Carbonic Acid and pka's

[ihearindiemusic]

I don't really understand why the relationship between pka and pkb in carbonic acid (H2CO3) is like this for the conjugate pairs...

pka1 + pkb2 =14
pka2 + pkb1 = 14


Why isn't it pka1 + pkb1 = 14? Isn't pkb1 the pkb that correlates to for the first conjugate base of carbonic acid?

Thanks!

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[Gauss44]

I don't really understand why the relationship between pka and pkb in carbonic acid (H2CO3) is like this for the conjugate pairs...

pka1 + pkb2 =14
pka2 + pkb1 = 14


Why isn't it pka1 + pkb1 = 14? Isn't pkb1 the pkb that correlates to for the first conjugate base of carbonic acid?

Thanks!

Because pka1 and pkb1 refer to the gain or loss of different protons. Here's a discussion that helped me: http://206.82.221.135/showthread.php?p=11975524

 

[Postictal Raiden]

Because if you add up both equations you get:

water ----> proton + hydroxide ion, which has a Ka value of 10^-14 (pKa = 14)

 

[Hemorrage]

I don't really understand why the relationship between pka and pkb in carbonic acid (H2CO3) is like this for the conjugate pairs...

pka1 + pkb2 =14
pka2 + pkb1 = 14


Why isn't it pka1 + pkb1 = 14? Isn't pkb1 the pkb that correlates to for the first conjugate base of carbonic acid?

Thanks!

I'm pretty sure its just a fact of nomenclature. Think about it this way, if you lose the 1st proton, you get a cBase that is somewhat powerful (lets say pKb2=5 which means pKa1=9), now, if you lose the 2nd proton, you get a cBase thats even more powerful of a base (pkb1 = 2 which means that pKa2 = 12).

So we see here that pKa1 < pKa2 which agrees with the fact that the first proton to leave is always the most acidic. So in order to cooperate with the fact that pKb1 < pKb2, we have to call the first reaction's pKb, pKb2.