Heats of combustion are very accurately known for hydrocarbons.93 For methane the value at 25C is 212.8kcalmol1 (890.4kJ mol1), which leads to a heat of atomization of 398.0kcal mol1 (1665kJ mol1) or a value ofE for the CH bond at 25C of 99.5 kcal mol1 (416kJ mol1). This method is fine for molecules like methane in which all the bonds are equivalent, but for more complicated molecules assumptions must be made. Thus for ethane, the heat of atomization at 25C is 676.1kcal mol1 or 2829 kJ mol1 (Fig. 1.11), and we must decide how much of this energy is due to the CC bond and how much to the six CH bonds. Any assumption must be artificial, since there is no way of actually obtaining this information, and indeed the question has no real meaning. If we make the assumption that E for each of the CH bonds is the same as E for the CH bond in methane (99.5 kcal mol1 or 416kJ mol1), then 6
99:5 (or 416) ¼ 597:0 (or 2498), leaving 79.1 kcalmol1 (331 kJ mol1) for the CC bond. However, a similar calculation for propane gives a value of 80.3 (or 336) for the
Heats of combustion are very accurately known for hydrocarbons.93 For methane the value at 25C is 212.8kcalmol1 (890.4kJ mol1), which leads to a heat of atomization of 398.0kcal mol1 (1665kJ mol1) or a value ofE for the CH bond at 25C of 99.5 kcal mol1 (416kJ mol1). This method is fine for molecules like methane in which all the bonds are equivalent, but for more complicated molecules assumptions must be made. Thus for ethane, the heat of atomization at 25C is 676.1kcal mol1 or 2829 kJ mol1 (Fig. 1.11), and we must decide how much of this energy is due to the CC bond and how much to the six CH bonds. Any assumption must be artificial, since there is no way of actually obtaining this information, and indeed the question has no real meaning. If we make the assumption that E for each of the CH bonds is the same as E for the CH bond in methane (99.5 kcal mol1 or 416kJ mol1), then 6
99:5 (or 416) ¼ 597:0 (or 2498), leaving 79.1 kcalmol1 (331 kJ mol1) for the CC bond. However, a similar calculation for propane gives a value of 80.3 (or 336) for the
การแปล กรุณารอสักครู่..
