What do we mean by #H^+# or #H_3O^+# in acid-base chemistry?

1 Answer
May 6, 2017

Answer:

Note that the #H^+# is a CONCEPTION, and not the reality.........

Explanation:

Careful experiment established the following equilibrium in aqueous solution:

#H_2O rightleftharpoons H^(+) + HO^-#

Which has an alternative representation:

#2H_2O rightleftharpoons H_3O^(+) + HO^-#

#K_w=[H_3O^+][HO^-]=10^(-14)# under standard conditions.......

So what is the #"acidium ion"#? As far as anyone knows it is a cluster of 3 or 4 or so water molecules with an extra #H^+#, to give, say...........

#H_7O_3^+# or #H_9O_4^+#; the extra #H^+# can tunnel between clusters of water molecules VERY rapdidly - think of a maul when you play rugby.

When acids are added to water solvent, we conceive that the acid protonates the solvent to give hydronium ions:

#HX(aq) + H_2O(l) rightleftharpoonsH_3O^+ + X^-#

The #H_3O^+# ion, the #"hydronium ion"# is thus a conceptual species, but this conception allows us to solve quantitative problems in stoichiometry. We write #H^+# or #H_3O^+# as a shorthand. The completion of the equilibrium, i.e. how far to OUR right as we read the page, is a measure of the strength of the acid. For strong acids, #HClO_4#, #H_2SO_4#, #HX# #(X!=F)#, the equilibrium lies almost entirely to the right.