Amphoteric Species

[Kivi]

H2 is two hydrogens, and numbers to the right are the overall charge

Which of the following is an amphoteric species?
A ) H2S
B ) HS-
C) H+
D) S-2

Obviously it is not C & D. Between A & B, I reasoned that water H2O is part of the same group as sulfur so H2S, just like water, would be amphoteric as well so I put A.

....But the answer is B. Where did I go from? Is H2S not amphoteric? Or was this just a bad question?

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[MedPR]

H2 is two hydrogens, and numbers to the right are the overall charge

Which of the following is an amphoteric species?
A ) H2S
B ) HS-
C) H+
D) S-2

Obviously it is not C & D. Between A & B, I reasoned that water H2O is part of the same group as sulfur so H2S, just like water, would be amphoteric as well so I put A.

....But the answer is B. Where did I go from? Is H2S not amphoteric? Or was this just a bad question?

Amphoteric means it can act as an acid or a base. Even though sulfur technically has access to d orbitals and can have up to 6 bonds, when it is bound to two hydrogens (H2S) it has happily picked up two electrons and is in a stable octet conformation. It is unlikely to accept another hydrogen (unlikely to act as a base). So H2S is probably not amphoteric.

HS-, however, can readily accept a proton to become H2S, so it can act as a base. It can also donate a proton to become S2-, thus making it an acid as well.

 

[Kivi]

Thanks, I understand that a bit more but why can we protonate oxygen in water, which already has a happy octet, to make hydronium?

I guess this is where I'm getting confused. Oxygen and Sulfur are in the same family/group so they should have similar chemical reactivity.

Could there be an analogous problem where HS- was switched with HO-, would OH- be considered amphoteric or would H2O be considered amphoteric?

 

[MedPR]

Thanks, I understand that a bit more but why can we protonate oxygen in water, which already has a happy octet, to make hydronium?

I guess this is where I'm getting confused. Oxygen and Sulfur are in the same family/group so they should have similar chemical reactivity.

Could there be an analogous problem where HS- was switched with HO-, would OH- be considered amphoteric or would H2O be considered amphoteric?

H2O would be the right answer if your two choices were OH and H2O.

I'm not exactly sure why H2S is different than H2O, but I imagine it has something to do with hydrogen bonding (cop out answer whenever a "why is water different?" question is asked).

 

[Godric]

Thanks, I understand that a bit more but why can we protonate oxygen in water, which already has a happy octet, to make hydronium?

I guess this is where I'm getting confused. Oxygen and Sulfur are in the same family/group so they should have similar chemical reactivity.

Could there be an analogous problem where HS- was switched with HO-, would OH- be considered amphoteric or would H2O be considered amphoteric?

From the way your wording it, OH- would be a base, while H2O is the amphoteric species. I could be wrong but thats my understanding of it.

 

[MedPR]

From the way your wording it, OH- would be a base, while H2O is the amphoteric species. I could be wrong but thats my understanding of it.

You're correct, but that's not really what he's asking.

He is saying that since HS- is amphoteric, why isn't OH- amphoteric as well?

 

[Kivi]

You're correct, but that's not really what he's asking.

He is saying that since HS- is amphoteric, why isn't OH- amphoteric as well?

Yep! That's what I'm asking, and also how the explanation of why water (H20) is amphoteric differs from how H2S being amphoteric?

 

[ljc]

Yep! That's what I'm asking, and also how the explanation of why water (H20) is amphoteric differs from how H2S being amphoteric?

This is just a guess, but I would bet it has something to do with water being smaller/more basic than sulfur. Sulfur is more happy to stand alone as a (-2) ion due to its larger size and lower basicity, as compared to oxygen. Conversely, sulfur is more acidic than water, so H2S prefers to shed its hydrogen more quickly than H2O

Considering this, I'd bet it's still technically possible for H2S to be amphoteric, though in this case, HS(-) would be the better answer.

 

[SSAfb3]

Honestly, at first glance I would've chosen A as well. But MedPR's response does a good job of summing up why it's not a base. It's been awhile, so hopefully I still understand this enough to explain, someone correct me if I'm wrong, but here's what I remember...

As for why HS(-) is an acid, but HO(-) is not, remember that within a column you get more acidic as you go down the column due to the size of the atom bonded to hydrogen being larger. The larger atom makes for a longer sp3 bond to H, and longer is weaker, so the bond is more easily broken, and thus more easily deprotonated. Conversely, acid strength across a row is dictated by electronegativity, and not by the size of the atom. Remember the most relevant electronegativity differences using FONClBrISC (F>O>N>Cl>Br>I>S>C).

When dealing with acids containing a couple elements other than H, polarity away from the H dictates strength. Acids containing many elements other than H need to take resonance into account first, followed by inductive forces of electron withdrawing and donating sub groups.

Last but not least, when it comes to alkynes and alkenes, throw everything out the window and remember that although the C-H bond is shorter in an alkyne that in an alkene than an alkane, the C holds the electrons much closer with each pi bond attached to it, so the alkyne C-H bond is more easily broken than the alkene than the alkane, even though it's a shorter bond.

Sorry for the digression into overall acid rules, hope it helped though!