Bond Energy

There are two types of binding energy. The energy required to break a bond produce the radical is called dissociation energy D. For example D for H2O → HO + H is 118 kcal / mol. However, this value is not selected as the binding energy OH in water because of D for HO → H + O is 100 kcal / mol. The average of these two values The (109 kcal / mol) was chosen as the binding energy E. For methane, the total energy change of CH4 into C + 4H (at 0 K) is 393 kcal / mol. This means E for the CH bond in methane is 98 kcal / mol at 0 K. In the molecular diatoms, of course D = E. The heat of combustion is known exactly is burning hothydrocarbons. For methane, combustion heat value at 25oC was 212.8 kcal / mol, which leads to heat atomisasinya value is 398.0 kcal / mol (At 25oC), or the value of E for CH bond is 99.5 kcal / mole at 25oC.  

This method good for molecules such as methane in which all ties equivalent, but for complex molecules that then must be made assumptions. So for ethane with atomisasinya heat at 25oC is 676.1 kcal / mol, then need more determination how much more energy for CC bond and how much for the six CH bonds. In this case, the assumption becomes important because there is no correct way to obtain such information, and indeed the question is not have a real understanding. If you made the assumption that E for each CH bond is the same as the CH bond in methane (99.5 kcal / mol), then for six CH bond in ethane is 6 x 99.5 = 597.0 kcal / mol, rest = 79.1 kcal / mol for C-C bond. Nevertheless, the same calculation for propanamenghasilkan 80.3 kcal / mol for CC bonds; to isobutana, the value is 81.6 kcal / mol. Review by using the heat atomization isomers also difficult. E value for CC bond in pentane, isopentane, and neopentana which calculated from heat atomization at 25oC in the same way each is 81.1, 81.8, and 82.4 kcal / mol, despite all these isomers have twelve C-H bond and four C-C bond.