Question

What is the strongest base that can exist in liquid ammonia as a solvent?

Short answer

Answer: The strongest base that can exist in liquid ammonia is the amide ion (NH2-).

Step-by-step solution

Understanding acidity and basicity in liquid ammonia

In water, acidity and basicity are defined based on the ionization of water: H2O ⇌ H+ + OH- However, in liquid ammonia, these concepts are defined based on the ionization of ammonia: 2NH3 ⇌ NH4+ + NH2- Ammonia acts as a Brønsted-Lowry base when it donates a lone pair of electrons to a proton forming the ammonium ion (NH4+). Ammonia also acts as a Brønsted-Lowry acid by accepting a pair of electrons from NH2- to form another NH3 molecule. Thus, the acidity and basicity in liquid ammonia are defined based on the ionization of ammonia into ammonium ions and amide ions.

Determine the ionization constant (Kb) for liquid ammonia

We need to know the ionization constant of ammonia in liquid ammonia to determine the strongest base that can exist. In the case of liquid ammonia, the ionization constant is called the "proton-transfer equilibrium constant" and denoted as Kb. As given in the problem: Kb = [NH4+] [NH2-] / [NH3]^2 The value of Kb is approximately 1.0 × 10^-33.

Determine the strongest base that can exist in liquid ammonia

The strongest base is the one that is least likely to give up its extra electron pair. In the case of liquid ammonia, this means the species having the minimum value of Kb. Since Kb is the equilibrium constant involving NH2-, the strongest base that can exist in liquid ammonia is the amide ion (NH2-). An important point to note is that the amide ion (NH2-) will exist when the ionization of ammonia is in equilibrium with the formation of ammonium ions and amide ions. This is because the species having the minimum value of Kb will be the one that is most likely to accept a pair of electrons from another species. In the case of liquid ammonia, this is the amide ion (NH2-).