Question

What is a conjugate acid and base pair? Explain with examples.

Answer 1

Hint : In order to explain a conjugate acid and base pair, we should have the knowledge of Bronsted-Lowry acid base theory. Conjugate base is formed by donating a proton from acid while conjugate acid is formed by accepting a proton from base. Hence conjugate acid –base pairs differ only in hydrogen ions $({{H}^{+}})$ .

Complete Step By Step Answer:
According to the Bronsted-Lowry acid base theory concept, acids are proton donors whereas bases are proton acceptors. In order to explain a conjugate acid and base pair. Let us consider the following reaction:
$CH_3COOH+H_2O\to CH_3CO{{O}^{-}}+H_3{{O}^{+}}$
In the above reaction, we can see that acetic acid $(CH_3COOH)$ donates a proton and obtains acetate ion $(CH_3CO{{O}^{-}})$ as a product. The product which is obtained from acid by donating a proton is called conjugate base. Hence, acetate ion $(CH_3CO{{O}^{-}})$ is the conjugate base of acetic acid. Similarly, water $(H_2O)$ accepts a proton and thus behaves like a base and obtains hydronium ion $(H_3{{O}^{+}})$ as a product. The product which is obtained from base by accepting a proton is called conjugate acid. Hence, hydronium ion $(H_3{{O}^{+}})$ is the conjugate acid of water (base).
Therefore, water $(H_2O)$ and hydronium ions $(H_3{{O}^{+}})$ are examples of conjugate acid-base pairs.

Additional Information:
Acetic acid is a weak acid i.e. partially dissociates in water to give hydrogen ions. On the other hand, water is amphoteric i.e. acts as both acid and base.

Note :
It is important to note that Bronsted-Lowry acid base theory describes the conjugate acid-base pair concept. According to this theory, conjugate base is formed by donating a proton from acid while conjugate acid is formed by accepting a proton from base. For instance, water $(H_2O)$ and hydronium ions $(H_3{{O}^{+}})$ are examples of conjugate acid-base pairs.