EK FL C/P #21

[jeep1010]

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Is this question just saying that resonance, polarity and radius are the primary determinants of acid strength? Or are they saying that H-bonding has no affect on acid strength?

I thought that H-bonding has some effect on acid strength. I know it for does for boiling point, I guess I may have thought it does for acid strength too?

 

[aldol16]

The answer hinges on the fact that both can H-bond.

 

[jeep1010]

I thought that H-Bonds were O, N, and F only? Or does Sulfur get included because its in the same column as Oxygen, as elements in the same column exhibit similar chemical properties? Would this mean that phosphorus and chlorine can H-Bond as well?

 

[aldol16]

I thought that H-Bonds were O, N, and F only? Or does Sulfur get included because its in the same column as Oxygen, as elements in the same column exhibit similar chemical properties? Would this mean that phosphorus and chlorine can H-Bond as well?

Ah, my mistake. I was thinking of H2O for some reason. Alright, so H-bonding doesn't change anything here because think of it this way. A hydrogen bond is formed, presumably, between the oxygen of a solvent H2O and the proton attached to the carboxylic acid. But think of it this way. If that proton came off the acid, it would still be solvated by H-bonding with the solvent waters - in fact, this is probably even better H-bonding because the waters can get closer and now you have two moieties instead of one (H+ and bicarbonate) so there's an entropic drive as well.

 

[jeep1010]

I may be confused on what you said above. Are you saying that with H-Bonding there will be more stability and less likelihood of a proton dissociating because it is H-bonded? Thus, not making the solution acidic because a proton cannot be released thereby increasing proton concentration?

 

[aldol16]

I may be confused on what you said above. Are you saying that with H-Bonding there will be more stability and less likelihood of a proton dissociating because it is H-bonded? Thus, not making the solution acidic because a proton cannot be released thereby increasing proton concentration?

No. H-bonding has no effect on acidity here. That's what I'm saying. Solvent molecules will associate with the proton whether it is bound (as H2CO3) or free. It's easier to solvate when it's free anyway and there's a strong entropic drive for the proton to be free. Therefore, H-bonding won't have any effect here because it's better for the proton to come off and be solvated than "H-bonding" when it's bound as H2CO3.

 

[rlaboss]

The answer hinges on the fact that both can H-bond.

Can we apply the same logic that acidity increases with increasing size of the atom to which the proton is bound to the size of the molecule to which the proton is bound (answer C)? Large size means the negative charge can be spread more, making the conjugate base more stable and therefore the acid more acidic than a small molecule that would have to have a very localized charge?

 

[aldol16]

Can we apply the same logic that acidity increases with increasing size of the atom to which the proton is bound to the size of the molecule to which the proton is bound (answer C)? Large size means the negative charge can be spread more, making the conjugate base more stable and therefore the acid more acidic than a small molecule that would have to have a very localized charge?

More size of the atom the H is bound to, not the entire molecule because the atom is what will be bearing the negative charge. So the part of the molecule that bears the negative charge is important. You shouldn't consider only size though - there are other, often competing factors as well. See this for a better explanation: http://www.chem.wisc.edu/courses/343/Lee/chapter 3 slides 3.pdf