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Suppose a reactant molecule has a double bond between two carbon atoms, and the product molecule has only a single bond between these two carbon atoms. When doing calculations with bond enthalpies, why does the double bond break completely and then reform into a single bond (rather than just one bond breaking in the double bond)?
Dr. Lavelle said in lecture that double bonds have to break completely in order to form a single bond between the same two atoms. It's probably because a single bond isn't actually half the energy of a double bond, so you can't just break one of the bonds in a double bond.
Just to give some evidence to what everyone else is saying. A carbon-carbon single bond has a length of 154 and energy of 348, a carbon-carbon double bond has a length of 134 and energy of 614, while a carbon-carbon triple bond has a length of 120 and energy of 839. Here we can see that a double bond is not just double the energy of a single bond, and a triple bond is not just triple the energy of a single bond.
If you imagine a double bond as the sigma and pi bond that we learned in 14A, I think it makes sense that you can't just halve the value of a double bond to get the value of a single bond because the first and second bonds hold the atoms together in different ways
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