[Blackcoat7]

I have some questions regarding titration of a weak dibasic acid and acidity in general. When looking at a titration curve of a dibasic acid that looks similar to a monoprotic acid curve, the magnitudes of Ka's (acid dissociation constants) are close to each other. What causes these magnitudes to be close to each other? Is it because it is a weak dibasic acid?

Why does an isomeric change in geometry from cis to trans affect pKa?

In looking at the Debye Huckel Equation log γ = (-0.509 * zi^2 * sqrt I)/1 + sqrt I, why is it, or not, reasonable to say that the activity of all ions of identical charge will be the same in a solution of a given ionic strength? Note: ai = γi * ci

[Borek]

What causes these magnitudes to be close to each other? Is it because it is a weak dibasic acid?

Sometimes they are close, sometimes they are not. Each case should be analyzed separately.

Why does an isomeric change in geometry from cis to trans affect pKa?

I believe you should be able to google the answer - just look for pages explaining differences between fumaric and maleic acid.

In looking at the Debye Huckel Equation log γ = (-0.509 * zi^2 * sqrt I)/1 + sqrt I, why is it, or not, reasonable to say that the activity of all ions of identical charge will be the same in a solution of a given ionic strength? Note: ai = γi * ci

No - activity doesn't depend on the activity coefficient alone, also on the concentration. Even if the activity coefficients are identical, activities don't have to be identical.

Please note, that the equation you listed is simplified. The more elaborate version takes into account ion radius, so the same charge doesn't guarantee the same activity coefficient.

[Blackcoat7]

Sometimes they are close, sometimes they are not. Each case should be analyzed separately.

So what you're saying is that other factors are involved? Like the base?

I believe you should be able to google the answer - just look for pages explaining differences between fumaric and maleic acid.

Aaa, I got it. So cis maleic acid makes a hydrogen bond with itself and is more favorable in making the maleic acid proton than trans fumaric acid.

No - activity doesn't depend on the activity coefficient alone, also on the concentration. Even if the activity coefficients are identical, activities don't have to be identical.

Please note, that the equation you listed is simplified. The more elaborate version takes into account ion radius, so the same charge doesn't guarantee the same activity coefficient.

Makes sense, thank you.

[Borek]

Sometimes they are close, sometimes they are not. Each case should be analyzed separately.

So what you're saying is that other factors are involved? Like the base?

I am saying there are so many factors involved there are no simple rules.

[AdiDex]

Hey Borek ,
Correct me if i am saying something wrong..

sometimes  curve of dibasic (Diprotic) acids is similar to monobasic ( monoprotic) acids....because the first dissociation constant of dibasic acids is small and the second dissociation constant will be too small in comparison to first dissociation constant. we can neglect it.
That why it looks like monobasic weak acid....

[Borek]

sometimes  curve of dibasic (Diprotic) acids is similar to monobasic ( monoprotic) acids....because the first dissociation constant of dibasic acids is small and the second dissociation constant will be too small in comparison to first dissociation constant. we can neglect it.
That why it looks like monobasic weak acid....

No.

Take sulfuric acid. Its first dissociation constant is quite large (around 1000), its second is much lower (around 0.01), but still HSO₄^- is a reasonably strong acid. They will both get neutralized, and the second proton starts to react before first endpoint becomes visible - that means both endpoints will combine and only one will be visible on the curve, despite both reacting.

In most cases when you see a diprotic acid giving a single endpoint, it doesn't mean second proton can be neglected, it means the separation of both endpoints is too low (but the stoichiometry of the neutralization is that of the diprotic acid).

[Blackcoat7]

When looking at the fractional composition αA2- =     ( K1K2) / [H+]2γ2±1γ±2 + [H+]γ±2 K1 + K1K2 of a dibasic acid how can we derive this from the acid equilibria expression Ka1 = (αHA- * αH+)/αH2A and Ka2 = (αA2- * αH+)/αHA-?

and

how can (CbVb)/Va+Vb  + [H+] = αHA- * Atot + 2αA2- * Atot + [OH-] be derived to obtain the fraction of -titration φ = (CbVb)/CaVa = αHA- + 2αA2- - ([H+] - [OH-]/Ca) / 1 + ([H+] - [OH-]/Cb)

?