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My general chemistry textbook explains that the reaction...

CH3COOH(aq) + NH3(aq)  NH4+(aq) + CH3COO-(aq)

...goes to completion. I understand that, as NH3 receives H+ ions from the acetic acid, more and more acetic acid ionizes. However, I thought that eventually NH3 would stop accepting H+ ions, as it is a weak base... Why does this reaction go to completion?

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Update:

I can see the reaction completing as follows:

NH3 + H2O   NH4+ + OH-
CH3COOH   CH3COO- + H+
H+ + OH-   H2O
______________________________________
NH3 + CH3COOH   NH4+ + CH3COO-

This is an explanation that makes more sense to me. As water molecules are formed between the OH- ions (which come from the reaction between NH3 and water) and H+ ions, more and more NH3 and CH3COOH are ionized until completion. However my textbook explains completion with this reaction:

H+(aq) + NH3(aq)   NH4+(aq)

...essentially claiming that "the H+ ions from the CH3COOH readily combine with NH3 molecules to form NH4+ ions."


So, who is right? In other words, does NH3 really accept H+ ions directly? Or does NH3 react with water first, so that the hydroxide ion produced (and NOT NH3) reacts with the H+?

[Dan]

Ammonia is not a weak base in the context of acetic acid - remember these things are relative. Consider the pKa of ammonium (~9) vs the pKa of acetic acid (4.8 ) - acetic acid is ~15000 times more acidic than ammonium, or from the other angle ammonia is ~15000 times more basic than acetate.

So ammonia can certainly directly deprotonate acetic acid. If you bubbled ammonia gas through glacial acetic acid you'd get ammonium acetate. Water is not required.