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Quick chemistry question

F

Fiveohhh

Diamond Member
I'm wondering how you can tell if a diprotic acid is a strong or weak acid from the titration curve. When monoprotic the equivalence point should have a pH of 7, but that doesn't seem to be the case for diprotic acids.

Any help would be appreciated. I have this lab due in 9 hours😀
 
And I just finished Chem 2 a week ago, took it in the summer, lol.

I can't think if how to tell for polyprotic acids, but may Google it a bit and will post if anything pops into my head. 🙂
 
Originally posted by: potoba
stronger acids give weaker conjugate bases
Click to expand...

not all diprotic acids are strong acids though.. phosphoric acid, for example, is triprotic, but is a weak acid 🙂


OP: i don't remember 😛 equivalence point is half of the H+ ions have been neutralized, correct? take a look at your titrant and the chemical equation to figure it out.

not to mention that diprotic acids release their H+ ions in stages - the first one comes off easily, then the second is typically a bit harder to neutralize.
 
Find the two halfway points, one should be below the first equivalence point and one between the two equivalence points. The pKa of an acid is where these two halfway points are. So the H2CO3 has a pKa of 6 and HCO3 has a pKa of 11...I'm just making up number.
 
Originally posted by: txrandom
Find the two halfway points, one should be below the first equivalence point and one between the two equivalence points. The pKa of an acid is where these two halfway points are. So the H2CO3 has a pKa of 6 and HCO3 has a pKa of 11...I'm just making up number.
Click to expand...

Maybe I'm missing something but how does that determine if a diprotic acid is a strong or weak one?
 
When you say "titration curve" I'm going to assume this: you have a beaker with some acid solution in it and a pH probe stuck into the solution. You titrate from a burette some base solution of known concentration. As you add base, you record periodically what volume of base has been added and what the pH meter says. That way you get a graph of pH versus volume. It starts from low pH at zero added base volume and the pH changes very little for a while, then rises much more rapidly (i.e., over a short span of base volume increase), then nearly levels out again. For a diprotic acid there should be two rises in the total curve.

Now, the term "strong acid" usually means that the acid molecule in water dissociates very easily so that the solution has a very high concentration of H+ ions. That also means, usually, that a very high proportion of the acid molecules are dissociated into ions. So the first clue is the pH at which the titration curve rises suddely - that's the equivalence point for that proton. If that happens at a very low pH then you had a very high H+ ion concentration before adding the base, and that could be considered a strong acid. The other clue is the slope of the rise of pH versus volume. If the rise is very fast, it indicates that the molecules were highly dissociated before any base was added, and this also indicates a stronger acid. But if the rise is more gradual over a wider range of added base volume, then the dissociation was incomplete and it took the addition of neutralizing base (OH-) ions to consume the H+ and force dissociation of more acid molecules. That would be considered a weak acid.

I once tried to analyze the acid content of a wine I was starting to ferment to be sure I had added the right amount of three fruit acids. But they were all weak acids and the titration curve had three very broad pH rises overlapping, so the curve looked more like a gentle slope up with almost no recognizable "knee" in it. Could not get the answer I wanted that way.
 
Well, a strong acid is generally one that completely dissociates in water. Hydronium ion has a pKa of about -2, so any strong acid should be one that has a pKa of -4 or so (since a pKa of -4 will completely protonate water, generating hydronium ion). So, since the the pKa of an acid will equal the pH of the midpoint (pH=pKa when [A-]=[HA]), any titration curve where the midpoint is around pH<2 should be a strong acid. Only polyprotic acid off the top of my head that fits the bill is H2SO4, pKa approx -6.
 
Originally posted by: Fenixgoon
Originally posted by: potoba
stronger acids give weaker conjugate bases
Click to expand...

not all diprotic acids are strong acids though.. phosphoric acid, for example, is triprotic, but is a weak acid 🙂


OP: i don't remember 😛 equivalence point is half of the H+ ions have been neutralized, correct? take a look at your titrant and the chemical equation to figure it out.

not to mention that diprotic acids release their H+ ions in stages - the first one comes off easily, then the second is typically a bit harder to neutralize.
Click to expand...

I said stronger, not strong!!!!
 
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