Calculate reaction enthalpy from average bond energies: ΔH = Σ(bonds broken) − Σ(bonds formed).
Estimating reaction enthalpy from bond energies offers a genuinely different calculation route than Hess's law or formation enthalpies — one grounded directly in the energy cost of breaking and forming individual chemical bonds — and is a standard AP Chemistry Unit 6 technique for reactions where tabulated ΔHf values aren't convenient.
The governing equation, ΔH = Σ(bonds broken) - Σ(bonds formed), reflects a simple physical truth: breaking any chemical bond always requires an energy input (endothermic), while forming any chemical bond always releases energy (exothermic). A reaction's overall enthalpy change is the net balance between the total energy spent breaking all reactant bonds and the total energy recovered forming all product bonds.
This framework lets you predict whether a reaction is exothermic or endothermic before doing any arithmetic at all, just by reasoning about relative bond strength: if the bonds being formed are collectively stronger than the bonds being broken, more energy is released than absorbed, and the reaction is exothermic — ΔH will come out negative once calculated. H2 + Cl2 → 2HCl is a clean illustration: breaking one H-H bond and one Cl-Cl bond costs energy, but forming two H-Cl bonds (each individually strong) releases more, giving a net exothermic result.
Average bond energies are tabulated values representing a typical bond strength across many different molecules containing that bond type — which is precisely why bond-energy calculations give only an estimate of ΔH, not an exact value the way Hess's law from measured formation enthalpies does. A specific C-H bond in methane doesn't have identically the same energy as a C-H bond in a more complex molecule, so bond-energy ΔH calculations carry inherent approximation that Hess's law calculations from directly measured data do not.
Methane combustion (CH4 + 2O2 → CO2 + 2H2O) and ammonia synthesis (N2 + 3H2 → 2NH3) are standard worked examples, each requiring careful bond-by-bond accounting of exactly which bonds break in the reactants and which form in the products.
This calculator walks through bond-by-bond energy accounting for real reactions, letting you predict exothermic or endothermic behavior before revealing the full calculation.