Question

Identify the conjugate acids and bases in the following equation:

(A) $\left( {{H_2}O,O{H^ - }} \right)$ and $\left( {NH_2^ + ,N{H_3}} \right)$
(B) $\left( {{H_2}O,O{H^ - }} \right)$ and $\left( {NH_4^ + ,NH_2^{2 - }} \right)$
(C) $\left( {{H_2}O,OH} \right)$ and $\left( {N{H_3},N{H_4}} \right)$
(D) $\left( {{H_2}O,O{H^ - }} \right)$ and $\left( {NH_4^ + ,N{H_3}} \right)$

Answer 1

Hint:All acids have a conjugate base and all bases have a conjugate acid. The substance formed after the removal of proton from an acid is called conjugate base and the substance or species formed after a base accepts a proton, is known as conjugate acid.

Complete step by step answer:
First of all let us understand the Bronsted-Lowry concept of acid and bases. This concept was put forward by Bronsted (a Danish Chemist) and Lowry (an English Chemist) independently and simultaneously in $1723.$ According to this concept, an acid is defined as a substance which has the tendency to give a proton $\left( {{H^ + }} \right)$ and a base is defined as a substance which has a tendency to accept a proton. In other words, an acid is a proton donor whereas a base is a proton acceptor. There are some results derived from this concept. Not only molecules but even the ions can act as acid or bases. According to this concept, water can act as both acid as well as base and hence is called amphoteric. The presence of hydroxyl $\left( {O{H^ - }} \right)$ groups are not essential for a substance to act as a base. According to this theory, when acid bases, ${H^ + }$ion, it leads to the formation of charged particles known as conjugate base and base accepts a proton to give a substance known as conjugate acid. There are two acid-base pairs in a reaction. These acid-base pairs are called conjugate acid-base pairs.
A conjugate pair of acid and base differ by a proton only.
Now according to given equation, we have
$N{H_3} + {H_2}O \to N{H_4}^ + + O{H^ - }$
We can clearly see that ammonia accepts a proton from water to give $N{H_4}^ + $. Water has lost a proton to give $O{H^ - }$. Since ammonia accepts a proton, so according to the Bronsted-Lowry concept, it will act as a base and the formed species $N{H_4}^ + $ will be conjugate acid. Similarly water loses a proton, so it will act as an acid and the formed conjugated species $O{H^ - }$ will be conjugate base.
Hence conjugate acid-base pair are $\left[ {N{H_3},N{H_4}^ + } \right];\left[ {{H_2}O;O{H^ - }} \right].$
So option (D) is the correct answer.

Note:
Always remember that the conjugate base of a strong acid will be weak, and the conjugate base of a weak acid will be strong. Similarly, the conjugate acid of a strong base will be weak, and the conjugate acid of a weak base will be strong.