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Author Topic: pKa values and dissociation of acids  (Read 2969 times)

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lo2

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pKa values and dissociation of acids
« on: February 16, 2012, 02:15:56 AM »

Hi there

I hope this post fits into the category!

I have become bit confused about pKa values and the level of dissociation of acids. For instance if we have acetic acid it has a pKa value of 4.76, so the degree of dissociation of this acetic acids as function of pH would look this:



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Btw what do you call this kind of diagram in English? In Danish we call it a "Bjerrum Diagram"

So this means that acetic acid would be almost 100 % dissociated at pH 7. But it is supposed to be a weak acid right, so how can this be?

So if you have some water (pH = 7) and you then add some acetic acid, the pH value will drop, but so will the dissociation rate of the acid. So the acid kind of "undissociates" itself by lowering the pH value?

And can you say that for all acids or bases, that they can be 100 % dissociated as long as the pH value is right?

And finally the lower the pKa value the stronger the acid is, right? So what about a base that has got a pKa value of around 11, is that a strong base? And how to find out?

I know there are a lot of questions and that they are probably quite fundamental, but this is something I have not been able to make sense of as of yet, so some answers would be most appreciated!

Cheers!
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fledarmus

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Re: pKa values and dissociation of acids
« Reply #1 on: February 16, 2012, 02:30:52 AM »

Yes, acetic acid is essentially 100% dissociated at pH7, but it is also essentially 100% associated at pH2. That is what makes it a weak acid. A strong acid would be dissociated in any water solution.

Since a water solution can only range between pH 0 and pH 14, and strong acids and strong bases are completely dissociated in water, you can think of a strong acid as anything with a pKa below about -2 and a strong base as anything with a pKa above about 16. Anything in between and you can find a pH where it is at least partially still associated in water.
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Borek

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Re: pKa values and dissociation of acids
« Reply #2 on: February 16, 2012, 04:05:24 AM »

So this means that acetic acid would be almost 100 % dissociated at pH 7. But it is supposed to be a weak acid right, so how can this be?

Why not? WHat if you have a salt - even weak acids are completely neutralized then.

Quote
So if you have some water (pH = 7) and you then add some acetic acid, the pH value will drop, but so will the dissociation rate of the acid. So the acid kind of "undissociates" itself by lowering the pH value?

In a way. It would work this way when you add acid very slowly, so that initial concentration is very small (small enough to not change pH from 7). Note that form the point of view of the acetic acid added to the solution equilibrium doesn't shift to the left, it shifts to the right.

Quote
And can you say that for all acids or bases, that they can be 100 % dissociated as long as the pH value is right?

Starting with the dissociation constant:

$$K_a = \frac {[H^+][A^-]}{[HA]}$$

and rearranging you can get

$$\frac{[HA]}{[A^-]} = 10^{pK_a-pH}$$

When pKa = pH, concentrations of both HA and A- is identical. When pH is three units below pKa, 99.9% of the acid is in the protonated form, when pH is three units above pKa 99.9% of the acid is in the form of the conjugate base.

Quote
And finally the lower the pKa value the stronger the acid is, right? So what about a base that has got a pKa value of around 11, is that a strong base? And how to find out?

pKa + pKb = pKw
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Borek

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Re: pKa values and dissociation of acids
« Reply #3 on: February 16, 2012, 04:06:20 AM »

Since a water solution can only range between pH 0 and pH 14

No, it is perfectly possible to have a solution with pH outside of the 0..14 range.
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lo2

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Re: pKa values and dissociation of acids
« Reply #4 on: February 16, 2012, 05:05:00 AM »

Ok thank you very much for the answers!

So is this not kind of a paradox, the more acetic acid that you add to a glass of water the lower the pH gets and so does the dissociation rate for acetic acid. So at a low pH relatively there is less acetic acid dissociated, but of course the amount must be bigger and hence the lower pH. Is this correctly understood?

And also what is that kind of diagram I showed in my initial post called in English?
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Borek

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Re: pKa values and dissociation of acids
« Reply #5 on: February 16, 2012, 05:20:05 AM »

is this not kind of a paradox, the more acetic acid that you add to a glass of water the lower the pH gets and so does the dissociation rate for acetic acid. So at a low pH relatively there is less acetic acid dissociated, but of course the amount must be bigger and hence the lower pH. Is this correctly understood?

I don't see a paradox here - it is exactly the way it works.

Quote
And also what is that kind of diagram I showed in my initial post called in English?

I would call it a speciation diagram, but my knowledge of English is far from being perfect.
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XGen

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Re: pKa values and dissociation of acids
« Reply #6 on: February 23, 2012, 02:56:19 PM »

I think a better way to look at the [HA] and [A-] concentrations isn't by adding in some acetic acid, then getting the pH, then adding more, and etc.. it is better understood by realizing that whenever acetic acid is added, an equilibrium is established at a pH where it is stable, and this happens every time more acid is added.
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