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There are amphiprotic (or amphoteric) substances, like $\ce{H2O}$ and $\ce {HCO3-}$, which act both as an acid and base.

If you mix a strong acid like $\ce{HCl}$ ($\mathrm{p}K_{\mathrm{a}} = -6.3$) with an even stronger acid, such as perchloric acid ($\ce{HClO4},~\mathrm{p}K_{\mathrm{a}} \approx -10 $) can it act as a base?

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  • $\begingroup$ I'm totally not sure what you mean by "If you mix a strong acid like HCl with an extremely strong acid, HCl, can it act as a base like them?". $\ce{HCl}$ is not an amphoteric acid. $\endgroup$ Commented Mar 29, 2018 at 9:50
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    $\begingroup$ That said, it will be protonated, if you push really hard. $\endgroup$ Commented Mar 29, 2018 at 13:14
  • $\begingroup$ @GaurangTandon yes, HCl is amphoteric: aanda.org/articles/aa/ps/2010/13/aa14959-10/aa14959-10.ps.gz $\endgroup$
    – DavePhD
    Commented Mar 29, 2018 at 20:42
  • $\begingroup$ @DavePhD Oh! I didn't know that. Well, that paper seems to be using some pretty extreme conditions though, to forcibly protonate $\ce{HCl}$. It doesn't seem to be as "amphoteric" as the regular bicarbonate anion... $\endgroup$ Commented Mar 30, 2018 at 1:43
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    $\begingroup$ @GaurangTandon That paper is reporting what occurs naturally in outer space. This table lists 12 common substances that are weaker bases than HCl. en.wikipedia.org/wiki/Proton_affinity_(data_page) $\endgroup$
    – DavePhD
    Commented Mar 30, 2018 at 11:57

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Well an acid can act as a base when there is a stronger acid present, But if you look at the molecular level acidity depends on how well the negative charge is balanced. so out of the two molecules present the one which can disperse(stabilize) the negative charge to a greater extent will act as acid.

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Yes.

For example, $\ce{H2SO4}$ can be protonated to $\ce{H3SO4+}$

See THE SULPHURIC ACID SOLVENT SYSTEM Canadian Journal of Chemistry (1960), vol. 38, pages 1363-1370.

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Chemists see this happen all the time when they use a mixture of nitric and sulfuric acids for nitration. The nitric acid acts as a Bronsted-Lowry base, but forms the powerful Lewis acid $\ce{NO_2^+}$, the nitryl ion, which then acts as the nitrating agent:

$\ce{NO_2(OH) + 2 H_2SO_4 -> NO_2^+ + H_3O^+ + 2 HSO_4^-}$

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