Explain reaction enthalpy using a bond ledger

Compare two energy amounts

Consider the fictional gas-phase reaction A2 + B2 → 2 AB. Separating bonded atoms requires energy; forming bonds from separated atoms releases energy. A reaction can therefore release energy overall even though breaking its initial bonds costs energy. The comparison must include both sides and their stoichiometric counts.

The assigned A–A bond dissociation energy is 400 kJ per mole of bonds, and B–B is 200 kJ per mole of bonds. At the initial A–B value of 360 kJ per mole of bonds, breaking reactant bonds for one mole of reaction costs 600 kJ. Forming two moles of A–B bonds releases 720 kJ.

Keep the sign and amount explicit

The estimate is ΔH = energy required to break bonds − energy released forming bonds. ΔH is the system enthalpy change. Here 600 − 720 = −120 kJ per mole of reaction as written. Negative means exothermic: under the appropriate constant-pressure heat-transfer conditions the reacting system releases heat to its surroundings.

The widget’s amount is moles of reaction as written, often called reaction extent. At 0.5 mol reaction, it consumes 0.5 mol each of A2 and B2 and forms 1 mol AB. Every energy amount halves. This is not the total mole count of every molecule present.

At A–B energy 200, the per-reaction difference is +200 kJ/mol reaction: endothermic. At 300, the model gives zero. At 400, it gives −200. These values describe assigned fictional bonds; choosing a slider value does not establish that a stable real molecule exists with those properties.

Distinguish an accounting path from a mechanism

The diagram passes through separated atoms to compare initial and final states. It is a bookkeeping path, not a claim that the actual reaction first breaks every bond and then forms every product. Enthalpy is a state function; different accounting paths between the same states must give the same net change when all terms are consistently included.

Bond energies are commonly gas-phase values, and average bond energies used for larger molecules are estimates that depend on chemical environment. Condensed-phase reactions can require additional phase and interaction terms. Do not use this simplified ledger as a precision prediction for arbitrary solutions or solids.

The difference between the two bars is not an activation energy. A favorable enthalpy change does not establish a fast reaction or thermodynamic spontaneity; activation barriers and entropy are separate questions. The model supplies neither a time axis nor a transition state.

Test scaling and the zero case

At zero reaction amount both total bars and total ΔH are zero, even if the per-mole value remains negative or positive. Zero amount is not evidence that the reaction itself is thermoneutral. Compare zero and one mole at the same A–B setting to make that distinction visible.

Transfer: with A–B energy 400 kJ/mol bonds and 0.5 mol reaction, calculate the breaking cost, formation release, net change and amount of AB formed.

Check the bond-energy transfer

Breaking costs 300 kJ. Forming 1 mol AB releases 400 kJ. Net change is −100 kJ, and 1 mol AB forms. This says nothing about reaction speed. Doubling the extent doubles all these extensive quantities without changing the −200 kJ/mol reaction value.

OpenStax bond strengths and reaction enthalpy explains the underlying accounting.