Solutions in Liquid Ammonia
The chemistry of the Ammoniated Electron.
One of the most visually stunning and chemically unique properties of s-block elements is their ability to dissolve in anhydrous liquid ammonia. Instead of a typical chemical reaction, the metals dissolve to form a mesmerizing deep blue solution with extraordinary physical and chemical properties.
1. The Dissolution Process
When an alkali metal ($M$) is dropped into liquid ammonia ($NH_3$), the metal atom ionizes. Because ammonia is an excellent solvent, it solvates (surrounds) both the metal cation and the free electron.
For Group 1 (Alkali Metals):
For Group 2 (Alkaline Earth Metals):
The species $[e(NH_3)_y]^-$ is called the Ammoniated Electron. It is literally a free electron floating in the solution, stabilized by a cage of ammonia molecules!
2. Properties of the Dilute Solution
Dilute solutions of s-block metals in liquid ammonia exhibit three defining characteristics:
- 1. Deep Blue Color: The solution turns a brilliant deep blue. This color is entirely due to the ammoniated electron. The free electron absorbs energy in the red region of the visible spectrum for its excitation, thereby transmitting the complementary blue color.
- 2. Highly Conducting: The solution conducts electricity remarkably well (better than any regular salt solution). The conductivity is due to the mobility of both the ammoniated cation and the ammoniated electron. (Note: The tiny, highly mobile ammoniated electron carries the vast majority of the current).
- 3. Paramagnetic Nature: Because there are millions of unpaired ammoniated electrons floating in the solution, dilute solutions are strongly paramagnetic (attracted to a magnetic field).
3. Effect of Increasing Concentration (The Trap)
The properties described above apply to dilute solutions. When the concentration of the metal is increased (typically beyond $3\text{ M}$), the solution undergoes a dramatic phase change. This is a massive favorite in competitive exams!
| Property | Dilute Solution | Concentrated Solution ($\gt 3\text{ M}$) |
|---|---|---|
| Color | Deep Blue | Bronze / Copper-like |
| Magnetic Nature | Paramagnetic (Unpaired $e^-$) | Diamagnetic (Paired $e^-$) |
| Appearance | Transparent Liquid | Metallic Luster |
In a highly concentrated solution, the ammoniated electrons are forced so close together that their spin magnetic moments cancel each other out. They pair up to form electron pairs ($[e_2(NH_3)_z]^{2-}$), causing the solution to lose its paramagnetism and become diamagnetic.
4. Chemical Decomposition (Formation of Amides)
The deep blue solution is metastable. It can be kept for a while if it is perfectly pure and extremely cold. However, upon standing for a long time, or upon the addition of a catalyst (like Iron, Platinum, or even rust), the solution decomposes.
The free ammoniated electron reduces the $NH_3$ molecule, liberating Hydrogen gas and forming a metal amide. Once this happens, the blue color fades away entirely.
For Alkali Metals (Group 1):
For Alkaline Earth Metals (Group 2):
(Note: $M_{(am)}$ indicates the metal dissolved in ammonia).
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