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

May 6, 2017

Note that the ${H}^{+}$ is a CONCEPTION, and not the reality.........

#### Explanation:

Careful experiment established the following equilibrium in aqueous solution:

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

Which has an alternative representation:

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

${K}_{w} = \left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right] = {10}^{- 14}$ under standard conditions.......

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

${H}_{7} {O}_{3}^{+}$ or ${H}_{9} {O}_{4}^{+}$; 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:

$H X \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {X}^{-}$

The ${H}_{3} {O}^{+}$ ion, the $\text{hydronium ion}$ is thus a conceptual species, but this conception allows us to solve quantitative problems in stoichiometry. We write ${H}^{+}$ or ${H}_{3} {O}^{+}$ 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, $H C l {O}_{4}$, ${H}_{2} S {O}_{4}$, $H X$ $\left(X \ne F\right)$, the equilibrium lies almost entirely to the right.