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[bajoneswadup]

Why is H2S more polar than HCl? TBR says any cpd with 2 lone pairs is more polar than any cpd of similar mass with 1 or 3 lone pairs. wtf?

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[chiddler]

Why is H2S more polar than HCl? TBR says any cpd with 2 lone pairs is more polar than any cpd of similar mass with 1 or 3 lone pairs. wtf?

RED LEADER REPORTING IN

you are mistaken, my friend! S is given an electronegativity of 2.5, while Cl is 3.16. Cl pulls electrons much more strongly than does S, and therefore HCl is more polar. Furthermore, HCl's polarity is clearly illustrated by its very acidic nature. It pulls electrons so strongly that the hydrogen can easily dissociate. If this were the case for H2S as well, then you would see similar acidity.

also i'm an imposter. i'm not a chemistry pro. but you read my advice AND NOW ITS TOO LATE

 

[bajoneswadup]

YES. So the last sentence of TBR gen chem book 2 pg. 81 is a mistake!? I knew I wasn't crazy.

 

[bajoneswadup]

NVM. Apparently H2S has bent geometry, so it's more polar than HCl. I guess this makes sense, although I'm still doubting everything I know abt chem in the back of my head.73)/782

 

[chiddler]

whoah you're right.

"The boiling point in H2S is greater than that of HCl, because the H2S molecule is more polar than HCl due to its bent geometry."

Why does the bent geometry contribute to its increased polarity? Especially when compared to the very polar chlorine molecule.

...is this accurate?

More thinking: The reason that H2S has a higher boiling point is that two lone pair electrons and two polarized hydrogen bonds means much more dipolar bonding occuring than the monoprotic HCl.

 

[bajoneswadup]

My 2 friends don't know either.. Umm maybe H2S is just some weird exception? Idk

Where is the blue and green leader when you need them?

 

[chiddler]

My 2 friends don't know either.. Umm maybe H2S is just some weird exception? Idk

Where is the blue and green leader when you need them?

they're busy with "life"

whatever THAT is...

 

[bajoneswadup]

whoah you're right.

"The boiling point in H2S is greater than that of HCl, because the H2S molecule is more polar than HCl due to its bent geometry."

Why does the bent geometry contribute to its increased polarity? Especially when compared to the very polar chlorine molecule.

...is this accurate?

More thinking: The reason that H2S has a higher boiling point is that two lone pair electrons and two polarized hydrogen bonds means much more dipolar bonding occuring than the monoprotic HCl.

OHhh. Wow ok thanks g.

 

[MedPR]

I imagine it has something to do with sulfur in H2S being able to "steal" the electron density from 2 hydrogens, as opposed to chlorine only being able to "steal" the electron density from 1 hydrogen.

Even though chlorine is more electronegative, it's not twice as electronegative.

Dipole moment is calculated by μ=δd. H2S has two polarity vectors (each HS bond).

 

[gettheleadout]

I imagine it has something to do with sulfur in H2S being able to "steal" the electron density from 2 hydrogens, as opposed to chlorine only being able to "steal" the electron density from 1 hydrogen.

Even though chlorine is more electronegative, it's not twice as electronegative.

Dipole moment is calculated by μ=δd. H2S has two polarity vectors (each HS bond).

Then why (would TBR) bring up the bond geometry at all?

 

[milski]

Then why (would TBR) bring up the bond geometry at all?

If it was a straight line arrangement (like CO2), the two vectors would cancel each other?

I don't have a good explanation for the original question. The H-Cl bond has a dipole moment of 1.1. I don't have HS in the table that I'm looking at, but H-N is 1.3 and S is less electronegative than N, so ballpark for H-S would be around 1.1. With the angle between the two bonds about 109 degrees, the sum ends up about 1.3. Ok, that actually looks like a plausible explanation.

 

[chiddler]

I imagine it has something to do with sulfur in H2S being able to "steal" the electron density from 2 hydrogens, as opposed to chlorine only being able to "steal" the electron density from 1 hydrogen.

Even though chlorine is more electronegative, it's not twice as electronegative.

Dipole moment is calculated by μ=δd. H2S has two polarity vectors (each HS bond).

wouldn't this reduce polarity?

Starting with one S-H: sulfur pulls electrons away from hydrogen and ends up with a dense electron cloud nearby. Add another H: Sulfur pulls electron density from the second H and the new electrons repel the electrons from the other hydrogen. Net result is reduced polarity for each hydrogen?

Furthermore, two hydrogens means two protons pulling electron density back from sulfur.

 

[idonotcare]

Giving it more thoughts:
1. If the question was asking about intermolecular force, ie which one has a higher boiling point, it will be H2S since S could hydrogen bond, while Cl can't.

2. If the question was asking purely about which one is more polar, H2S has dipole moment of 0.97D and HCl has dipole moment of 1.05D, so HCl will be more polar (tho I wouldn't know this without looking it up, seems unlikely to be tested on the MCAT)

 

[MedPR]

Then why (would TBR) bring up the bond geometry at all?

Think about the vectors. Two parallel vectors in the same direction cancel out (not really in this case since there would still be increased electron density), while the two bent vectors sum in the y direction.

 

[ilovemedi]

So what's more important for BP - molecular weight or polarity? Because it says HI has a higher BP than HCl due to weight, tho HCl is obviously more polar... Is weight more important in these situations?

Also using that argument, you could say H2S is heavier, thus higher BP than HCl.. Aside from polarity.

 

[MedPR]

So what's more important for BP - molecular weight or polarity? Because it says HI has a higher BP than HCl due to weight, tho HCl is obviously more polar... Is weight more important in these situations?

Also using that argument, you could say H2S is heavier, thus higher BP than HCl.. Aside from polarity.

I don't know if there is a trend.

 

[gettheleadout]

If it was a straight line arrangement (like CO2), the two vectors would cancel each other?

I don't have a good explanation for the original question. The H-Cl bond has a dipole moment of 1.1. I don't have HS in the table that I'm looking at, but H-N is 1.3 and S is less electronegative than N, so ballpark for H-S would be around 1.1. With the angle between the two bonds about 109 degrees, the sum ends up about 1.3. Ok, that actually looks like a plausible explanation.

Think about the vectors. Two parallel vectors in the same direction cancel out (not really in this case since there would still be increased electron density), while the two bent vectors sum in the y direction.

I understand the geometry of the electron density cloud, but I was interpreting TBR's explanation of "due to its bent geometry" as the geometry literally causes the difference in the strengths of the dipoles, not the fact that there are two S-H bonds. In reality the geometry of SH2 allows this greater polarity (because it allows a non-canceling arrangement of dipoles), but does not cause it. That's my problem with the explanation.

 

[MedPR]

I understand the geometry of the electron density cloud, but I was interpreting TBR's explanation of "due to its bent geometry" as the geometry literally causes the difference in the strengths of the dipoles, not the fact that there are two S-H bonds. In reality the geometry of SH2 allows this greater polarity (because it allows a non-canceling arrangement of dipoles), but does not cause it. That's my problem with the explanation.

I don't know if my thought process is going to make any difference, but here goes.

The polarity causes the bent geometry. The lone pairs repel the hydrogens, just like the lone pair in NH3 repels the hydrogens. If you look at CO2, it is linear and non-polar. Obviously linear =/= non-polar, but if CO2 was bent, there would be a net dipole equal to the sum of the two vectors. It's not the same case as H2S, since in H2S the central atom is the most polar one, but the idea is the same as far as vector addition goes.

 

[gettheleadout]

I don't know if my thought process is going to make any difference, but here goes.

The polarity causes the bent geometry. The lone pairs repel the hydrogens, just like the lone pair in NH3 repels the hydrogens. If you look at CO2, it is linear and non-polar. Obviously linear =/= non-polar, but if CO2 was bent, there would be a net dipole equal to the sum of the two vectors. It's not the same case as H2S, since in H2S the central atom is the most polar one, but the idea is the same as far as vector addition goes.

How does the polarity of the S-H bonds cause a bent geometry? The fact that there are four filled orbitals causes tetrahedral electron group geometry and the fact that two are bonding orbitals by necessity means the molecular geometry is bent. It's not like the lone pairs repel the hydrogens because they're partially positive any more than the lone pairs would repel other lone pairs, it's just VSEPR arrangement.

 

[Lunasly]

How does the polarity of the S-H bonds cause a bent geometry? The fact that there are four filled orbitals causes tetrahedral electron group geometry and the fact that two are bonding orbitals by necessity means the molecular geometry is bent. It's not like the lone pairs repel the hydrogens because they're partially positive any more than the lone pairs would repel other lone pairs, it's just VSEPR arrangement.

I just read this in TBR and could not quite understand it. 🙁