Question

What are the conjugate acid and conjugate base respectively for the following reaction?
${{\text{H}}_2}{\text{O}} + {{\text{H}}_2}{\text{O }} \to {\text{ }}{{\text{H}}_3}{{\text{O}}^ + } + {\text{O}}{{\text{H}}^ - }$
(A) ${{\text{H}}_3}{{\text{O}}^ + }{\text{,O}}{{\text{H}}^ - }$
(B) ${\text{O}}{{\text{H}}^ - }{\text{,}}{{\text{H}}_3}{{\text{O}}^ + }$
(C) Both A and B
(D) None of the above

Answer 1

According to Bronsted Lowry acid base theory, acid is a proton donor and base is a proton acceptor. When an acid loses a proton, a conjugate base is obtained. When a base accepts a proton, a conjugate acid is obtained.

Complete step by step answer:
When an acid ionizes, it gives a proton. Consider ${\text{HA}}$ as an acid. Write a balanced chemical equation for the ionization of the acid ${\text{HA}}$ .
${\text{HA }} \to {\text{ }}{{\text{H}}^ + }{\text{ + }}{{\text{A}}^ - }$
Thus, the ionization of ${\text{HA}}$ gives ${{\text{H}}^ + }$ (proton) and ${{\text{A}}^ - }$. Here, ${{\text{A}}^ - }$ is the conjugate base of the acid ${\text{HA}}$. An example of Bronsted Lowry acid is hydrogen chloride.
It can be represented as ${\text{HCl}}$. Write a balanced chemical equation for the ionization of the hydrogen chloride.
${\text{HCl }} \to {\text{ }}{{\text{H}}^ + }{\text{ + C}}{{\text{l}}^ - }$
Thus, the ionization of ${\text{HCl}}$ gives ${{\text{H}}^ + }$ (proton) and ${\text{C}}{{\text{l}}^ - }$ . Here, ${\text{C}}{{\text{l}}^ - }$ is the conjugate base of the acid ${\text{HCl}}$ .
Now consider the reaction for the autoionization of water.
${{\text{H}}_2}{\text{O}} + {{\text{H}}_2}{\text{O }} \to {\text{ }}{{\text{H}}_3}{{\text{O}}^ + } + {\text{O}}{{\text{H}}^ - }$
In this reaction, two water molecules undergo a disproportionation reaction to form one hydronium ion and one hydroxide ion.
A hydroxide ion is obtained when a water molecule loses a proton. Thus, water molecules act as a Bronsted lowery acid and hydroxide ion acts as its conjugate base.
${{\text{H}}_2}{\text{O }}\left( {{\text{acid}}} \right){\text{ }} \to {\text{ }}{{\text{H}}^ + } + {\text{O}}{{\text{H}}^ - }\left( {{\text{conjugate base}}} \right)$
A hydronium ion is obtained when a water molecule accepts a proton.
Here, the water molecule acts as a base and hydronium ion is its conjugate base.
${{\text{H}}_2}{\text{O }}\left( {{\text{base}}} \right){\text{ + }}{{\text{H}}^ + } \to {\text{ }}{{\text{H}}_3}{{\text{O}}^ + }\left( {{\text{conjugate acid}}} \right)$
Write the reaction of autoionization of water, also write conjugate acid and conjugate base.
${{\text{H}}_2}{\text{O}}\left( {{\text{acid}}} \right) + {{\text{H}}_2}{\text{O}}\left( {{\text{base}}} \right){\text{ }} \to {\text{ }}{{\text{H}}_3}{{\text{O}}^ + }\left( {{\text{conjugate acid}}} \right) + {\text{O}}{{\text{H}}^ - }\left( {{\text{conjugate base}}} \right)$
Hence, the conjugate acid and conjugate base are ${{\text{H}}_3}{{\text{O}}^ + }{\text{,O}}{{\text{H}}^ - }$ respectively.

Thus, the correct answer is the option (A).

Note: Bronsted Lowry acid base theory is similar to Arrhenius concept of acid and base. The Arrhenius concept is limited to aqueous solutions. However, Bronsted Lowry theory can also be applied to nonaqueous solutions. In Arrhenius theory, a base gives a hydroxide ion. In Bronsted Lowry theory, a base accepts a proton.