Is there also a leveling effect for weak acids in water?

The leveling effect says that any acid stronger than the hydronium ion ($$\ce{H3O+}$$) will completely dissociate in water. Hence no differentiation of acid strength is possible between eg HBr and HCl in water.

I was wondering why no one ever talks about the leveling effect for acids weaker than water. Isn't it so that these acids would not dissociate at all and hence no differentiation is possible on this side either?

Also, what is the reasoning behind the fact that acids stronger than the hydronium ion will completely dissociate in water? Why can't it be an equilibrium (say if it's only slightly stronger than hydronium)?

• "Dissociation" is somewhat incorrect term. When you dissolve, say, gaseous HBr in water, the water molecule plucks proton from HBr. So dissociation is, in fact, acid-base reaction. That means, if you dissolve an acid whose acidity is 2 units of pKa(or more) lower, than the acidity of hydronium, it will be, in practice, completely "dissociated". Oct 9, 2018 at 15:20
• Ok, I appreciate the comment, dissociate is indeed the wrong term. However, as you say, complete 'reaction' will only occur if the pKa differs more than 2 units, so the statement that all acids stronger than H3O+ can be considered to react completely is not really correct? Also what about the weaker acids than H2O? Oct 9, 2018 at 15:25
• Your point about strong acid is entirely valid. Of course there's equilibrium, but in diluted acids it can be approximated as complete dissociation. On the other hand this talk about weak, doesn't seem to make sense. Oct 9, 2018 at 16:30
• For weak acids you have to think also of the conjugate base. So $\ce{HA}$ might not be a great proton donor, but $\ce{A-}$ can be a great proton acceptor.
– MaxW
Oct 9, 2018 at 18:09
• First of all, I personally don't get why people are so eager to downvote questions of people who are genuinely interested, without argumentation too. If an acid is weaker than water it's pKa is larger than 14 and by default the pKb of the conjugate base smaller than 0. This means that virtually no acid will be transformed into it's conjugate base. And this is the same for all acids weaker than water, it's like the levelling for strong acids but to the other side of the equilibrium. Oct 9, 2018 at 19:30