Acetic acid completely dissociates in liquid ammonia and I would like to know the $\mathrm{p}K_\mathrm{a}$ value for the acid in that solvent. Can anybody point me to a reference?
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$\begingroup$ I'm not sure such reference exists, but maybe... who knows. One thing is that's a bit obscure solvent, second that acetic acid is strong in something like that and it's difficult to estimate pKa of strong acids. $\endgroup$– MithoronCommented Feb 28, 2021 at 17:50
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$\begingroup$ Is acetic acid soluble in liquid ammonia? $\endgroup$– Martin - マーチン ♦Commented Mar 17, 2021 at 23:21
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$\begingroup$ Yes, it is soluble as the question implies. $\endgroup$– ScepticalChymistCommented Mar 17, 2021 at 23:27
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Sorry to necropost, but I think I might have something on this. This might not be completely accurate (I'm an amateur) but here I go, I guess.
The hydrolysis of ammonium in water is illustrated as: $$ \ce{NH4^+(aq) + H2O(l) <=> H3O^+(aq) + NH3(aq)} $$ As ammonium has an aqueous pKa of 9.25, the equilibrium constant of this reaction is $ 5.62*10^{-10} $.
Reversing the equation results and modifying the phase symbols to fit an ammoniacal solution in the following ($ (am) $ just means dissolved in liquid ammonia): $$ \ce{H3O^+(am) + NH3(l) <=> NH4^+(am) + H2O(am)} $$ with an equilibrium constant of $ 1.78*10^9 $, which indicates that hydronium completely dissolves in ammonia.
But acetic acid does not completely disassociate in water. The acid ionization of acetic acid in water is: $$ \ce{HC2H3O2(aq) + H2O(l) <=> H3O^+(aq) + C2H3O2-(aq)} $$ with an aqueous pKa of 4.76 and an equilibrium constant of $ 1.74*10^{-5} $. Combining the equations and multiplying their equilibrium constants leads to some pretty interesting results. The following is the disassociation constant of acetic acid in ammonia: $$ \ce{HC2H3O2(am) + NH3(l) <=> NH4^+(am) + C2H3O2-(am)} $$ Knowing that the reaction between liquid ammonia and hydronium has an equilibrium constant of $ 1.78*10^9 $ and that the acid ionization constant of acetic acid in water is $ 1.74*10^{-5} $, the acid ionization constant of acetic acid in ammonia can be calculated to be approximately 30,900, which corresponds to an ammoniacal pKa of -4.49, indicating that acetic acid disassociates completely in ammonia, making it a strong acid.
