Protonation Question

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Protonation Question

Postby jonathanshi_1A » Sun Nov 27, 2016 9:09 pm

Regarding Polyprotic Acids/bases, do you always assume that they have 2 protonations? I understand that the 3rd protonation is extremely hard because it's harder to lose a H+ ion from a negatively charged ion.

Rochelle Ellison 2H
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Re: Protonation Question

Postby Rochelle Ellison 2H » Mon Nov 28, 2016 10:28 am

Polyprotic acids are acids that can donate more than more than 1 H+ ion and their conjugate bases can accept more than 1 H+ ion. You typically do not need to assume how many H+ ions will be lost/accepted as you can tell how many H+ ion the acid will donate by determining how many acidic hydrogen atoms the acid has. These acidic hydrogen atoms are typically written in front of the formula for inorganic acids or at the end of the carboxyl group for organic acids. So H2CO3 can lose 2 and H3PO4 can lose 3. Typically when calculating the pH of polyprotic acids you can take only the first deprotonation into account, except for sulfuric acid which is a strong acid so the first deprotonation is complete and therefore the second deprotonation is significant. To check if the second or third deprotonation is significant you can use the rule that if Ka1/1000 is greater than Ka2 or Ka2/1000 is greater than Ka3 then Ka2 or Ka3 is not significant and can be disregarded when calculating the pH.

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