Question

Why is `HCl` a Bronsted-Lowry acid?

Answer 1

Because it gives away protons readily.

Explanation:

Bronsted-Lowry theory states that an acid is a molecule that will drop off `H^+` ions, and a base is a molecule that will pick them up again.

For example,

`H_2O + HCl -> H_3O^+ + Cl^-`

In this situation, water is gaining a proton (`H^+` ion), so it is a base, while `HCl` is giving one away, so it is an acid, according to Bronsted-Lowry theory. This is because `"HCl"` is a stronger acid than `"H"_3"O"^(+)`.

However, in rare cases, it shouldn't be ruled out that `HCl` can be amphoteric, meaning it can act as an acid or a base. For example, in the reaction

`HBr + HCl -> Br^(-) + H_2Cl^+`

then hydrochloric acid is accepting a proton, meaning it is acting like a base. This is only plausible because `"HBr"` is a stronger acid.

But it depends on the `"pKa"` of `"H"_2"Cl"^(+)` relative to `"HBr"`. If the `"pKa"` of `"HBr"` is higher, then the reaction wouldn't go to completion, as `"Br"^(-)` would then want to grab a proton from `"H"_2"Cl"^(+)`.