Question

Which is a stronger acid: `HCl` or `HF`?

Answer 1

In aqueous solution hydrochloric acid is unquestionably the STRONGER acid.

Explanation:

`HX + H_2O rightleftharpoons H_3O^+ + 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 `Cl^-`, 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. `NaCl` salts give a neutral solution; `NaF` salts give a slightly BASIC aqueous solution. Is this consistent with the acidities?