# How do we identify a strong acid in aqueous solution?

May 18, 2017

#### Answer:

As you are no doubt aware, this distinction is usually used in the context of $\text{acid/base chemistry}$.

#### Explanation:

A strong acid is completely ionized in aqueous solution; viz. for the hydrohalic acids, $H X \left(X \ne F\right) :$

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

The strength of an acid depends upon the completion of this equilibrium; i.e. how far the equilibrium to the right. $H I$, $H B r$, and $H C l$ ARE STRONG ACIDS by this criterion, because dissociation/ionization are almost complete.

On the other hand, weaker acids, incompletely ionize:

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

This equilibrium lies to the left as we face it due to (i) the intrinsic strength of the $H - F$; and (ii) to the unfavourable entropy effect of the solvated ${F}^{-}$ conjugate base.

For ${H}_{2} S {O}_{4}$ BOTH conjugate bases are stabilized, and this acts as a diacid in water:

${H}_{2} S {O}_{4} \left(a q\right) + {H}_{2} O \left(l\right) \rightarrow H S {O}_{4}^{-} + {H}_{3} {O}^{+}$

$H S {O}_{4}^{-} + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s S {O}_{4}^{2 -} + {H}_{3} {O}^{+}$