Electronegativity differences between atoms in a bond

[kobe200LATE]

Assuming the same hybridization of the atoms in the bond, is it safe to assume that a bond between atoms with a higher difference in electronegativity is stronger than a bond between atoms with a smaller difference in electronegativity?

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[Dr Gerrard]

I dont know man, I would assume the less the difference in electronegativity, the stronger the bond.

I think a C-C bond is much stronger than an H-F bond, because the latter readily dissociates.

 

[Bernoull]

Assuming the same hybridization of the atoms in the bond, is it safe to assume that a bond between atoms with a higher difference in electronegativity is stronger than a bond between atoms with a smaller difference in electronegativity?

For covalent bonds this is totally wrong. Difference in ENs gives bond polarity. The strongest covalent bonds are nonpolar (atoms have "identical" ENs), polar covalent bonds are weaker, as polarity increases, the bond eventually breaks making 2 ions..

Remember covalent bonding is simply sharing of e's. EN is when one atom is greedy and wants more than its fair share of the e's, the greedier the atom, the less e' density there's left to share/bond, thus weakening the covalent bond..

 

[Charles_Carmichael]

Yup, polarity between covalent bonds makes the bond weaker since less energy is required to break the bond heterolytically (although more energy is needed to break the bond homolytically). For example, electronegativity can make a hydrogen more acidic since electron density is pulled away from the hydrogen towards the electronegative atom.

 

[kobe200LATE]

This is what I thought previously also, but I recently completed a passage in which they said that a C=O bond is a stronger bond than C=C bond. I then read a post on this forum in which someone said that it was because there was a greater difference in electronegativity.

So I guess my real question is, why is a C=O bond stronger than a C=C bond if it's not because of the difference in electronegativity?

 

[boaz]

Nice question.

When we speak of "bond strength" we are referring to the energy required to break the bond and leave each atom with one electron each. So the bond strength depends on the energy required to break it, which in turn depends upon the stability of the products. The stability of the products depends on several factors. For example, the C-O bond is stronger than C-N because when the C-N bond breaks the result is a nitrogen atom with a half-filled p-orbital. Since an O atom has more than half filled it is relatively less stable than N in this respect.

Now, a double bond like C=O would need to go through 2 successive breaks. In the first, the pi-bond breaks, resulting in a diradical. One elctron goes to C; the other to O. But we know that the bond was polarized--there was more electron density near the O than the C. This is for a good reason: the O has a higher effective nuclear charge than C, so the negative charge is stabilized by being closer to O. But now that the bond broke, leaving exactly equal electron density on O and C, the stability is gone.

Contrast that with a C=C bond. Since the electron density is equally shared between the 2 C's, there is no loss in stability in that respect since after the bond breaks the electron density is still equally distributed between them.

Hope that helps!

 

[Charles_Carmichael]

ishchayill got to it before I did. Nice explanation! But yes, the bond energy is the energy required to break the bond homolytically (ie. breaking the bond generates free radicals) so if one atom has more of the electron density, it's harder to break that bond homolytically but it's easier to break the bond heterolytically (where ions are the result). Hope this helps.