Reaction Enthalpy from Average Bond Enthalpies

Add the average enthalpies of all bonds broken in the reactants and subtract the sum for product bonds formed: ΔrH≈ΣD(broken)−ΣD(formed). For H₂+Cl₂→2HCl, tabulated values give 436+243−2·431=−183 kJ/mol. The negative sign indicates an exothermic reaction. Count each bond with its stoichiometric coefficient. This is an estimate because a given bond type can have different values in different molecules. For accurate work, standard formation enthalpies are preferable. Keep bond counts and stoichiometric coefficients tied to the same balanced reaction equation. Treat the result as an estimate, not a measured value.

Breaking a bond requires energy input, so its contribution is positive. Forming new bonds releases energy, so that amount is subtracted. This gives the sign rule “broken minus formed.” In HCl formation, breaking H−H and Cl−Cl costs 679 kJ per mole of reaction. Forming two H−Cl bonds releases 862 kJ. The difference, −183 kJ/mol, is negative. Reversing the order gives the wrong sign and would misclassify an exothermic reaction as endothermic. Each bond enthalpy is positive; the sign emerges only from balancing bond breaking against bond formation.

No. Tables average bonds of the same type across different molecules and chemical environments. A C−H bond in methane need not have exactly the same enthalpy as a C−H bond elsewhere. The estimate can therefore differ from calorimetry or a calculation with standard formation enthalpies. Bond enthalpy usually refers to homolytic separation of gas-phase molecules. Check physical states and whether a table gives average or molecule-specific values. The method is useful for signs and approximate magnitudes, not exact thermochemistry. Averaging explains why precise tabulated values still cannot guarantee an exact reaction enthalpy for a particular compound.

Both methods follow energy conservation but use different data. The bond-enthalpy method decomposes a reaction into bonds and uses average tabulated D values. Hess’s law instead sums standard formation enthalpies of the actual substances with their stoichiometric coefficients. The Hess calculation is usually more accurate when reliable formation data are available. Bond enthalpies are useful for comparing structural changes or when only bond data are provided. Do not mix the two kinds of tables because they describe different reference processes. Both methods must use the same standard states and reaction direction for signs and values to be comparable.

Common errors include counting a double bond once when the table separates its components, or forgetting a stoichiometric coefficient. Read the balanced equation and sketch reactant and product structures before calculating. A double bond contains one σ and one π bond if the table lists them separately, so follow its convention. Also keep amounts referenced to one mole of reaction. In the example, two H−Cl bonds form, so subtract 2·D(H−Cl). Finally, check that the sign matches the expected energy picture. Reversing the written reaction also reverses the sign of its estimated enthalpy change.