Why is Acetic acid a weak acid?

[kevo246]

So I understand the definition of a weak acid and strong acid by its ability to fully dissociate in solution. A strong acid has a very stable conjugate base, and a weak acid will have a "fairly" stable conjugate base. But when looking at Acetic Acid, I would predict its conjugate base to be quite stable due to the presence of oxygen and its resonance ability. Any clarification would help.Thanks

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[ilovemcat]

So I understand the definition of a weak acid and strong acid by its ability to fully dissociate in solution. A strong acid has a very stable conjugate base, and a weak acid will have a "fairly" stable conjugate base. But when looking at Acetic Acid, I would predict its conjugate base to be quite stable due to the presence of oxygen and its resonance ability. Any clarification would help.Thanks

There's really no way to tell if an Acid is "strong" without knowing the pKa of the acid. However, you should and can tell if an acid is stronger given a list of choices by simply comparing the conjugate bases of those acids. The more stable the base, the stronger the acid.

For the MCAT however, you should atleast know the 7 most common strong acids as this can be useful for answering several questions: HCl, HClO4, HClO3, HI, HBr, H2SO4, and HNO3. All of these have negative pKa values. Everything else (usually) is considered to be weak by comparison. I say usually because some Organic Acids like: Methanesulfonic acid and Benzenesulfonic acid are "strong" because they have negative pKa values.

 

[kevo246]

awwww....I see. Good point with the pKa values. One concept I didn't consider. As well, I must continue to evaluate these acids RELATIVE to another acid. Lower pKa=more acidic. Good CALL

 

[ilovemcat]

awwww....I see. Good point with the pKa values. One concept I didn't consider. As well, I must continue to evaluate these acids RELATIVE to another acid. Lower pKa=more acidic. Good CALL

Yep. Also, remember pKa is inversely related to Ka values (like pH is to H concentration). They could very easily give those instead. The larger the Ka value, the more acidic the acid.

 

[kevo246]

Yep. Also, remember pKa is inversely related to Ka values (like pH is to H concentration). They could very easily give those instead. The larger the Ka value, the more acidic the acid.

But of course.....one concept I mustn't forget.

So in conclusion, one cannot determine if an acid will fully dissociate in solution just by looking at the compound characteristics, ie resonance, induction effect. Therefore, other than the KNOWN strong acids, we can only judge an acids strength relative to another acidic compound and rank them according to stability of conjugate base. (more stable conjugate base=stronger acid)

 

[StudyShy]

You haven't memorized the ionization constatns for acids and their conjugate bases yet? j/k 🙂

 

[apumic]

You haven't memorized the ionization constatns for acids and their conjugate bases yet? j/k 🙂

While this is obviously a joke, you SHOULD have some intuition about this. You ought to know that H3O+ ~0 (really -1.7) and HCl would have a pKa <<0 (strong acid). You should know pKa (HI)>>pKa (HBr)>>pKa(HCl) by periodic trends. You should know HF and H2O would compare as well as H2S and H2O. Further, you should know the common organic compounds from ochem (i.e., they're covered in ochem so they're definitely fair game w/o having to give a table) -- in other words, you should have a general idea of what a pKa might look like for a phenol, an H alpha to carbonyl, alkyl Hs (terminal), carboxylic acids, etc., and know how different common functional groups (e.g., alkyl groups, halides, etc.) modify these properties. This all comes from a conceptual understanding. IF you simply memorize that halides make something more acidic, they WILL find a way to make the answer choice most obviously correct to you incorrect!

 

[StudyShy]

I think that over time, acids and bases become more second nature. They are definitely something to have a good handle on.

I'll just focus on learning the aa's. j/k again

 

[lolarogers]

Acetic acid is a weak acid because, like all other weak acids, it only disassociates partially in water (also known as incomplete ionisation).

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