Question

In the equation $HCl(g) + {H_2}O(l) \to {H_3}{O^ + }(aq) + C{l^ - }(aq),$ which species is a Bronsted-Lowry acid?

Answer 1

Bronsted Lowry's acid-base theory further adopts the Arrhenius definition of acid and base, because substances no longer need to be composed of hydrogen ions $({H^ + })$ or hydroxide $(O{H^ - })$ to be classified as acids or bases. Acid is a substance that can release protons (as in Arrhenius theory) and base is a substance that can accept protons. On the basis of this definition we will identify the Bronsted-Lowry acid in the given chemical equation.

Complete Step By Step Answer:
We know that according to the Bronsted-Lowry theory, an acid is a species which donates protons $({H^ + })$ and a base is a species which accepts protons. So we can say that the substance with one less proton on the product side than the proton on the reactant side will be known as Bronsted-Lowry acid. Now, the given chemical equation is:
$HCl(g) + {H_2}O(l) \to {H_3}{O^ + }(aq) + C{l^ - }(aq)$
In the above equation we can see that HCl is donating its proton and that proton is accepted by ${H_2}O$ to form hydronium ions $({H_3}{O^ + })$ .
The remaining chloride anion Cl− is called an acid conjugate base, because theoretically it can form an acid by accepting protons.
Therefore, HCl acts as a Bronsted-Lowry acid in the above chemical equation.

Note:
There is one more type of theory regarding acid and base which is known as Lewis acid and base theory. It defines an acid as an electron donor species while base as an electron acceptor species. Therefore, we should not get confused between Bronsted-Lowry acid-base concept and Lewis acid-base concept.