bond dissociation energies

Meaning: Bond dissociation energy (BDE) is the enthalpy change needed to break one mole of a particular bond in gaseous molecules, with each atom keeping one electron (homolytic cleavage). Breaking a bond requires energy, so BDE is positive, and forming that bond releases the same amount. For the same two atoms, strength generally rises with bond order (C≡C > C=C > C–C), and shorter bonds tend to be stronger. Table values are often averages over many molecules.

Equation: ΔH°rxn ≈ Σ BDE(bonds broken) − Σ BDE(bonds formed), where ΔH°rxn and BDE are in J/mol (usually quoted in kJ/mol).

On the MCAT: Passages give a table of bond energies and ask you to estimate ΔH for a combustion or other reaction, or to rank bonds by strength or length. The exam also asks which bond breaks first in a radical reaction (usually the weakest, such as O–O in peroxides). A negative ΔH means the bonds formed are stronger overall than the bonds broken.

Common confusion: Students flip the sign or think breaking bonds releases energy. Breaking is endothermic and forming is exothermic, so the broken bonds go first in the subtraction. BDE is also a thermodynamic quantity, not an activation energy.