Dipole Moment

[MedPR]

Found this on mcatquestion. I lost the page, but this is the gist of it. Values aren't exact either, but close enough to answer.

Dipole moment is defined as charge*separation distance.

If KCl has a dipole moment of 10, and HF has a dipole moment of 2, what would the dipole moment of HCl be?

A. 9
B. 1.5
C. 5
D. 0.1

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[chiddler]

Found this on mcatquestion. I lost the page, but this is the gist of it. Values aren't exact either, but close enough to answer.

Dipole moment is defined as charge*separation distance.

If KCl has a dipole moment of 10, and HF has a dipole moment of 2, what would the dipole moment of HCl be?

A. 9
B. 1.5
C. 5
D. 0.1

HF is close at 2. Must be higher. KCl is far at 10. Must be lower.

C?

 

[MedPR]

HF is close at 2. Must be higher. KCl is far at 10. Must be lower.

C?

It's actually B. The explanation basically said that since Fluorine is more electronegative (than Chlorine) it can create a greater separation of charge so HF should have a greater dipole moment than HCl.

Initially I thought it was A because KCl is 10 or something... So looking at the given equation:

K+ and H+ and F- and Cl- all have the same charges. Cl- is larger than F- though, so it has greater charge separation. K+ is bigger than H+, so it also affords more charge separation. So HCl dipole moment should be smaller than KCl, but larger than HF. This was my logic, and its wrong. I'm still not seeing why.

 

[chiddler]

It's actually B. The explanation basically said that since Fluorine is more electronegative (than Chlorine) it can create a greater separation of charge so HF should have a greater dipole moment than HCl.

Initially I thought it was A because KCl is 10 or something... So looking at the given equation:

K+ and H+ and F- and Cl- all have the same charges. Cl- is larger than F- though, so it has greater charge separation. K+ is bigger than H+, so it also affords more charge separation. So HCl dipole moment should be smaller than KCl, but larger than HF. This was my logic, and its wrong. I'm still not seeing why.

wow. i'm very curious. and confused.

ok here's something to consider: K's electronegativity is 0.8, H is 2. thinking. Oh and one is covalent and other is ionic. Definitely should of thought of that in the beginning...

OK lets find the differences in electronegativity to determine the magnitude of polarity in the bonds.

KCl - K = 0.8, Cl = 3.16. The difference is 2.34

HF - H = 2.08, F = 3.98. The difference is around 2.

so! KCl is more polar than HF. HF is more polar than HCl. So HCl is the lowest on the list.

Also, I just realized my above calculations are pointless. To learn from this is that just because it's F doesn't make it super strongest dipole.

 

[hellocubed]

wow. i'm very curious. and confused.

ok here's something to consider: K's electronegativity is 0.8, H is 2. thinking. Oh and one is covalent and other is ionic. Definitely should of thought of that in the beginning...

OK lets find the differences in electronegativity to determine the magnitude of polarity in the bonds.

KCl - K = 0.8, Cl = 3.16. The difference is 2.34

HF - H = 2.08, F = 3.98. The difference is around 2.

so! KCl is more polar than HF. HF is more polar than HCl. So HCl is the lowest on the list.

Also, I just realized my above calculations are pointless. To learn from this is that just because it's F doesn't make it super strongest dipole.

How would we solve this without using any number values?
Considering, in the context of the question, we are not given any numbers.

A glance at the period table would make it seem that HCl has a greater difference in Electronegativity than HF.

http://www.chemicalelements.com/graphics/table.gif

Unless someone could do me a favor and help interpret the table to come out with the solution...

 

[johnwandering]

good effort, but I don't believe we are given those numbers?

 

[chiddler]

yeah the important realization is that KCl is an ionic compound (very highly polarized) and HF is a covalent (highly but not very highly polarized).

That's also why i finished my post with the realization that the calculations were unnecessary!

@hellocubed, you're mistaken. HF has more electronegative difference.

 

[SaintJude]

So among the H-(F,O,N,Cl) H-F is the only bond with a highly electronegative atom that's covalent,right? Is this related to the same reason that HF is weak acid?

 

[Dasypus]

So among the H-(F,O,N,Cl) H-F is the only bond with a highly electronegative atom that's covalent,right? Is this related to the same reason that HF is weak acid?

H-F is covalent, but so is HCl and HBr and HI and H2O and H3N. They're all soluble in water. The thing that makes HF a weak acid is that it's the only one of the hydrogen-halides that can make hydrogen bonds (H-F to H-F). Because it can do that, it 'keeps to itself' a bit more than HCl or HBr or HI. Not fully solvated, so a weak acid.

 

[MedPR]

The solution as others have alluded to is based in the fact that HCl and HF are both polar covalent bonds, while KCl is an ionic bond. In other words, HCl is more comparable to HF than it is to KCl. With that in mind, since HF is more electronegative than HCl, the dipole moment in HF is slightly greater than HCl. Given the answer choices, all you have to do is pick the one that is slightly less than the given value for HF.

 

[phaloz]

The solution as others have alluded to is based in the fact that HCl and HF are both polar covalent bonds, while KCl is an ionic bond. In other words, HCl is more comparable to HF than it is to KCl. With that in mind, since HF is more electronegative than HCl, the dipole moment in HF is slightly greater than HCl. Given the answer choices, all you have to do is pick the one that is slightly less than the given value for HF.

That's my intuition of it, but it seems like a rather arbitrary way of solving it. I don't know how to otherwise though. If someone knows a systematic method to approach this problem please share.

 

[SaintJude]

H-F is covalent, but so is HCl and HBr and HI and H2O and H3N. They're all soluble in water. The thing that makes HF a weak acid is that it's the only one of the hydrogen-halides that can make hydrogen bonds (H-F to H-F). Because it can do that, it 'keeps to itself' a bit more than HCl or HBr or HI. Not fully solvated, so a weak acid.

How do you know they're all covalent ( apart from the well known water) without knowing the values for electronegativity? Being soluble in water isn't related to covalent nature is it it? I mean NaCl is soluble in water and it's an ionic compound.

 

[Dasypus]

How do you know they're all covalent ( apart from the well known water) without knowing the values for electronegativity? Being soluble in water isn't related to covalent nature is it it? I mean NaCl is soluble in water and it's an ionic compound.

I admit, I mostly know this because I know that H forms covalent bonds with non-metals, the annoying FONClBrSCH. This is not exclusive (SiH4 is covalent, and LiAlH4 has aluminum covalently bonded to hydrogen, for instance, so don't rule out all metal(oids)) but it is nonetheless something I committed to memory along the way. Basically, hydrogen follows its own rules, because it's teenie and a bit pathetic, in a way lithium and sodium aren't.

 

[drechie]

The solution as others have alluded to is based in the fact that HCl and HF are both polar covalent bonds, while KCl is an ionic bond. In other words, HCl is more comparable to HF than it is to KCl. With that in mind, since HF is more electronegative than HCl, the dipole moment in HF is slightly greater than HCl. Given the answer choices, all you have to do is pick the one that is slightly less than the given value for HF.

Yes, but Cl is bigger than F, so wouldnt that increase the distance between the charges. Where H-Cl >>> H-F (in terms of bond distance?)