Buffer/acid base titration question.

[blackhole123]

=Lets say you titrate a weak acid, HA, with a strong base, NaOH.

 

Now, before the equivalence point, the pH is calculated like a buffer because HA + OH- -> A- + H2O. So you have a mixture of a an acid and its conjugate base. My question is, why doesn't this act like a neutralization reaction like a strong acid and strong base? In order to get OH- you have to dissociate NaOH, so you also have Na+ in solution. Why doesn't the Na+ react with the A- to form a salt like a strong acid and strong base would?

 

I'm also confused about buffers in general. You have an acid and its conjugate base... why don't they react? How can they coexist in solution?

 

Help, I seem to have a fundamental misunderstanding of acid-base chemistry!

 

EDIT:

 

WOW I'm dumb. I've been stressing over this for hours, thinking I didn't understand chemistry anymore. But obviously, the A- is in solution so its not going to form a precipitate because salts dissociate in water. Na+ is just a spectator ion.

 

Still not totally sure on the second question though.

 

Well, actually I guess they couldn't really react and change pH. If HA reacts with A- it would just form another A-....

Edited October 14, 2010 by blackhole123

[cypress]

=Lets say you titrate a weak acid, HA, with a strong base, NaOH.

 

Now, before the equivalence point, the pH is calculated like a buffer because HA + OH- -> A- + H2O. So you have a mixture of a an acid and its conjugate base. My question is, why doesn't this act like a neutralization reaction like a strong acid and strong base?

 

It actually does in the sense that whether the the substrate is strong or weak, a similar molecular equivalent of the titrant is required, but the difference is that with a strong substrate, the beginning pH will be off the chart in one direction or another because the strong substrate readily dissociates on its own account so that as the titration progresses, the pH will also move greatly. However with a weak substrate (buffer), the pH will remain relatively stable because the "buffer" dissociates only as a response to addition of the titrant in proportion to the amount added until the buffer is consumed. When the initial solution contains both a strong substrate and a buffer, pH will again move slowly until the buffer is fully dissociated/ionized and then it will move greatly, but more to the point, it will require the molar equivilent of both the strong and the weak substrates.

 

In order to get OH- you have to dissociate NaOH, so you also have Na+ in solution. Why doesn't the Na+ react with the A- to form a salt like a strong acid and strong base would?

 

In general the capability to form a solid rather than remain ionized is always possible, but in practice it occurs less frequently with strong acids and bases because the propensity to remain ionized is a primary driver for what allows the compound to be strongly acidic or basic. Therefore it is not as common for the ions of strong acids and bases to precipitate a salt (precipitated salts are often referred to as minerals). Keep in mind that it does happen from time to time so don't treat this observation as a rule.

 

I'm also confused about buffers in general. You have an acid and its conjugate base... why don't they react? How can they coexist in solution?

 

Help, I seem to have a fundamental misunderstanding of acid-base chemistry!

 

I think you have a fairly good understanding. Your questions illustrate a solid foundation and you are questioning the simplifications that are provided to introduce this topic. As you are finding, the simplifications tend to be a bit misleading and your questions are around these apparent contradictions.

 

They coexist because they are in equilibrium, continually changing forms as H+ (protons) leave and reattach. The concentration ratio is the ratio that is required given the conjugate base and acid forms relative propensity to hold/release protons so that the rates are balanced. They don't react because they are chemically similar and have little chemical interest in each other. Think of them as two gentlemen suitors (sorry my age is showing) taking turns on the dance floor with the ladies (the proton).

 

Sorry for the slow response, was helping my daughter with homework as well so I did not see your edits until after I finally finished this.

Edited October 14, 2010 by cypress

[mississippichem]

Well, actually I guess they couldn't really react and change pH. If HA reacts with A- it would just form another A-....

 

Yes, that's the idea of a buffer. The buffer supplies a excess of some "common ion" to continually participate in a associative/dissociative equilibrium with all the [ce]H^+[/ce] and [ce] OH^- [/ce] floating around in solution that would otherwise alter the pH drastically. I think you've got the idea reasonably well. Buffers used to confuse me a few years back.