# Why is HCl a Bronsted-Lowry acid?

Apr 7, 2016

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,

${H}_{2} O + H C l \to {H}_{3} {O}^{+} + C {l}^{-}$

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

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

$H B r + H C l \to B {r}^{-} + {H}_{2} C {l}^{+}$

then hydrochloric acid is accepting a proton, meaning it is acting like a base. This is only plausible because $\text{HBr}$ is a stronger acid.

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