Breaking Bonds

[Parietal Lobe]

Ok, so I know that bond energy is the amount of energy it takes to break a bond. In my Orgo class, I learned that it's harder to break a double or triple bond than a single bond which makes sense since double and triple bonds have a sigma and one or two pi bonds. But why then does it take more energy to break a low energy, stable bond (i.e. single bond)? If double and triple bonds are harder to break, wouldn't they require more energy than breaking a single bond?

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[DrMattOglesby]

I am not quite sure where your confusion lies so I'll just say this much for now:

-Pi bonds are easier to break than sigma bonds.

-(Pi bonds + sigma bonds) are harder to break than sigma bonds alone.

Let me know if you need more clarification on either of those statements.

 

[Revilla]

I am not quite sure where your confusion lies so I'll just say this much for now:

-Pi bonds are easier to break than sigma bonds.

-(Pi bonds + sigma bonds) are harder to break than sigma bonds alone.

Let me know if you need more clarification on either of those statements.

I think he's asking why single bonds have a higher bond energy than double bonds. But OP, I think you have to be very careful about when to use "sigma bond" and "single bond." Keep in mind that double bonds also have sigma bonds.

 

[DrMattOglesby]

okay just for the sake of clarification:

-The question is, "Why are sigma bonds harder to break than pi bonds?"

That is the best light in which I could interpret the topic. Please correct me if I am not addressing the question you had in mind...

 

[Parietal Lobe]

Forget sigma and pi bonds. Here's my question. If single bonds are low energy and easier to break than double bonds, why do they (the single bonds) have a higher bond energy? Isn't bond energy the energy needed to break a bond? Why do single bonds require more energy to break when double bonds are harder to break?

 

[Bacchus]

There are varying degrees of bonds. Some X-X vs X-X bonds may be "harder" or "easier" to break depending on the atoms involved. There is no clear trend. a BF bond has a higher bond energy than a CC double bond. It has to due with things such as electronegativity, atomic sizes, overlap, etc.

 

[ODorDO]

i believe general chem text books explain it very well (geometry: sp, sp2, sp3, thermodynamics, and others i cant think of on top of my head).

good luck

 

[RSAgator]

You're confusing yourself a bit.

Lets say you have H2C==CH2

In this case you have 3 sp2 orbitals and 1 p orbital per carbon.

In the carbon carbon double bond, the sp2 orbitals form the first bond, and due to their partial s character they overlap well resulting in a high bond strength. The second bond is formed by the p orbital, which has much less overlap resulting in a weaker bond.

The double bond is a combination of both the first single sp2-sp2 bond, as well as the second p-p bond, and so even though there is less energy required to break JUST the p-p bond, breaking a double bond requires breaking BOTH the p-p and the sp2-sp2 bonds. Thus, overall, the double bond requires more energy to break than the single bond, but the p-p bond alone requires less energy to break than the sp2-sp2 bond.

The picture above illustrates the idea behind a hybrid orbital, and shows all the orbitals of interest. The p orbitals overlap side by side and so have little bond strength. The sp(x) orbitals overlap on the fat end, resulting in much more overlap and thus much more bond strength. The double bond contains one sp2-sp2 bond (strong overlap) and one p-p bond (weak overlap).

And finally, to use mathematics to illustrate the point:

sp2-sp2 = 4
p-p = 2

sp2-sp2 + p-p = 6

(sp2-sp2 + p-p) > (sp2-sp2) > (p-p)