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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: OrganicH2O on June 24, 2020, 07:23:23 PM

Title: If the pKa of water is 14, are other pKa's also wrong??
Post by: OrganicH2O on June 24, 2020, 07:23:23 PM

It's commonly stated that the pKa of water is actually 14, even though countless Sophomore texts list it as 15.6. Even in the the classic "Evan's pKa table" it is listed as 15.6.  P-Chem is really not my thing, so I'm not sure I can fully follow the explanation with thermodynamics.

I am just curious to ask anyone who actually understands the thermodynamics here: does that mean other common pKa's are also slightly off? If this is the case, at least relative acidities would stay roughly the same. E.g. if methanol's pKa was also 1.5 lower, methanol and water would still have about the same pKa.

Title: Re: If the pKa of water is 14, are other pKa's also wrong??
Post by: Babcock_Hall on June 24, 2020, 08:41:26 PM

I am a little pressed for time right now, but there is are at least two articles in J. Chem. Ed. that deal with this subject.
J. Chem. Educ. 2017, 94, 6, 690–695 is the more recent one.  https://pubs.acs.org/doi/10.1021/acs.jchemed.6b00623

Title: Re: If the pKa of water is 14, are other pKa's also wrong??
Post by: Borek on June 25, 2020, 03:57:02 AM

Basically it depends on whether you assume pure water activity is 1, or it equals water molar concentration of 1000/18. Matter of convention. 14 is typically much safer to use as most Ka/Kb tables report values assuming Kw of 14 (Ka+Kb=Kw).

Compare https://www.chembuddy.com/?left=pH-calculation&right=water-ion-product

Title: Re: If the pKa of water is 14, are other pKa's also wrong??
Post by: hollytara on June 25, 2020, 03:08:02 PM

Here is a really simplistic explanation. 

At pH = 14, based on Kw, [H+] = 10^-14 and [OH-] = 10^0 = 1

 at pKa, [HA] and [A-] should be equal - but at pH 14, [OH-] is 1 M but [H2O] is ~54 M

So pKa is not 14.

What about 15.6?  Here at pH 15.6, [H+] = 10^-15.6 and [OH-] = 10^1.6 = 39.8 M. Is [OH-] = [H2O]?  If you take the molarity of pure H2O at 55.5M and divide by 2, [OH-] = [H2O] would be at 27.75M which would be a pH of 15.4 

I think this difference between 15.6 and 15.4 has to do with the water activity at this extremely high [OH-]

Title: Re: If the pKa of water is 14, are other pKa's also wrong??
Post by: OrganicH2O on June 30, 2020, 08:47:46 PM

Thank you, all for the resources/ideas. I will have to do some independent work to grasp all this.