# How do we assess whether an acid is strong in aqueous solution?

Sep 29, 2016

Acidity is an experimental phenomenon, and strong acids behave as strong acids do in water.

#### Explanation:

In general when hydrogen is bound to a strongly electronegative atom, then that hydrogen is acidic. However, this criterion fails when we consider the acidity of say $H F$, a weak acid, with a very electronegative conjugate base, ${F}^{-}$.

As with every other reaction, we look at the reaction itself:

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

The extent of reaction depends on (i) the strength of $H - A$ bond, and (ii) the solvation of the ${A}^{-}$ conjugate base. (This is an enthalpy effect and an entropy effect,

i.e. $\Delta {G}^{\circ} = \Delta {H}^{\circ} - T \Delta {S}^{\circ}$

Anions for which the negative charge is delocalized, i.e. $N {O}_{3}^{-}$, or $S {O}_{4}^{2 -}$ tend to have strong conjugate acids. This is in contrast to $H F$, whose conjugate base, ${F}^{-}$ is strongly polarizing and tends to induce solvent order. $H F$ is thus a weaker acid than $H C l$ or $H B r$ on the basis of this entropy effect.

So, you just have to learn the common strong acids.