# Which is a stronger acid: HCl or HF?

Jul 27, 2016

In aqueous solution hydrochloric acid is unquestionably the STRONGER acid.

#### Explanation:

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

Two factors are important here: (i) the strength of the $H - X$ bond; and (ii) charge density of the anion, ${X}^{-}$. The $H - X$ bond is definitely stronger for $H - F$; the ${F}^{-}$ ion is smaller and more polarizing than $C {l}^{-}$, and more likely to cause solvent order.

Both enthalpy and entropy thus conspire to reduce the acidity of $H - F$ with respect to the lower hydrogen halides.

Note that we can illustrate this effect practically. $N a C l$ salts give a neutral solution; $N a F$ salts give a slightly BASIC aqueous solution. Is this consistent with the acidities?