5
$\begingroup$

Is water an Arrhenius acid? We define Arrhenius acidity with respect to water, but what will water itself be? It can donate $\ce{H+}$ as well as $\ce{OH-}$.

$\endgroup$
12
$\begingroup$

We define Arrhenius acidity with respect to water

That definition is new to me, whatever it is. The definition I know is:

An Arrhenius acid is a substance that dissociates in water to form $\ce{H+}$ ions.

An Arrhenius base is a substance that dissociates in water to form $\ce{OH-}$ ions.

The key concept to answer the question is the self-ionisation of water given in equation $(1)$.

$$\ce{H2O <<=> H+ + OH-}\tag{1}$$

This ionisation happens in water, and the ion product as written in equation $(2)$ is $10^{-14}$.

$$K_\mathrm{w} = [\ce{H+}] [\ce{OH-}] = 10^{-14}\tag{2}$$

Thus, water qualifies as a substance that dissociates in water to form $\ce{H+}$ ions. It also qualifies as a substance that dissociates in water to form $\ce{OH-}$ ions. It is both an Arrhenius acid and an Arrhenius base and thus the only Arrhenius amphoteric compound.

$\endgroup$
  • 1
    $\begingroup$ Aren't there other molecules which can donate both $OH^-$ and $H^+$? (I can imagine some organic ones, though I have no examples at hand.) Those would also be Arrhenius amphoteric, right? $\endgroup$ – Paŭlo Ebermann Oct 3 '16 at 21:16
  • 1
    $\begingroup$ @PaŭloEbermann I’m pretty sure there aren’t. Frankly, I don’t know a single organic molecule that dissociates to give $\ce{OH-}$. (Side note: Use mhchem for chemical expressions; it prevents italics and inserts better spacing.) $\endgroup$ – Jan Oct 3 '16 at 21:31
  • $\begingroup$ What about this? $\endgroup$ – DHMO Oct 3 '16 at 22:39
  • $\begingroup$ @DHMO What about that? $\endgroup$ – Jan Oct 3 '16 at 22:40
  • 1
    $\begingroup$ @DHMO It claims that the pH value of water will be neither acidic nor basic. The post itself clearly says that water is both an Arrhenius acid and an Arrhenius base. $\endgroup$ – Jan Oct 3 '16 at 22:43
-2
$\begingroup$

water isn't the only arrhenius amphoteric compound. If I am remember correctly Aluminium can also react with water to form both an acid and a base.

$\endgroup$
  • 1
    $\begingroup$ Can you provide some examples of acids it creates? Not lewis ones, ofcourse. $\endgroup$ – Sanika Khadkikar Oct 17 '16 at 3:17
  • 2
    $\begingroup$ Aluminium cannot be an Arrhenius acid. There is no hydrogen that it could dissociate away. $\endgroup$ – Jan Oct 18 '16 at 0:13

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.