HTML Help…

The balanced equation for the combustion of ethane in oxygen is C₂H₆ + 7O₂ –> 2CO₂ + 3H₂O. Therefore, we need to destroy 12 C-H bonds and 7 O-O bonds. This requires (12413) + (7498) = 8442 KJ of energy. The joining of the bonds requires 4 C-O bonds and 6 H-O bonds. This requires (4805) + (6464) = 6004KJ of energy. This means we have an endothermic reaction losing 2438 joules.

The balanced reaction for ethanol combustion in oxygen is C₂H₅OH + 3O₂ –> 2CO₂ + 3H₂O Therefore, we need 3 oxygen molecules, and 2 carbon dioxide and 3 water molecules are made. We need to figure out the energy it takes to break an O-H bond.

I think it would be 2C₂H₆ + 7O₂ –> 4CO₂ + 6H₂O. Therefore, we need 24 C-H bonds and 7 O-O bonds. This is (24×413) + (7×498) = 13398KJ. The output is 8 C-O bonds and 12 H-O bonds, which is (8×805) + (12×464) = 12008KJ. I still don’t know why this calculation seems to be endothermic, even though it must be wrong.

Q1. The balanced equation of the combustion reaction between ethane and oxygen is 2C₂H6 + 7O₂ -> 4CO₂ + 6H₂O. To break down two molecules of ethane, 12 C-H bonds and two C-C bonds would need to be broken, which would require 5650 (12×413+2×347) kJ. To break down seven molecules of oxygen, seven O=O bonds would need to be broken, which would require 3486 (498×7) kJ. In total, 9136 (5650 + 3486) kJ will be spent in this combustion reaction.

Forming four carbon dioxide molecules will create eight C=O bonds, which will produce 6440 (805×8) kJ. Forming six water molecules will create 12 H-O bonds, which will produce 5568 (464×12) kJ. In total, 12008 (6440 + 5568) kJ will be produced in this combustion reaction. Overall, there is an energy profit of 2872 (12008 – 9136) kJ when reacting two ethane molecules with oxygen. To compare ethane’s energy profit with methane’s energy profit, the energy profit of combusting two methane molecules would first need to be calculated. The energy profit of combusting two methane molecules is 1636 (2×818) kJ. 2872 kJ is more energy than 1636 kJ, which means ethane produces more energy when reacting with oxygen compared with methane.

Q2. The balanced out equation of an ethanol combustion reaction is C₂H₅OH + 3O₂ -> 2CO₂ + 3H₂O. Therefore, three oxygen molecules will be needed, two carbon dioxide molecules will be produced, and three water molecules will be produced. We would need to know how much energy breaking/forming a C-O bond will need/yield.