bond energy and activation energy

sgstudent

Is the energy required to break the bonds of a compound in order for the reaction to occur (bond energy) the same as activation energy? Thanks for the help!

Borek

Do you need to break the bonds entirely to reach the transition state?

sgstudent

I think so, when we do the calculating it is enthalpy to break bonds+enthalpy to form bonds. When we calculate it the bond breaking part is the bond energy so all the bonds have to break. So
I'm guessing that the
activation energy is the energy required to break the
bonds. But then again, when a substitution reaction only the chlorine gas' and one C-H bond gets broken. So is the activation energy the energy to break only certain bonds?Thanks!

Borek

What is a transition state?

sgstudent

from what I read I think its the intermediate part of the reaction meaning its the reactants st the peak of the activation energy 'mountain'. So not all the bonds are broken? Because in the reactions I see are pike H2+O2-->2H2O so the transition state all the bonds are broken. But when I see my sister's O level notes there are questions with organic reactions so I don't thing its possible for all the bonds to break. But she still calculated it such that all the bonds are broken so I'm white confused here... thanks for tbe help!

Gokul43201

I think you are picturing a process like this:

reactants --> (bond breaking) --> transition state = individual species with all relevant bonds broken --> (bond formation) --> products

But does it have to follow that sequence?

Borek

sgstudent

I'm guessing that not all the bonds have to break since in some reaction only certain bonds are required to be broken like in the CH4+Cl2 case. But then the part that scares me is on finding the activation energy since when such reaction occurs only the involved bonds are the activation energy. Is there a way to determine it? When I draw out the full structural formula it gives some insights but still its quite difficult...thanks for the help guys!

Borek

Experimentally, or through QM only, you can't find it from typical thermodynamic data. And this is nothing unusual. Or perhaps I should put it differently - it is rather unusual to be able to calculate activation energy just from the thermodynamical data describing bond energies.

sgstudent

Hi Borek I suddenly started thinking about this again and I read the chemguide explanation. So what I got from it was that when the reactants collide with activation energy and correct orientation, the old bonds are partially broken at simultaneously new bonds are partially formed. But what happens after that transition state? Will more energy be absorbed to continue breaking those bonds?

Thanks and sorry for the super long wait before this reply its because I just thought about it after revising and reading up on some online notes. Thanks :)