Polyprotic pKa and pKb

[MedPR]

So TBR says pKa1 + pKb2 = 14, and pKa2 + pKb1 = 14 since the first proton lost from the acid is the second proton gained by the conjugate base.

On page 262 in gen chem, however, it says that H2PO4- pKa2 = 7.2 and HPO4- pKb2=6.8

Is that a typo? Should it say pKa2 = 7.2 and pKb1=6.8?

Right below it says:

H2CO3 pKa1=6.4 and HCO3- pKb2=7.6.. which makes sense.

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[Pisiform]

I think its correct, pKb1 will go with pKa3

 

[Pisiform]

to be more clear, pKb1 would be defined by: PO43- + H2O ----> HPO4- + OH-

so pKb1 is not compatible with pKa2 but is with pKa3

 

[MedPR]

I think its correct, pKb1 will go with pKa3

to be more clear, pKb1 would be defined by: PO43- + H2O ----> HPO4- + OH-

so pKb1 is not compatible with pKa2 but is with pKa3

I'm still really confused, sorry.

I understand that H3PO4 is pka1, H2PO4- is pka2, and HPO4 2- is pka3.

But if you wrote out all of the deprotonation reactions, H2PO4- is the conjugate base of H3PO4, but it is also the conjugate acid of HPO4 2-.. So what pka and/or pkb do you assign to H2PO4 -?


H3PO4 + H2O -----------> H2PO4- + OH-
H2PO4- + H2O -----------> HPO4 2- + OH-
HPO4 2- + H2O ----------> PO4 3- + OH-

 

[Pisiform]

H3PO4 + H2O -----------> H2PO4- + H+ is defined by pKa1
H2PO4- + H2O -----------> HPO4 2- + H+ is defined by pKa2
HPO4 2- + H2O ----------> PO4 3- + H+ is defined by pKa3

PO43- + H2O ---> HPO4 2- + OH- defined by pKb1

HPO4 2- + H2O ----> H2PO4 - + OH- defined by pKb2

H2PO4- + H2O ----> H3PO4 + OH- is defined by pKb3

from this you can see

pKb1 + pKa3 = 14

pKa1 + pKb3 = 14

pKa2 + pKb2 = 14

 

[Gengar]

I'm confused by the reactions you wrote

Edit: I was referring to the OP's second post

H3PO4 ---------> H+ + H2PO4- ... pka1
H2PO4- ----------> H+ + HPO4 2- ... pka2
HPO4 2- --------> H+ + PO4 3- ... pka3

PO4 3- + H2O -------> OH- + HPO4 2- ... pkb1
HPO4 2- + H2O --------> OH- + H2PO4- ... pkb2
H2PO4- + H2O -------> OH- + H3PO4 .... pkb3

hope that helps, correct me if i'm wrong

 

[Gengar]

neverrrrrmind lol

 

[MedPR]

H3PO4 + H2O -----------> H2PO4- + H+ is defined by pKa1
H2PO4- + H2O -----------> HPO4 2- + H+ is defined by pKa2
HPO4 2- + H2O ----------> PO4 3- + H+ is defined by pKa3

PO43- + H2O ---> HPO4 2- + OH- defined by pKb1

HPO4 2- + H2O ----> H2PO4 - + OH- defined by pKb2

H2PO4- + H2O ----> H3PO4 + OH- is defined by pKb3

from this you can see

pKb1 + pKa3 = 14

pKa1 + pKb3 = 14

pKa2 + pKb2 = 14

Got it! I never thought of it that way before.

Thank you

 

[Ssina]

yeah the numbering switches so have to write them out

 

[BerkReviewTeach]

Got it! I never thought of it that way before.

Just remember that the first proton lost will be the last one gained. If the acid is diprotic, then pKa1 and pKb2 represent a conjugate pair. If the acid is triprotic, then pKa1 and pKb3 represent a conjugate pair. If the acid is tetraprotic, then pKa1 and pKb4 represent a conjugate pair.

PS: another great job by pisiform.