonds costs energy and forming bonds releases it, so a reaction’s enthalpy is estimated as the total energy needed to break all the reactant bonds minus the total energy given out when the product bonds form. A positive result is endothermic (the products are held less tightly), a negative result is exothermic (stronger bonds form than break). Because tabulated bond energies are averages across many molecules, this is an estimate that ignores physical state and resonance, and it applies to gas-phase species.
Worked example
For H₂ + Cl₂ → 2 HCl, bonds broken are H–H (436) and Cl–Cl (242) = 678 kJ/mol; bonds formed are two H–Cl (2 × 431) = 862 kJ/mol. ΔH ≈ 678 − 862 = −184 kJ/mol — exothermic. (The default figures give 2,648 − 2,800 = −152 kJ/mol.)
Frequently asked questions
Why break-minus-form rather than the reverse?
Energy is put in to break bonds (a positive cost) and released when bonds form (a negative, energy-out term). Summing energy-in minus energy-out gives the net change: if forming releases more than breaking costs, ΔH is negative and the reaction is exothermic.
How accurate is a bond-energy estimate?
It is a first approximation, typically within a few tens of kJ/mol of the true value. Tabulated bond energies are averages, so molecules with resonance (like benzene) or unusual environments deviate more. For precise work use standard enthalpies of formation instead.
Does physical state matter?
Yes — bond energies assume gaseous species. Condensing to a liquid or solid releases additional energy not captured here, so this method is most reliable for gas-phase reactions.
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This calculator is for educational purposes. Double-check important results before acting on them.