Question

What change will be caused by addition of a small amount of HCl to a solution containing fluoride ions and hydrogen fluoride?

Answer 1

We interrogate the equilibrium:

`HF(aq)+ H_2O(l) rightleftharpoons F^(-) + H_3O^+`

Explanation:

For `HF`, `K_a=10^(-3.17)`

`HF` is a weak hydrohalic acid given (i) the relative strength of the `H-F` bond, and (ii) more importantly, the large, entropically disfavoured charge density of the anion.

Addition of hydrogen chloride will drive the equilibrium to the left.

Note that fluoride salts of alkali metals give SLIGHTLY basic aqueous solutions, due to the equilibrium:

`F^(-) +H_2O(l) rightleftharpoonsHF(aq) + HO^-`