1
$\begingroup$

Since the six strong acids dissociate completely into their ions, and the reaction goes to completion, does this mean that acids such as $\ce{HI}$ and $\ce{HCl}$ have the same acidic strength?

I know that 0.10 M $\ce{HI_{(aq)}}$ and 0.10 M $\ce{HCl_{(aq)}}$ both contain 0.10 M $\ce{H3O+_{(aq)}}$, which leads me to believe they are equally strong acids. Is this correct?

$\endgroup$
3
$\begingroup$

There's something called the "solvent leveling effect". In short, there's a lowest pKa in a particular solvent, based on the basicity of the conjugate base.

In water, you're limited by $\ce{OH-}$. So given the same concentration of $\ce{HI}$ and $\ce{HCl}$ in water, they will indeed have the same pH.

In other solvents, you may have a difference between acids, depending on the pKa.

$\endgroup$
  • $\begingroup$ I don't think there is a lowest pKa in a particular solvent. This study dx.doi.org/10.1063/1.1739994 quantifies the pKas of nitric and perchloric acid in water by NMR. There is also the method of measuring HCl vapor pressure pubs.rsc.org/en/content/articlepdf/1936/tf/tf9363200743 . Maybe there is a lowest pH or at least a lowest p[H+] just due to the fact that there are only so many H3O+ ions that can fit in a given volume. No way p[H3O+] can be less than -log 55. $\endgroup$ – DavePhD Dec 4 '15 at 13:33
2
$\begingroup$

No, not all strong acids are the same strength

There are more than just 6 strong acid.

The acid dissociation constant ($K_a$) quantifies the strength of an acid. pK is negative log base 10 of $K_a$.

Trifluoromethanesulfonic acid pk <-13

Perchloric acid pK = <-9

HI pK = -9.3

HBr pK = -9

HCl pK = -6

sulfuric acid pK = -3

chloric acid pK = -1

nitric acid pK = -1

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.