bond enthalpies

Meaning: Bond enthalpy is the enthalpy change to break one mole of a particular bond in gaseous molecules, giving gaseous atoms or fragments. Breaking a bond is endothermic (positive); forming the same bond releases the same amount. A larger value means a stronger bond. For the same two atoms, triple > double > single, and shorter bonds are generally stronger. Tabulated values for bonds in larger molecules are averages over many compounds, so results are estimates.

Equation: ΔH°rxn ≈ Σ(bond enthalpies of bonds broken) − Σ(bond enthalpies of bonds formed). ΔH°rxn is the reaction enthalpy change in J/mol. Each bond enthalpy is in J/mol (tables usually give kJ/mol) and is multiplied by the number of moles of that bond.

On the MCAT: A passage gives a table of bond enthalpies and a reaction, such as combustion or hydrogenation of an alkene. Draw the structures, count every bond broken and formed, and sum. A negative result means the products have stronger total bonding, so the reaction is exothermic.

Common confusion: Students use products − reactants, as they do with enthalpies of formation. For bond enthalpies the order is broken − formed, because breaking costs energy. The answer is also approximate and assumes gas-phase species.