pKa v. pKa1 and pKa2

Acidity
Basicity
The Conjugate Seesaw

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Maricruz Diagut 3J
Posts: 12
Joined: Fri Sep 25, 2015 3:00 am

pKa v. pKa1 and pKa2

Postby Maricruz Diagut 3J » Thu Dec 03, 2015 2:09 am

Can someone please explain what the difference between pKa v. pKa1 and pKa2 is?

Heerali Patel 3A
Posts: 26
Joined: Fri Sep 25, 2015 3:00 am

Re: pKa v. pKa1 and pKa2

Postby Heerali Patel 3A » Thu Dec 03, 2015 1:46 pm

pKa1 and pKa2 are the negative logs of the acidity constants for the first and second stage in which a polyprotic acid loses a proton. pKa(overall) is the negative log of the overall acidity constant for the overall ionization reaction of the polyprotic acid. For example, using H2CO3 as the polyprotic acid:

H2CO3(aq) + H2O(l) <=> HCO3-(aq) + H3O+(aq) Ka1=[H3O+][HCO3-]/[H2CO3] pKa1=-log(Ka1)
HCO3-(aq) + H2O(l) <=> CO3^2-(aq) + H3O+(aq) Ka2=[H3O+][CO3^2-]/[HCO3-] pKa2=-log(Ka2)
__________________________________________
H2CO3(aq) +2H2O(l) <=> CO3^2-(aq) +H3O+(aq) Ka(overall)=Ka1 Ka2=[H3O+]^2[CO3^2-]/[H2CO3] pKa(overall)=-logKa(overall)
Last edited by Heerali Patel 3A on Thu Dec 03, 2015 1:53 pm, edited 2 times in total.

Chem_Mod
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Re: pKa v. pKa1 and pKa2

Postby Chem_Mod » Thu Dec 03, 2015 1:47 pm

Ka refers to the equilibrium if an acid only has 1 proton to give. Ka1 and Ka2 are for polyprotic acids and refer to the first deprotonation and second deprotonation reactions.

Alondra Loera 1A
Posts: 20
Joined: Fri Sep 25, 2015 3:00 am

Re: pKa v. pKa1 and pKa2

Postby Alondra Loera 1A » Thu Dec 03, 2015 9:47 pm

Just like the pH, the pKa tells you of the acid or basic properties of a substance.
pKa <2 means strong acid
pKa >2 but <7 means weak acid
pKa >7 but <10 means weak base
pKa >10 means strong base

pKa1 is when an acid can undergo more than 1 ionization the pka1 is the disassociation constant for the first ionization
pKa2 is the disassociation constant for the second ionization

Kai_Chiu 1F
Posts: 19
Joined: Fri Sep 29, 2017 7:03 am

Re: pKa v. pKa1 and pKa2

Postby Kai_Chiu 1F » Sat Dec 09, 2017 11:34 am

So, pKa1 and pKa2 only really matter when the problem is asking for second and first ionization? E.g. in problem 12.35, it simply asks for Ka value and gives a pKa1. I could just take 10^-pKa1 and get the answer? I do not have to worry about the 1 subscript?


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