Acid-Base gen chem question

[evoviiigsr]

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I know that for a diprotic acid, only the first H has an effect on pH. I was wondering, for a diprotic BASE, does only the first OH effect pH as well?

 

[mrmilad]

I am almost certain that the first protonation is the one that yields the highest results in solution, so I would say that diprotic bases follow the same trend as diprotic acids. However diprotic bases usually react with water to make the OH- so just keep in mind that the OH- doesnt necessarily have to be part of the diprotic base ie:
Major reaction: CO3-2 + H20 --> HCO3- + OH- Large Kb
Kb1 = [HCO3-][OH-]/[CO3-2]

Minor reaction: HCO3- + H2O ---> H2CO3 + OH- Small Kb
Kb2 = [H2CO3][OH-]/[HCO3-]

 

[evoviiigsr]

I am almost certain that the first protonation is the one that yields the highest results in solution, so I would say that diprotic bases follow the same trend as diprotic acids. However diprotic bases usually react with water to make the OH- so just keep in mind that the OH- doesnt necessarily have to be part of the diprotic base ie:
Major reaction: CO3-2 + H20 --> HCO3- + OH- Large Kb
Kb1 = [HCO3-][OH-]/[CO3-2]

Minor reaction: HCO3- + H2O ---> H2CO3 + OH- Small Kb
Kb2 = [H2CO3][OH-]/[HCO3-]

I thought that OH has to be part of the base... In your first example, you have just CO3 reacting with water, but CO3 can't exist by itself, it comes as H2CO3, where hydronium and hyroxide are both formed.

 

[mrmilad]

Carbonate is acting as a base in that situation, it would happen if Carbonic acid which is a diprotic acid was deprotonated twice, which would give you a CO3-2 ion, this is now the conjugate base of bicarbonate ion, and is a base. It would deprotonate water creating an OH- donating e- which is the definition of a lewis base, and possibly it is an arrhenius base as well.

 

[glamqueen]

If it is a Lewis base, it is definitely an arrhenius base too, but then again -your question is probably answered by now...

 

[J ROD]

I know that for a diprotic acid, only the first H has an effect on pH. I was wondering, for a diprotic BASE, does only the first OH effect pH as well?

Yes.

 

[BerkReviewTeach]

I know that for a diprotic acid, only the first H has an effect on pH. I was wondering, for a diprotic BASE, does only the first OH effect pH as well?

Yes and no actually. It depends on the base.

You'll note that carbonate has two pKb values, which impies that the protons are gained one at a time. This correlates to the conjugate acid, which dissociates one proton at a time. So, when carbonate is added to water, we need only consider the gain of the first proton (correlating to pKb1).

However, when you consider hydroxide salts such as KOH, Ca(OH)2, etc... at their saturation point, they are assumed to dissociate completely, which means that both hydroxide anions are released from the lattice. As such, they do not obey the "one at a time" principle. So a 0.001 M Ba(OH)2 solution (if barium hydroxide were completely soluble) would have a pH of 11.3 rather than 11.0 (the pH expected if it were to only release one hydroxide per molecule).

So the answer to your question is that the conjugate base of a polyprotic acid will in fact gain protons on at a time and have distinctly different pKb values for each gain of a proton, but hydroxide and oxide salts do not fit that behavior pattern.