Question

Liquid (anhydrous) ammonia, NH3(l), with a boiling point of –33 °C, is often used as an alternate solvent to water. It forms strong hydrogen bonds and the polar molecules can solvate ionic compounds. Anhydrous ammonia also undergoes autoionization. Therefore, there can be acidic, neutral, or basic species in solutions of liquid ammonia.

a) Write the chemical equation, using the Brønsted‐Lowry perspective, that describes the autoionization of NH3(l). (Hints: for water this equation is 2H2O(l) ⇌ H3O+(aq) + OH–(aq); X(am) denotes a solute solvated by ammonia)

b) Identify the species in pure NH3(l) that can act as Brønsted acids and as Brønsted bases. (There are more than one for each case.)

c) Write the equilibrium expression for the autoionization constant Kam of liquid ammonia.

d) At 50°C, the autoionization constant of ammonia Kam is 1030. What are the conditions for “neutral”,

“acidic”, and “basic” liquid ammonia solutions at 50°C?

e) Propose a quantity, analog to the pH, that indicates the acidity of a liquid ammonia solution.

f) For each of these compounds: NH4Cl, LiNH2, KOH, N(CH3)3, when dissolved in liquid ammonia, identify whether it gives rise to an "acidic" solution or a "basic" solution. Write out the chemical equilibrium that shows how it behaves as an acid or a base in NH3(l).

Answer 1

a) Write the chemical equation, using the Brønsted‐Lowry perspective, that describes the autoionization of NH3(l). (Hints: for water this equation is 2H2O(l) ⇌ H3O+(aq) + OH–(aq); X(am) denotes a solute solvated by ammonia)

NH3(g) + H2O <--> NH4+ + OH-

b) Identify the species in pure NH3(l) that can act as Brønsted acids and as Brønsted bases. (There are more than one for each case.)

NH3 + H+ --> NH4+ this is a base, since it accepts H+ ions

NH3 --> NH2- + H+ --> this must be an acid, since it is donating H+

c) Write the equilibrium expression for the autoionization constant Kam of liquid ammonia.

Ka = [NH2-][NH4+]/[NH3]

d) At 50°C, the autoionization constant of ammonia Kam is 1030. What are the conditions for “neutral”,

Ka = 10^30

for "neutral" balance ... [NH2-] = 10^15 = [NH4+] = 10^15

for "acidic" balance ... [NH2-] < 10^15 nad [NH4+] > 10^15

for "basic" balance ... [NH2-] > 10^15 nad [NH4+] < 10^15

e) Propose a quantity, analog to the pH, that indicates the acidity of a liquid ammonia solution.

pNH = 15 --> neutral