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

1 Answer
Jul 27, 2016

Answer:

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?