Which is a Weaker Acid?

[Whiteshoes]

Which of the following acids has the LARGEST pKa value?
Large pKa= weak acid

In essence which one is the weakest acid?

A. HCLO2
B. HIO2
C. HCLO3
D. HIO3

BOTH C & D are out because more Oxygen = more acidic.
However I always that acidity increases as you move to the RIGHT and DOWN.
for example HI is more acidic than HCL

According to Berkeley Review the answer is B, which would mean I is less acidic than Cl

I googled it and found a similar question answer off of yahoo answers that would be opposite from berkeley's answer.

http://answers.yahoo.com/question/index?qid=20110331103803AAWSsA9

Please help clarify. thanks

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[Cruzerthebruzer]

Interesting, I would've guessed A as well due to the periodic trends of acidity. Curious to hear the answer on this one too.

 

[Liquidice07]

No joke, I just went over this same question in the tbr about an hour ago.

The way I understood the answer explanation was....

You have to keep in mind the "trends" associated with oxyacids (where the acidic H is bonded to an oxygen which is bonded to a central atom).
The more electronegative the central atom is, the more acidic that proton will be.
Thus, Cl is more e.n. than I
Therefore, Cl is more acidic which gives I the larger pKa



EDIT
Though I have found this section in the tbr and the buffer section (the following section from this one) to be the most confusing and questionable out of all I have studied yet.
I am reading through my textbook on the buffer stuff cuz the tbr is contradicting itself in that section

 

[Whiteshoes]

No joke, I just went over this same question in the tbr about an hour ago.

The way I understood the answer explanation was....

You have to keep in mind the "trends" associated with oxyacids (where the acidic H is bonded to an oxygen which is bonded to a central atom).
The more electronegative the central atom is, the more acidic that proton will be.
Thus, Cl is more e.n. than I
Therefore, Cl is more acidic which gives I the larger pKa



EDIT
Though I have found this section in the tbr and the buffer section (the following section from this one) to be the most confusing and questionable out of all I have studied yet.
I am reading through my textbook on the buffer stuff cuz the tbr is contradicting itself in that section

Yeah That's what i figure. I guess moving RIGHT and DOWN is only for Haloacids (HBr) not Oxyacids (H2SO4). for Oxyacids it's electronegativity and number of Oxygens.

So just to verify this would mean that answer in yahoo answers is incorrect right? because we are dealing with oxyacids.

 

[loveoforganic]

I believe you all's rationale is correct

 

[Temperature101]

Which of the following acids has the LARGEST pKa value?
Large pKa= weak acid

In essence which one is the weakest acid?

A. HCLO2
B. HIO2
C. HCLO3
D. HIO3

BOTH C & D are out because more Oxygen = more acidic.
However I always that acidity increases as you move to the RIGHT and DOWN.
for example HI is more acidic than HCL

According to Berkeley Review the answer is B, which would mean I is less acidic than Cl

I googled it and found a similar question answer off of yahoo answers that would be opposite from berkeley's answer.

http://answers.yahoo.com/question/index?qid=20110331103803AAWSsA9

Please help clarify. thanks

Edit...

 

[whiteshadodw]

size of anion/bond length only matters for haloacids. for all all else, inductive effect. except in ochem when resonance can and usually does trump inductive effect.

 

[BeatMCAT]

Heres the structure of the acid.http://en.wikipedia.org/wiki/Chlorous_acid

You first always deprotonate and look at the conjugate bases. You compare first the electronegativity or size (depending if comparing row or column). Here one O- is not different than the other. Then look for resonance. both get same resonance structure, and then you look for inductive effect. Chlorine as mentioned is more electronegative, so higher inductive effect, and therfore respective acid is more acidic.

Always follow this method. You'll always get the right answer.
deprotonate-->look at conjugate base-->size or electronegativity-->resonance-->inductive. Resonance almost ALWAYS beats inductive effect.

 

[hvairline]

With oxyacids acid strength depends a)on the number of oxygens bonded to the central atom and b)the electronegativity of the central atom . So since EN Cl> EN I we have that chlorine due to a higher electronegativity attracts electrons away from the O-H bond further polarizing it and making the bond weaker thus increasing acid strength. For acids of the type X-H we have that acidity increases as you move from left to right across a period(increase in electronegativity) and from up to down across a group(greater radius=> greater stability of conjugate base => greater tendency to release H)