Bond dissociation energy (BDE) is the energy needed to break 1 mole of a specific bond in a gaseous molecule. Mean bond energy is the average over all bonds of that type in various molecules. For $delta_H$ calculation: list ALL bonds in reactant molecules (energy input to break them) and ALL bonds in product molecules (energy released forming them). $delta_H$ = total energy to break - total energy to form. Example: CH4 + 2O2 -> CO2 + 2H2O. Break: 4 C-H + 2 O=O. Form: 2 C=O + 4 O-H. $delta_H$ = [4(413) + 2(498)] - [2(799) + 4(463)] = 2648 - 3450 = -802 kJ. Actual value is -890 kJ — discrepancy because mean bond energies are averages.