Question

Like water, ammonia undergoes autoionization in liquid ammonia: $$\mathrm{NH}_{3}+\mathrm{NH}_{3} \rightleftharpoons \mathrm{NH}_{4}^{+}+\mathrm{NH}_{2}^{-}$$ (a) Identify the Bronsted acids and Brønsted bases in this reaction. (b) What species correspond to \(\mathrm{H}^{+}\) and \(\mathrm{OH}^{-},\) and what is the condition for a neutral solution?

Short answer

In the given autoionization of ammonia: Bronsted acid is NH3 giving out a proton to form NH4+ (the conjugate acid). Bronsted base is also NH3 that accepts a proton to form NH2- (the conjugate base). The species corresponding to H+ is NH4+ and to OH- is NH2-. The condition for a neutral solution is that the concentrations of NH4+ and NH2- are equal, thus securing dynamic equilibrium.

Step-by-step solution

Identify the Bronsted Acids and Bases

In the given equilibrium reaction, one molecule of ammonia (\(\mathrm{NH}_{3}\)) is donating a proton (H\(^{+}\)); so it is acting as a Bronsted acid. The resulting ion, the ammonium ion (\(\mathrm{NH}_{4}^{+}\)), is the conjugate acid. The other molecule of ammonia is accepting a proton (H\(^+\)); hence it acts as a Bronsted base. The resulting ion, the amide ion (\(\mathrm{NH}_{2}^{-}\)), is the conjugate base.

Identify the Species Corresponding to H+ and OH-

The species corresponding to \(\mathrm{H^{+}}\) (Hydronium ion) is \(\mathrm{NH_{4}^{+}}\) (Ammonium ion) as it is the ion produced after NH3 (the Bronsted acid) donates a proton. The species corresponding to \(\mathrm{OH^{-}}\) (Hydroxide ion) is \(\mathrm{NH_{2}^{-}}\) (Amide ion) as the Bronsted base (NH3) accepts a proton to become NH2-.

Condition for a Neutral Solution

A solution of ammonia will be neutral when the concentrations of the ammonium ions (\(\mathrm{NH_{4}^{+}}\)) and the amide ions (\(\mathrm{NH_{2}^{-}}\)) are equal. This is because in a neutral solution, the amount of acidic species (ammonium ions) must be equal to the amount of basic species (amide ions). This condition represents a state of dynamic equilibrium in the solution.

Key concepts

Bronsted Acids

In the context of a chemical reaction, particularly involving proton transfer, a Bronsted acid is defined as any substance that can donate a proton ( H + ). This concept is central to understanding many reactions, including those occurring in liquids like ammonia.
In the autoionization of ammonia, one ammonia molecule ( NH ₃ ) donates a proton, acting as the Bronsted acid. After donating, it is transformed into an ammonium ion ( NH ₄ + ).
This transformation highlights the proton donation capability, hallmarking its role as an acid according to Bronsted definitions.

• NH ₃ acts as a Bronsted acid.
• The conjugate acid formed is NH ₄ + .

This simplified understanding helps in recognizing acid behavior in various chemical environments.

Bronsted Bases

A Bronsted base, the partner in proton transfer, is any species capable of accepting a proton. This forms the core basis of many chemical dynamics. In the ammonia autoionization, this role is played by another ammonia molecule.
Here, NH ₃ accepts a proton from the donating NH ₃ molecule, acting as a Bronsted base. This results in the formation of the amide ion ( NH ₂ - ), its conjugate base.
Recognizing Bronsted bases is crucial for understanding reactions in which proton exchange is key.

• NH ₃ serves as the Bronsted base.
• The resulting conjugate base is NH ₂ - .

These reactions underscore the dual role ammonia can play, depending on the match-up with its counterpart species.

Neutral Solution

The concept of a neutral solution is pivotal when discussing equilibrium in chemistry. A solution is deemed neutral when its acidic and basic constituents are balanced. In the context of ammonia, this means having equal amounts of ammonium ions ( NH ₄ + ) and amide ions ( NH ₂ - ).
This balance signifies that the solution exhibits no overall acidic or basic characteristics. It reflects a balance similar to that in water, where H + and OH - concentrations are equal.
A neutral solution in ammonia distinctly indicates equal participation and dissociation events between the ions.

• A neutral state leads to neither acidic nor basic dominance.
• The presence of equal amounts of NH ₄ + and NH ₂ - achieves neutrality.

Understanding this neutral condition can help in comprehending solution equilibria, especially under autoionization conditions.

Dynamic Equilibrium

Dynamic equilibrium is a fascinating state in chemical reactions, characterized by the continued, equal rate of forward and reverse reactions. In autoionization, such as that of ammonia, dynamic equilibrium reaches when the transformation from NH ₃ to NH ₄ + and NH ₂ - , and vice versa, occur at the same rate.
This equal and concurrent reaction rate means the concentrations of all species remain constant over time, even though reactions continue to take place.
This equilibrium provides stability and allows us to predict the behavior of the solution under different conditions.

• Both forward and reverse this process occur simultaneously at equal rates.
• Concentration remains constant while reactions are ongoing.

Recognizing dynamic equilibrium in chemical processes helps in anticipating the system's responses to external changes.