# Is "HCl" a strong acid? What about "HF"?

Sep 18, 2015

Yes.

#### Explanation:

$H C l$ is a strong acid in water as well as in it's natural gaseous form, which means in a solution every molecule is ionized, whereas HF is a mild acid so in a solution there's some of it ionized and some it as HF.

The equations
$H C l \rightarrow {H}_{\left(a q\right)}^{+} + C {l}_{\left(a q\right)}^{-}$
$H F \leftrightarrow {H}_{\left(a q\right)}^{+} + {F}_{\left(a q\right)}^{-}$

This is partly because $C {l}^{-}$ is a very stable ion - it has a complete octet for instance - , so when it breaks away from the molecule it can stay that way.

While the same could apply to ${F}^{-}$ for the same reasons, fluorine is very electronegative and as such the bond fluorine makes with the proton has a tendency to be restored.

In water, the only other strong acids are $H B r$, $H I$, $H N {O}_{3}$ and the first proton of ${H}_{2} S {O}_{4}$.

Sep 18, 2015

In aqueous solution, $H C l$ is classified as a very strong acid, undergoing almost complete ionization. On the other hand $H F$, is classified as a weak acid.

#### Explanation:

For the reaction,

$H X r i g h t \le f t h a r p \infty n s {H}^{+} + {X}^{-}$, where $X$ = $C l$, the equilibrium lies strongly to the right. For $X$ = $F$, the equilibrium lies to the left. Hydrochloric acid, therefore, is by definition a strong acid.

As physical scientists, however, we should seek quantitative measures of acidity. The $p {K}_{a}$ of HCl $\approx - 7$; whereas $p {K}_{a}$ of HF = $3.14$.