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Topic: Base Strength: OH- vs. SH-  (Read 31213 times)

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Offline Omega Glory

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Base Strength: OH- vs. SH-
« on: April 09, 2011, 04:26:32 PM »
Why is OH- a stronger base than SH-?

Aren't SH-'s electrons farther from the nucleus and therefore freer to abstract a proton?  And isn't OH- more electronegative to begin with, making it want to hold those electrons closer?

Offline enahs

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Re: Base Strength: OH- vs. SH-
« Reply #1 on: April 09, 2011, 05:11:13 PM »
Being more electronegative means it is happier with a negative charge on it.

But you also have to look at what happens when it does get that extra proton, it becomes H2O or H2S. Which one is more acidic and why?

Offline Omega Glory

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Re: Base Strength: OH- vs. SH-
« Reply #2 on: April 09, 2011, 07:05:37 PM »
Exactly--oxygen is more electronegative, so OH- is happier with a negative charge on it.  Ergo, why would it want to donate a lone pair of electrons to abstract a proton, making it neutral and therefore less happy?

As for H2O and H2S, I would assume H2S is more acidic because sulfur's atomic radius is larger, lengthening and weakening the H-S bond strength, making dissociation easier.

Offline enahs

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Re: Base Strength: OH- vs. SH-
« Reply #3 on: April 09, 2011, 09:28:15 PM »
You can not look at the acids and bases separately, they are one in the same thing.

OH- is happy to have an extra electron on it, so it is happy to form that species. Once it is formed, it is very reactive.
SH- is also very reactive, but it want to form less because it is a less stable molecule.

In other words, H2O only sorta wants to give up a H, but H2S wants to give up a H a lot more.

Acids and their conjugate based, and bases and their conjugate acids have to be looked at together. And many times, the stability (or lack of relatively) of the other form you are looking at is the important part.

Remember, these are all equilibrium, even OH-  ::equil:: H2O is in equilibrium, despite the fact that you can get a strong base in the form of NaOH. I

Offline Omega Glory

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Re: Base Strength: OH- vs. SH-
« Reply #4 on: April 09, 2011, 09:48:40 PM »
I understand what you are saying, that weak acids are conjugate to strong bases, and vice-versa.  Yet I am still having trouble reconciling what I know of oxygen with the basicity of the hydroxide anion.  I still cannot grasp how an atom as electronegative as oxygen, which therefore holds its electrons very tightly, can so easily donate them in order to abstract a proton.  My confusion is further compounded by the fact that oxygen is a period 2 element.  Its electrons should therefore be held closer to the nucleus in a smaller shell, and should logically be less available for bonding.

Is there any way to reconcile these facts with the basicity of the hydroxide anion?  Am I missing a logical leap, or does the fact that OH- is conjugate to a weak acid simply trump little things like electronegativity and atomic radius?

Offline enahs

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Re: Base Strength: OH- vs. SH-
« Reply #5 on: April 10, 2011, 12:23:45 AM »
I would say that the stability of the conjugate acid is by far the driving force here.

But, there are a lot of period 2 properties that do not "jive" with the overall trend including this one. Because the atoms in the period 2 are so small, you are trying to force a lot of negative charge close together, closer to other negative charges, and like charges repeal each other.

The neutral atom of oxygen, yes, is more electronegative then S. But once you add and extra electron to oxygen, its negative charge density is much larger then for sulfur, because sulfur has more room to spread out the negative charge density. As such, oxygen is more happier to form a bond and get ride of some of that negative charge density.

But, if you looked at just that alone you would think it would not want to even form in the first place; it is a delicate balance between electronegativety, charge density, and conjugate pairs.

Offline rabolisk

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Re: Base Strength: OH- vs. SH-
« Reply #6 on: April 11, 2011, 09:08:39 AM »
I would just add that HI > HBr > HCl> HF in terms of acid strength, although electronegativity would suggest the exact opposite. The reason for this is polarizability.

Offline AWK

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Re: Base Strength: OH- vs. SH-
« Reply #7 on: April 11, 2011, 09:50:56 AM »
I would just add that all above discussion concerns water solutions.
AWK

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