What is the entity of an acid? (A formula unit?)

For instance, is the entity of #HCl# a formula unit, if we assume that #H# acts as a metal?

1 Answer
Feb 17, 2017

Answer:

Well, certainly we can add #HCl(g)# to an aqueous solution......

Explanation:

In water, we conceive that an acid is a species that increases the concentration of the characteristic cation of the solvent, the hydronium ion, #H_3O^+#:

#HCl(g) + H_2O(l) rarr H_3O^(+) + Cl^(-)#

In water, hydrolysis of the #H-Cl# molecules occurs to give hydronium ions and halide ions. We represent the hydronium ion as #H_3O^+# or even as #H^+#. As far as anyone knows, this hydronium ion is a cluster of #"2-3 water molecules"# with an extra proton associated with the cluster, i.e. to give #H_7O_3^+# or #H_5O_2^+#. The extra proton is highly mobile, and can tunnel rapidly across the solution. If you have ever played rugby, think of a maul where the forwards cluster together and pass the pill from hand to hand. Of course I am comparing a macro scenario to quantum tunnelling.

Now in solution, we can represent the #"acidium ion"# as #H^+# or #H_3O^+#. This is a convenient fiction that works..........and we can speak of and calculate moles of #H_3O^+#.

Capisce?