Question

In the equation `HCl(g) + H_2O(l) -> H_3O^+(aq) + Cl^(-)(aq)`, which species is a Bronsted-Lowry acid?

Answer 1

Hydrochloric acid, `"HCl"`.

Explanation:

As you know, a Bronsted - Lowry acid is a proton donor, which means that you can identify a Bronsted - Lowry acid in a reaction by looking at which chemical species lost a proton.

A proton is simply a hydrogen ion, `"H"^(+)`, so the species that has one less proton on the products' side than it does on the ractants' side will be the Bronsted - Lowry acid.

In your case, you have

`color(red)("H")"Cl"_text((aq]) + "H"_2"O"_text((l]) -> "H"_3"O"_text((aq])^(color(red)(+)) + "Cl"_text((aq])^(-)`

Notice that hydrochloric acid, `"HCl"`, starts with a proton that ends up on a water molecule, which acts as a Bronsted - Lowry base, and forms hydronium ions, `"H"_3"O"^(+)`.

The remaining chloride anion, `"Cl"^(-)`, is called the acid's conjugate base because it can theoretically reform the acid by aaccepting a proton.

Since hydrochloric acid donated a proton, it acted as a Bronsted - Lowry acid.