Question

One of the following is a Bronsted acid but not a Bronsted base
A.${H_2}S$
B.${H_2}O$
C.$HC{O_3}^ - $
D.$N{H_3}$

Answer 1

Hint:In this question, we have to identify which is a Bronsted acid but not a base. A bronsted acid is an acid which releases ${H^ + }$ ions and a bronsted base is a base that accepts ${H^ + }$ ions. A conjugate base has one less hydrogen ion or proton than the acid we started with. A conjugate acid has one more hydrogen ion or proton than the base we started with.

Complete step by step answer:
Now we will discuss the given options one by one.
The first option is ${H_2}S$ (hydrogen sulphide).
If ${H_2}S$ (hydrogen sulphide) dissociates and it will release a proton. The one which releases proton $({H^ + })$ is a bronsted acid but if ${H_2}S$ (hydrogen sulphide) accepts a proton; it will not make many compounds. Therefore, ${H_2}S$ (hydrogen sulphide) acts as a bronsted acid but not a bronsted base.
${H_2}S \rightleftharpoons H{S^ - } + {H^ + }$
The second option is ${H_2}O$ (water). Water dissociates and releases a proton $({H^ + })$ to give ${H^ + }$ and $O{H^ - }$ acts as a bronsted acid. If water accepts a proton it will give ${H_3}{O^ + }$ and acts as a bronsted base.
${H_2}O + {H^ + } \to {H_3}{O^ + }$
${H_2}O \rightleftharpoons {H^ + } + O{H^ - }$
The third option is $HC{O_3}^ - $ (bicarbonate ion). Bicarbonate ion will also dissociate and releases a proton $({H^ + })$ to give carbonate ion $(C{O_3}^{2 - })$ and acts as a bronsted acid. If it accepts a proton it will give carbonic acid $({H_2}C{O_3})$ and acts as a bronsted base.
$HC{O_3}^{2 - } \rightleftharpoons C{O_3}^{2 - }$
$HC{O_3}^{2 - } + {H^ + } \to {H_2}C{O_3}$
The fourth option is $N{H_3}$ (ammonia). Ammonia also dissociates and releases proton $({H^ + })$ to give $N{H_2}^ - $ and acts as a bronsted acid. Ammonia also accepts a proton to give ammonium cation $(N{H_4}^ + )$ and acts as a bronsted base.
$N{H_3} + {H^ + } \to N{H_4}^ + $
$N{H_3} \rightleftharpoons N{H_2}^ - + {H^ + }$
After discussing it we can conclude that ${H_2}S$ (hydrogen sulphide) acts as a bronsted acid but not a bronsted base.
Hence, the correct option is (A).

Note:
We take an example for the explanation of the Bronsted-Lowry acid-base concept.
$HCl + N{H_3} \rightleftharpoons N{H_4} + C{l^ - }$
Hydrochloric acid is a bronsted- lowry acid because it donates a hydrogen ion or proton.
Ammonia is a bronsted- lowry base because it accepts the hydrogen ion or proton.
The Bronsted-Lowry theory comes with the concept of conjugate acid-base pair.