Question

Water acts as a Bronsted acid as well as a Bronsted base. Give one example each to illustrate this statement.

Answer 1

Hint: According to Bronsted-Lowry theory, acid is a substance which donates an \[{{\text{H}}^ + }\] ion or a proton and forms its conjugate base and the base is a substance which accepts an \[{{\text{H}}^ + }\] ion or a proton and forms its conjugate acid.

Complete step by step answer:
 Bronsted-Lowry acid is a substance that donates a proton to the other compound and forms its conjugate base.
.\[\text{Acid }\leftrightarrows\text{ Proton + Conjugate base}\].
Consecutively, if a conjugate base accepts a proton, corresponding acid will be formed.
Similarly, base is a substance which accepts a proton from another compound and forms a conjugate acid.
\[\text{Base + Proton }\leftrightarrows\text{ Conjugate acid}\]
Consecutively, a conjugate acid can donate a proton and corresponding base will be formed.
Bronsted acids undergo dissociation to yield protons and therefore increase the concentration of ions in the solution. On the other hand, Bronsted bases accept protons from water (the solvent) to yield hydroxide ions. So, the concentration decreases.
Water acts as an acid and as a base too i.e. water is amphoteric in nature. In a reaction between acetic acid and water, it acts as a base i.e. will accept a proton.
\[\text{C}{{\text{H}}_{3}}\text{COOH + }{{\text{H}}_{2}}\text{O }\leftrightarrows\text{ C}{{\text{H}}_{3}}\text{CO}{{\text{O}}^{-}}\text{ + }{{\text{H}}_{3}}{{\text{O}}^{+}}\]
Here, \[{\text{C}}{{\text{H}}_3}{\text{CO}}{{\text{O}}^ - }\] becomes the conjugate base.

In reaction with ammonia, water will act as a base i.e. will accept a proton.
\[{{\text{H}}_{2}}\text{O + N}{{\text{H}}_{3}}\leftrightarrows\text{ }\,\text{ O}{{\text{H}}^{-}}\text{ + NH}_{4}^{+}\]
In the reaction, water will donate a proton to ammonia. The hydroxide ion will be the conjugate base of water which is acting as an acid.
Hence, water acts as a Bronsted acid as well as a Bronsted base.

Note: An advantage of the Bronsted-Lowry definition of acids and bases is its ability to explain the acidic or basic nature of ionic species. But at the same time, this theory also fails to explain how compounds which lack hydrogen also show acidic properties.