general chemistry - covalent bonds and electrostatic forces

[Monkeymaniac]

Electrostatic forces between two oppositely charged atoms hold the atoms together by an ionic bond.

But I am a bit confused as to if such force exists in a polar covalent bond as well. For example, H of H-F would have a partial positive and F of H-F would have a partial negative charge. Are there electrostatic forces between the partially charged atoms (even though the bond is covalent)?

Then is the fact that H-F is stronger than H-Cl bond explained by this? That, H-F is stronger because it has a stronger "pseudo-ionic bond" than H-Cl does?

Another interesting fact that I found.

Bond Bond energy (kJ/mol)
H--H 436
F--F 158
H--F 568

It seems that H-F is stronger than either H2 or F2. Does the electrostatic force play any part in this?

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[matth87]

Electrostatic forces between two oppositely charged atoms hold the atoms together by an ionic bond.

But I am a bit confused as to if such force exists in a polar covalent bond as well. For example, H of H-F would have a partial positive and F of H-F would have a partial negative charge. Are there electrostatic forces between the partially charged atoms (even though the bond is covalent)?

Then is the fact that H-F is stronger than H-Cl bond explained by this? That, H-F is stronger because it has a stronger "pseudo-ionic bond" than H-Cl does?

Another interesting fact that I found.

It seems that H-F is stronger than either H2 or F2. Does the electrostatic force play any part in this?

Dipole formation is an intramolecular force that makes the bond stronger.

 

[Monkeymaniac]

Dipole formation is an intramolecular force that makes the bond stronger.

I see. So is that intramolecular force a kind of electrostatic force?

 

[matth87]

I see. So is that intramolecular force a kind of electrostatic force?

From a physics standpoint if your making the partial charges then you could say you have some electrical force between the H and the F from

Force Electric = KQq/r^2

 

[Bernoull]

Anytime you have 2 charged entities, electrostatic forces are present. This includes electrons and nuclei so all both ionic and covalent involve couloumb forces.

Actually for polar covalent (asymmetric) bonds, the stronger the polarity the weaker the bond. For covalent bonds, the strength of the bond is dependent on the degree of orbital overlap between the atoms. Greater electronegativity differential b/t similar sized bonding atoms means more polar bonds and less orbital overlap hence weaker covalent bonding. Another important factor is atomic radius, larger atom(s) overlap their orbitals to a lesser extent and bond strength is less.

In short the partial atomic charges in polar covalent bonds are not important in determining bond strength. Degree of molecular orbital overlap is the overriding factor.

I'm could find similar bond energy values but i found bond lengths. Given the inverse relation b/t bond length and strength, these value fit nicely with the orbital overlap explanation given above.

H--H 31pm
F--F 57pm
H--F 91pm
H--Cl 127pm

 

[Monkeymaniac]

Actually for polar covalent (asymmetric) bonds, the stronger the polarity the weaker the bond.

I don't think this is true. Take the series of hydrogen halides for example. To list them in the order of increasing bond strength, it's H-I < H-Br < H-Cl < H-F.
Here, H-F is most polar, yet it's the strongest.

I'm could find similar bond energy values but i found bond lengths. Given the inverse relation b/t bond length and strength, these value fit nicely with the orbital overlap explanation given above.

Also, it's not always true. Bond length also depends on the radius of the atoms. Here's the list of bond length and energy for your reference.

Bond Length(pm) Energy(kJ/mol)
F--F 142 158
Cl-Cl 199 243
H-Cl 127 432
H-H 74 436
H-F 92 568

 

[Bernoull]

I don't think this is true. Take the series of hydrogen halides for example. To list them in the order of increasing bond strength, it's H-I < H-Br < H-Cl < H-F.
Here, H-F is most polar, yet it's the strongest.

If you read the next two sentences after what you quoted, this question won't be necessary. I'm quite familiar with the hydrogen halide series. U have at least 2 variables w/in the series. Bond polarity and atomic size. Both affect the degree of molecular orbital overlap which dictates covalent bond strength.

I said:
"Actually for polar covalent (asymmetric) bonds, the stronger the polarity the weaker the bond. For covalent bonds, the strength of the bond is dependent on the degree of orbital overlap between the atoms. Greater electronegativity differential b/t similar sized bonding atoms means more polar bonds and less orbital overlap hence weaker covalent bonding. Another important factor is atomic radius, larger atom(s) overlap their orbitals to a lesser extent and bond strength is less."


Also, it's not always true. Bond length also depends on the radius of the atoms. Here's the list of bond length and energy for your reference.

It's really a platitude to say a rule has exceptions, that's a foregone conclusion. For the mcat u'll be wise to know the rules as opposed to the exceptions. If you can memorize both, then more power to you!!

FYI, bond length by definition means distance b/t the atomic nuclei therefore it includes atomic radius.

Bond Length(pm) Energy(kJ/mol)
F--F 142 158
Cl-Cl 199 243
H-Cl 127 432
H-H 74 436
H-F 92 568

.

 

[Monkeymaniac]

Anytime you have 2 charged entities, electrostatic forces are present. This includes electrons and nuclei so all both ionic and covalent involve couloumb forces.

Actually for polar covalent (asymmetric) bonds, the stronger the polarity the weaker the bond. For covalent bonds, the strength of the bond is dependent on the degree of orbital overlap between the atoms. Greater electronegativity differential b/t similar sized bonding atoms means more polar bonds and less orbital overlap hence weaker covalent bonding. Another important factor is atomic radius, larger atom(s) overlap their orbitals to a lesser extent and bond strength is less.

In short the partial atomic charges in polar covalent bonds are not important in determining bond strength. Degree of molecular orbital overlap is the overriding factor.

I'm could find similar bond energy values but i found bond lengths. Given the inverse relation b/t bond length and strength, these value fit nicely with the orbital overlap explanation given above.

H--H 31pm
F--F 57pm
H--F 91pm
H--Cl 127pm

Yes, every rule has exceptions, but it was misleading in that you gave exceptinos as examples to the rule.. The essence of the orbital overlap explanation you gave were that convalent bond strength is dictated mostly by the degree of molecular orbital overlap, that a strogner bond would have more orbital overlap and thus a shorter length in general. One could be mislead by the example and the inverse relationship you mentioned just before giving out the example. H-H has the shortest bond length but it isn't the stongest bond among the four. H-F isn't the shortest bond but it is the strongest bond among the four. By the way, the bond lengths you listed there are way off, both in relative and absolute scales.

 

[boaz]

From a physics standpoint if your making the partial charges then you could say you have some electrical force between the H and the F from

Force Electric = KQq/r^2

Well said. Now we can extend this to reason why H-F is stronger than H-I. Since the radius of F- is much smaller than that of I-, the H-F bond is much shorter. From the equation above, note that force (i.e. "bond strength") is inversely proportional to the square of the distance between the charges.