TBR Titrations Clarification?

[justadream]

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TBR Gen Chem says that titration curves have an initial cusp when the reagent being titrated is weak (the "lip-o-weakness").

Can someone clarify why this happens (and why this happens only when the reagent being titrated is weak)?

Also, how important is it to know/memorize how the titration curves shift when concentration/strength change?

 

[orangetea]

I am pretty sure the initial cusp is due to the fact that you have a weak acid. Weak acids are not as likely to dissociate as strong acids and so it will take a while (as in you have to keep increasing your base concentration to get it going. However that could have been worded a lot better.


I would HIGHLY recommend that you know those graphs like the back of your hand for strong/weak acids AND bases. Acid base are HIGHLY likely to come up since they are a huge factor in a lot of physiological systems and can be applied to a of topics. I just googled the titration curves and a lot of them were labeled so I went through them and memorized/learned why certain things are where they are and concentrations are each point.

🙂

 

[sillyjoe]

I am pretty sure the initial cusp is due to the fact that you have a weak acid. Weak acids are not as likely to dissociate as strong acids and so it will take a while (as in you have to keep increasing your base concentration to get it going. However that could have been worded a lot better.


I would HIGHLY recommend that you know those graphs like the back of your hand for strong/weak acids AND bases. Acid base are HIGHLY likely to come up since they are a huge factor in a lot of physiological systems and can be applied to a of topics. I just googled the titration curves and a lot of them were labeled so I went through them and memorized/learned why certain things are where they are and concentrations are each point.

🙂

Did you memorize the effects of changing concentrations?

 

[orangetea]

Did you memorize the effects of changing concentrations?

I hope I understand that correctly.

So I memorized (or rather reasoned) what concentration of of the weak/acid base are more predominant at certain points on the graph. So for example for diprotic acids I went over at what point is HCO3 more prevalent vs CO3.

As far as shifting goes I went over how the pH at equivalent point would look for a weak acid/base and strong acid/base.

#brainfried so i hope that made sense 🙂

 

[justadream]

@orangetea

I thought the lip-o-weakness effect was due to changes happening quickly (not slowly).

For example, when you titrate a weak acid with a strong base, when you initially add a little but of base, the pH increases dramatically.

 

[orangetea]

yess we are both saying the same thing but like I said I didn't word it correctly. So for example if you look at graph for the weak acid titration as you said there is that initial cusp due to dramatic pH increase. The reason for that is that you have a weak acid and you are titrating it with a base. Weak acids do not dissociate as easily and so initially (before the buffering region) the pH goes up due to the addition of the base. That's what I meant by it takes a while(from the point of adding the base to reaching the buffering region) to get it going.

That's how I had understood it from the TBR book 🙂

 

[Czarcasm]

@orangetea

I thought the lip-o-weakness effect was due to changes happening quickly (not slowly).

For example, when you titrate a weak acid with a strong base, when you initially add a little but of base, the pH increases dramatically.

My understanding for the slight curvature ('lip-o-weakness') was because a weak acid has a weak conjugate base (which occasionally can react). A strong acid on the other hand has a neutral conjugate. What this basically means is, with a strong acid, what becomes neutralized essentially stays neutralized. For weak acids, you have to consider the reverse reaction occurring. In other words, it's slightly more difficult to attain a certain pH because you have this competing reaction. You overcome this reaction by introducing more base (or acid) so that it favors one side over the other. That's my interpretation anyways. It makes sense to me, but I'm not entirely sure if that's the appropriate reasoning. At the end of the day though, I don't really think it's all that import to recognize. Other things are far more important. For instance, where equivalence is occurring: above, below or at pH 7 for instance is more meaningful.

 

[justadream]

@Czarcasm

How does that interpretation account for the fact that the change is rapid but then diminishes? Isn't what you are saying the explanation for why the equivalence point for a weak acid + strong base titration is greater than 7?

 

[Czarcasm]

@Czarcasm

How does that interpretation account for the fact that the change is rapid but then diminishes? Isn't what you are saying the explanation for why the equivalence point for a weak acid + strong base titration is greater than 7?

No, that's entirely different. The equivalence point for a weak acid & strong base titration is above 7 only because the salt that's produced is basic. The rapid increase for a strong base/acid titration is in large part because the pH scale is logarithmic and because there is a direct correspondence between strong base added and pH increase;

Better yet, think of it this way. When you add strong base to a strong acid, it's as if you're simply decreasing the number of H+ molecules in solution. This is because the salt is neutral and part of the initial H+ that was floating in the solution is neutralized. Having fewer H+ and because it's now less concentrated, this essentially correlates to a higher pH value.

It's rapid initially because, for example: going from a loss of 0.001 moles to 0.01 mole H+ (a 10 fold increase) corresponds to 1 pH unit. 0.01 moles to 0.1 moles (another 10 fold increase), corresponds to another pH unit. 0.1 moles to 1 moles (10 fold increase), corresponds to another pH unit. As you can see here, the mole difference is increasing dramatically and so essentially, it's more difficult to attain a 10-fold increase in change. Notice here, because we're dealing with a strong acid/base titration, we're disregarding any mildly reactive conjugate base present that could further slow this rapid change (because it reacts slightly to produce a few protons, slowing the change in pH). Eventually you reach a point where the solution is fully neutralized (pH 7), and from there, adding additional [OH-] will only further increase the pH from 7 (because pOH is decreasing).