Second Deprotonations of Diprotic Acids

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Katrina Jang 2H
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

Second Deprotonations of Diprotic Acids

Postby Katrina Jang 2H » Sat Nov 28, 2015 7:24 pm

Why can the second ionization of diprotic acids be ignored if Ka2 << Ka1?

Anne Cam 3A
Posts: 71
Joined: Fri Sep 25, 2015 3:00 am

Re: Second Deprotonations of Diprotic Acids

Postby Anne Cam 3A » Sat Nov 28, 2015 7:55 pm

If Ka2 is significantly less than Ka1, that means the concentration of products is less than that of the reactants for the Ka2 compared to Ka1. In other words, the second deprotonation of the acid is not favored and thus unlikely to occur.

joannali1027
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

Re: Second Deprotonations of Diprotic Acids

Postby joannali1027 » Sun Nov 29, 2015 6:34 pm

To add on, it's because once you lose one proton, it gets much harder for another Hydrogen atom to leave when the molecule is now negatively charged. Because of this, the Ka will be smaller as it favors the reactants. However, H2SO4 is an exception where the first is fully deprotonized and is a strong acid in its first deprotonation.

Daniella Ching 4C
Posts: 20
Joined: Fri Sep 25, 2015 3:00 am

Re: Second Deprotonations of Diprotic Acids

Postby Daniella Ching 4C » Sun Nov 29, 2015 10:29 pm

Is Ka2 typically greater than Ka1 for most polyprotic acids? Is there a trend? Or would we have to calculate the Ka1 and Ka2 for each polyprotic acid in order to tell?

Anne Cam 3A
Posts: 71
Joined: Fri Sep 25, 2015 3:00 am

Re: Second Deprotonations of Diprotic Acids

Postby Anne Cam 3A » Mon Nov 30, 2015 10:05 am

Generally there is a trend, because as joannali1027 mentioned, after removing one proton (H+) from a polyprotic acid, the molecule has a negative charge (ex. H2PO4- from H3PO4 losing 1 proton). Negative charges attract positive charges, so the negatively charged molecule is more unlikely to let go of another, positively charged proton. So further deprotonation (product formation) is not favored, and Ka2 would be smaller since [products]<[reactants].


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