Question

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

Answer 1

Yes.

Explanation:

`HCl` 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
`HCl rarr H_((aq))^+ + Cl_((aq))^-`
`HF harr H_((aq))^+ + F_((aq))^-`

This is partly because `Cl^-` 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 `HBr`, `HI`, `HNO_3` and the first proton of `H_2SO_4`.

Answer 2

In aqueous solution, `HCl` is classified as a very strong acid, undergoing almost complete ionization. On the other hand `HF`, is classified as a weak acid.

Explanation:

For the reaction,

`HX rightleftharpoons H^+ + X^-`, where `X` = `Cl`, 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 `pK_a` of HCl `~~ -7`; whereas `pK_a` of HF = `3.14`.