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)?

  • $\begingroup$ "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". $\endgroup$ – SimonE Oct 9 '18 at 15:20
  • $\begingroup$ 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? $\endgroup$ – Stikke Oct 9 '18 at 15:25
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    $\begingroup$ 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. $\endgroup$ – Mithoron Oct 9 '18 at 16:30
  • $\begingroup$ 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. $\endgroup$ – MaxW Oct 9 '18 at 18:09
  • $\begingroup$ 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. $\endgroup$ – Stikke Oct 9 '18 at 19:30

To answer your first question, weak acids don't get levelled in water. Each solvent has their acid-base equilibrium window, and water has one that is between 0-14, meaning that any solvent with a pKa within that range can be distinguished in water. There are other solvents with a ore narrow acid-base discrimination Window, such as methanol can acid, with a range between -10 to about 3, which will be unable to discriminate between weak acids. The reasoning behind the acid-base discrimination window is a bit long winded (and maybe a little confusing), so I will try to break it down as much as I can.

Say you have an acid in water, then the pKa + pKb = pKw This is true for any acid or base in water. If pKa<0 or pKb<0 then then most of the compound will be dissociated in water, as an acid or base, respectively. This is undesirable, especially if the pKa/pKb<<0. As you have noted in your comments, when the pKa of a compound is less than 2 units compared to water, then in water only 1% of it will remain in the undissociated form. If pKa>pKw, you will obtain a negative pKb which will behave more like a base in water. It follows that compounds with a 0< pKa< pKw will fulfill the condition where both pKa and pKb> 0. Now guess what pKw is? 14. Hopefully this might explain to you why water is able to distinguish between weak acids with a pKa between 0 to 14.

  • $\begingroup$ Ok I think now I totally got it. I know it's a matter of semantics, and not that important. But including pKa's and pKb's in my reasoning: If the pKa is smaller than 14, say at least 2 units below as well. Then in this case the weak acid would only dissociate around 1% right? And so below this pKa weak acids can not be distinguished as you said. So that's what I called a leveling effect for weak acids in my original question. $\endgroup$ – Stikke Oct 11 '18 at 9:31

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