Question

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

Answer 1

`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 `HO^-` represents the basic principle.

Explanation:

For water we could write the autoprotolysis as:

`H_2O rightleftharpoons H^+ + HO^-`

Alternatively we could write:

`2H_2O rightleftharpoons H_3O^+ + HO^-`.

Both protium ion, `H^+`, and hydronium ion, `H_3O^+`, 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_7O_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^(-) rarr 1/2H_2(g)uarr`