Liquid ammonia, NH 3 (l), is an ionizing solvent like water. This means that NH

Question

Liquid ammonia, NH3(l), is an ionizing solvent like water. This means that NH3(l) can dissolve many different ionic compounds and can undergo 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).

(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 mass-action expression for the autoionization constant of liquid ammonia.

(d) At -50°C, the autoionization constant of ammonia is 10-30. What are the conditions for “neutral”, “acidic”, and “basic” liquid ammonia solutions at -50°C?

(e) Identify two different compounds so that, when each is dissolved in liquid ammonia, one gives rise to an "acidic" solution and the other to a "basic" solution. Write out the chemical equilibrium that shows how one behaves as an acid, and the other behaves as a base in NH3(l).

Explanation / Answer

a). Autoionization of NH3 (l)

It is the self ionization reaction of ammonia

NH3 + NH3 --- > NH4+ + NH2-

This is called as auto ionization of ammonia.

b).

Bronsted acid :

NH+ (aq) NH3

Bronsted base :

NH3 , NH2

c)

Mass action expression for autoioniztion of ammonia is analogous to water.

Keq = [NH4+ ][ NH2-]

Since concentration of NH3 is almost constant so

Keq = [NH4+][NH2-]

d)

At -50 deg C the keq value for ammonia is very low that means

NH4+ and NH2- product is almost 0 and so there is only liquid NH3 which is basic.

e)

HCl is dissolved in NH3 making acidic solution

HCl + NH3 --- > NH4+ + Cl-

NaOH makes basic

NaOH + NH3 -- > NaNH2 + H2O

Solution is basic.

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