Solubility in Liquid Ammonia

Liquid ammonia is a fascinating solvent with unique properties that allow it to dissolve a wide range of substances, including many metals. The solubility of materials in liquid ammonia is of particular interest in the study of Group 1 elements (alkali metals) due to the remarkable reactions and solutions they form.

Properties of Liquid Ammonia

Before diving into the solubility of substances in liquid ammonia, let's first understand some of its key properties:

Molecular Formula: ( NH_3 )
Boiling Point: (-33.34^\circ C)
Dielectric Constant: High (around 22 at -34°C), which is conducive to ionization.
Self-Ionization: Ammonia can self-ionize to a small extent, similar to water:

[ 2NH_3 \rightleftharpoons NH_4^+ + NH_2^- ]

Solvent Properties: It can act as a solvent for a variety of compounds due to its polar nature and ability to form hydrogen bonds.

Solubility of Group 1 Metals in Liquid Ammonia

Group 1 metals, also known as alkali metals, are highly reactive and can dissolve in liquid ammonia to form deep blue solutions. This color is due to the presence of solvated electrons, which are electrons that are free to move in the solution.

Reaction of Alkali Metals with Liquid Ammonia

When an alkali metal dissolves in liquid ammonia, the following reaction occurs:

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

Where:

( M ) is the alkali metal (e.g., Li, Na, K, etc.).
( [M(NH_3)_x]^+ ) is the metal cation solvated by ammonia molecules.
( [e(NH_3)_y]^- ) is the solvated electron.

Properties of Solutions

The solutions formed by dissolving alkali metals in liquid ammonia have several interesting properties:

Color: Deep blue, due to solvated electrons.
Conductivity: These solutions are good electrical conductors because of the presence of free electrons.
Paramagnetism: The solutions are paramagnetic, again due to solvated electrons.
Diamagnetic: At higher metal concentrations, the solutions become diamagnetic and colorless as the solvated electrons pair up.

Table of Differences and Important Points

Property	Solubility in Water	Solubility in Liquid Ammonia
Solvent Polarity	High	High
Solvation	Hydration (water molecules)	Ammoniation (ammonia molecules)
Conductivity	Depends on the ion	High due to free electrons
Color of Solutions	Colorless (usually)	Deep blue due to solvated electrons
Magnetic Properties	Typically diamagnetic	Paramagnetic at low concentrations, diamagnetic at high concentrations

Examples

Sodium in Liquid Ammonia: When sodium is dissolved in liquid ammonia, it forms a deep blue solution due to the presence of solvated electrons. The reaction can be represented as:

[ Na + xNH_3 \rightarrow [Na(NH_3)_x]^+ + [e(NH_3)_y]^- ]

Conductivity: A solution of potassium in liquid ammonia will conduct electricity because of the movement of the solvated electrons and the potassium cations.
Magnetic Behavior: A dilute solution of lithium in liquid ammonia will exhibit paramagnetism due to the unpaired solvated electrons. However, as the concentration of lithium increases, the solution may become diamagnetic as the electrons pair up.

Conclusion

The solubility of Group 1 metals in liquid ammonia results in solutions with unique properties, such as deep blue color, high conductivity, and interesting magnetic behavior. These properties are largely due to the presence of solvated electrons, which are a key feature of these solutions. Understanding the behavior of solutes in liquid ammonia is important for various applications in synthetic chemistry and materials science.