Bond Energy Calculator
Estimate a reaction enthalpy from the bond energies of bonds broken and formed.
Energy absorbed to break all bonds in the reactants.
Energy released when all bonds in the products form.
Reaction Enthalpy (ΔH)
Calculation Steps
How Bond Energies Predict Reaction Enthalpy
During a chemical reaction, bonds in the reactants must break and new bonds form in the products. Breaking bonds absorbs energy (endothermic), while forming bonds releases energy (exothermic). The overall enthalpy change of the reaction is the difference between these two amounts.
ΔH = Σ(bonds broken) − Σ(bonds formed)
- • Bonds broken = total bond energy of reactant bonds (kJ/mol)
- • Bonds formed = total bond energy of product bonds (kJ/mol)
- • ΔH = enthalpy change of the reaction (kJ/mol)
Exothermic vs. Endothermic
The sign of ΔH tells you the direction of energy flow. When more energy is released forming product bonds than is absorbed breaking reactant bonds, the reaction gives off heat and ΔH is negative. The opposite case absorbs heat overall.
Exothermic
ΔH < 0 (releases heat)
Endothermic
ΔH > 0 (absorbs heat)
Note: Bond energies are average values measured across many molecules, so this method gives an estimate of ΔH rather than an exact value. It applies to gas-phase reactions and ignores changes of state. For precise thermochemistry, use standard enthalpies of formation or calorimetric data.