Bond enthalpy, or bond dissociation energy, is defined as which of the following?

• A. The energy released when bonds form.
• B. The energy required to break a bond.
• C. The energy change for the overall reaction.
• D. The energy required to break all bonds in a molecule.

Answer: (B)

Bond enthalpy is the energy needed to break a bond, usually measured per mole and for a single bond in the gas phase. This energy corresponds to homolytic cleavage, producing radical fragments, and it is positive because breaking bonds requires input of energy. The reverse process—bond formation—releases energy, so its energy change is the negative of the bond dissociation energy. The overall reaction enthalpy depends on all bonds broken and formed, not on a single bond, and breaking all bonds in a molecule would just be the sum of all individual bond dissociation energies, not the definition itself.