Question

Discuss the following observations: (a) Zinc dissolves in a solution of sodium amide in liquid \(\mathrm{NH}_{3}\) with liberation of \(\mathrm{H}_{2} ;\) careful addition of ammonium iodide to the resulting solution produces a white precipitate which dissolves if an excess of ammonium iodide is added. (b) Addition of \(\mathrm{K}\) to \(\mathrm{H}_{2} \mathrm{O}\) results in a vigorous reaction; addition of K to liquid \(\mathrm{NH}_{3}\) gives a bright blue solution, which over a period of time liberates \(\mathrm{H}_{2}\)

Short answer

Zinc dissolves in sodium amide, forming complexes and releasing H_2 ; with NH_4I , it forms a precipitate. Potassium reacts with water violently and forms a blue solution with liquid NH_3 , eventually liberating H_2 .

Step-by-step solution

Analyze the Reaction of Zinc in Sodium Amide Solution

When zinc dissolves in a sodium amide ((nh2)^-) solution in liquid ammonia ((nh3)), it forms a complex and liberates hydrogen gas ((h2)). The reaction can be represented as:\[Zn + 2NaNH_2 \rightarrow Na_2[Zn(NH_2)_4] + H_2 \uparrow\]This indicates zinc is acting as a reducing agent, reducing sodium amide while itself being oxidized.

Understand the Role of Ammonium Iodide

Ammonium iodide ( (nh4i) ) is carefully added to the zinc solution, producing a white precipitate, likely zinc iodide ( (zni2) ). However, with excess ammonium iodide, the precipitate dissolves due to the formation of a soluble complex, such as ammonium zinc iodide. This suggests a dynamic equilibrium between the precipitate and the soluble complex.

Examine Potassium's Reaction with Water

When potassium ((k)) is added to water, it reacts vigorously, producing potassium hydroxide and hydrogen gas:\[2K + 2H_2O \rightarrow 2KOH + H_2 \uparrow\]This reaction is highly exothermic, releasing energy and resulting in the liberation of hydrogen gas.

Understand Potassium's Reaction with Liquid Ammonia

Adding potassium to liquid ammonia results in a bright blue solution due to the formation of solvated electrons, widely described as dissolving metal ammonia solutions:\[K + (x+y)NH_3 \rightarrow [K(NH_3)_x]^+ + e^-(NH_3)_y\]Over time, these solvated electrons react, liberating hydrogen gas: \[2e^-(NH_3) + 2NH_3 \rightarrow H_2 \uparrow + 2NH_2^-\]

Key concepts

Zinc and Sodium Amide Reaction

In the chemical interaction between zinc and sodium amide in liquid ammonia, zinc acts as a reducing agent. It donates electrons to sodium amide, leading to the liberation of hydrogen gas. This process demonstrates common features of redox chemistry, where one substance is oxidized while another is reduced.
When zinc reacts with sodium amide, a complex is formed along with hydrogen gas. The overall chemical equation is:

• \[Zn + 2NaNH_2 \rightarrow Na_2[Zn(NH_2)_4] + H_2 \uparrow\]

In this scenario, zinc gains a positive charge by losing electrons, thus being oxidized, while sodium amide is reduced.
Ammonium iodide plays an intriguing role when added to the resulting solution. Initially, it forms a white precipitate of zinc iodide. Yet, upon adding more ammonium iodide, this precipitate dissolves because of the formation of a soluble complex ion, like ammonium zinc iodide. This transformation represents a shift in equilibrium—a dynamic balance between the solid precipitate and its dissolved form.

Potassium and Water Reaction

Potassium is a highly reactive metal, and its interaction with water exemplifies a classic vigorous reaction in chemistry. When potassium comes into contact with water, it reacts energetically, producing potassium hydroxide and hydrogen gas.
The balanced chemical equation representing this reaction is:

• \[2K + 2H_2O \rightarrow 2KOH + H_2 \uparrow\]

This reaction is exothermic, meaning it releases heat. So much heat, in fact, that the released hydrogen gas can ignite spontaneously.
Potassium's violent reaction with water serves as an excellent illustration of the reactivity of alkali metals, which increases down the group in the periodic table.

Solvated Electrons in Liquid Ammonia

When potassium is dissolved in liquid ammonia, a fascinating blue solution is formed. This bright blue color arises from the presence of solvated electrons. These electrons are free and mobile, dissolved in the ammonia solvent, giving rise to the term "dissolving metal ammonia solutions."
The initial reaction can be depicted as:

• \[K + (x+y)NH_3 \rightarrow [K(NH_3)_x]^+ + e^-(NH_3)_y\]

These solvated electrons are responsible not only for the characteristic color but also for the solution's various chemical properties, including conductivity and reactivity.
Over time, these solvated electrons in the ammonia can engage in further reactions:

• \[2e^-(NH_3) + 2NH_3 \rightarrow H_2 \uparrow + 2NH_2^-\]

The stability of these particles and consequent release of hydrogen demonstrates yet another aspect of redox and electrochemistry in inorganic reactions.