[Pomum]

Hi,

The question is the following:

"If the fuel blend was vaporized before combustion, predict whether the amount of energy released would be greater, less or the same. Explain your answer."

The mark scheme states that it would be greater because there are fewer intermolecular forces.

Could someone explain this to me?

Thanks,
Pomum

[Holz888]

I THINK that this is how it works. I vaguely remember an explanation, but please correct me if I'm wrong. In order to combust, the molecules must be separated completely, and not interacting with any others. Therefore, usually, some energy would first be used to break all the bonds, thus meaning less is given out. Consider an exothermic reaction: the energy required to break all of the bonds is less than the energy given out when they are made, so the total energy is given out. Say you have more bonds to break (or interactions in the case of intermolecular forces), the total amount of energy given out will be lower than the first reaction. Sorry, I don't know how clear I'm coming across, so I'll do a few pretend figures.

Reaction with many intermolecular forces:
Energy required to break ALL bonds & interactions = 50 kJ
Energy given out when bonds are formed = 100 kJ
100 - 50 = 50 kJ given out

Reaction with few intermolecular forces:
Energy required to break ALL bonds & interaction = 10 kJ
Energy given out when bonds are formed = 100 kJ
100 - 10 = 90 kJ given out