What are H^+ and H_3O^+ in water in the context of acidity?

Feb 18, 2016

${H}^{+}$ is a representation; in water we don't have too much idea what it is. It is indeed a reactive species. And it represents the acidium species in water, just as $H {O}^{-}$ represents the basic principle.

Explanation:

For water we could write the autoprotolysis as:

${H}_{2} O r i g h t \le f t h a r p \infty n s {H}^{+} + H {O}^{-}$

Alternatively we could write:

$2 {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + H {O}^{-}$.

Both protium ion, ${H}^{+}$, and hydronium ion, ${H}_{3} {O}^{+}$, are REPRESENTATIONS of what we conceive to be the acidium ion in water, the CHARACTERISTIC cation of the solvent (water). So what is the actual species? It is likely ${H}_{7} {O}_{3}^{+}$ or similar, that is a CLUSTER of 3 or 4 water molecules, WITH AN EXTRA PROTON, ${H}^{+}$. And likewise, the base is one or more water molecules LESS a PROTON.

This acid cluster may be very short-lived, with the proton being shunted to another cluster of water molecules (if you have ever played rugby and I assume you have, think of a rolling maul).

This acidium cation is in fact a reactive species; with an electron source (a metal) it can be reduced to form dihydrogen gas:

${H}^{+} + {e}^{-} \rightarrow \frac{1}{2} {H}_{2} \left(g\right) \uparrow$