What is the acid principle in water? Is there such a thing as #H^+#?

1 Answer
Nov 14, 2016

Answer:

In aqueous solution, we REPRESENT the acidium ion as #H^+# or #H_3O^+#.

Explanation:

The acidium ion is the characteristic cation of the solvent; and for water we represent this species as #H^+# or #H_3O^+#. So what is it really?

As far we know, the acidium species is a cluster of 3 or 4 water molecules, with an EXTRA proton attached to give #H_7O_3^+# or #H_9O_4^+#. The extra proton can be readily exchanged between water clusters, so that it could rapidly tunnel across the solution. If you have ever played rugby, think of a maul where the forwards exchange the pill to the back of the maul and keep driving.

Nowadays, it is common to represent the acidium ion in water as #H_3O^+#, the #"hydronium ion"#. It is more of a representation than an actual species.

Other solvent systems exist. In liquid ammonia, the characteristic cation, the acid, is #NH_4^+#, and the characteristic anion is amide ion, #NH_2^-#. In a more acidic medium, say hydrogen fluoride, these become #H_2F^+# and #F^-#.