According to the Brønsted-Lowry definition of acids and bases, what is a conjugate acid-base pair? Provide an example.
Solution: Here, we are going to discuss about conjugate acid-base pair.
According to Bronsted-Lowry theory, acid is a substance that is capable of donating a hydrogen ion, H+ and bases are substances capable of accepting a hydrogen ion, H+. In short, acids are proton donors and bases are proton acceptors.
Let us consider the example of dissolution of NH3 in H2O represented by the following equation.
In this reaction, water molecule acts as proton donor and ammonia molecule acts as proton acceptor and are thus, called Bronsted-Lowry acid and base, respectively. In the reverse reaction, H+ is transferred from NH4+ to OH-. In this case, NH4+ acts as a Bronsted acid while OH- acted as a Bronsted base. Such pairs of acid and base that differs only by one proton is called a conjugate acid-base pair. Therefore, OH- is called the conjugate base of an acid H2O and NH4+ is called conjugate acid of the base NH3.
If Bronsted acid is a strong acid, then its conjugate base is a weak base and vice-versa. It may be noted that conjugate acid has one extra proton and each conjugate base has one less proton.