SP and SP3 stability vs reactivity

[591489]

I understand that an sp hybridized orbital will have more s character and will thus be lower in energy than an sp3 hybridized orbital. My question is, if the sp orbital is lower in energy, why does breaking a triple bond release more energy than breaking a single bond?

Thanks

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[brood910]

The energy released upon breaking a bond is not equal to the energy you are mentioning.
Potential energy is the one that increases as stability of the bond decreases, not bond energy/bond dissociation energy.

 

[Czarcasm]

There's a few distinctions that need to be made: sigma vs. pi bonds and hybridization.

When two atoms are involved in a triple bond, there will always be 1 sigma bond and 2 pi bonds (p to p orbital bond). The sigma bond can vary (either a hybridized orbital or atomic orbital) but the pi bonds will always consist of two bonding p-orbitals. Take ethyne for example (HCCH). The two carbons are sp hybridized (being bonded two two atoms) and the sigma bond between the carbons is simply described as an sp-orbital bonded to an sp-orbital. There are also two pi bonds (the p orbitals of the carbons bonding together). These pi bonds are orthogonal to one another.

When you break a triple bond, you are breaking both 2 pi-bonds and 1 sigma bond.
When you break a single bond, you are only breaking the sigma bond.

The KEY thing to realize here is this:
Single bonds: 1 sigma bond
Double bonds: 1 sigma bond + 1 pi bond
Triple bonds: 1 sigma bond + 2 pi bonds

In all three scenarios, there will always be only 1 sigma bond.

Obviously because there are more bonds involved, triple bonds will require more energy to break. But, if you were simply comparing the sigma bonds between two orbitals, then you would consider the hybridization and make the distinction you are referring to. Sigma bonds that have more s characteristic will be stronger (because they are closer to one another on average), and therefore require more energy input to break these bonds.

Hope this helps.

 

[Czarcasm]

Sorry to keep going on, but I'm assuming you were doing one of those Enthalpy of Reaction questions that require you to use BDE to find the enthalpy. The key thing to realize for those values is that all of those BDE represent the energy required to BREAK a bond (whether its the BDE of a single bond, pi bond, etc). Bonds that are formed release energy. If the overall energy released in forming bonds is greater than the energy input to break the bonds, the reaction will release energy overall and be exothermic. Likewise, if the products release less energy than the energy required to break the bonds, the reaction will consume energy overall and be endothermic.

In this scenario, let's take the same example above: ethyne (HCCH) and somehow react it to produce ethane (H3CCH3). In this scenario, we are breaking two pi bonds in the reactant (HCCH), which consists of a triple bond and producing four new sigma bonds to carbon. Because sigma bonds are always stronger than pi bonds (due to more s characteristic), we can then say that not only will the product be more stable, but it will be exothermic as well (releasing energy in the progress).

Sorry, just wanted to clarify things a bit.