Problem 165 - A Complete Resource Book in Chemistry for JEE Main

Question

Metallic sodium dissolves in liquid ammonia to form a deep blue coloured solution. The deep blue colour is due to formation of (a) solvated electron, e \(\left(\mathrm{NH}_{3}\right) \mathrm{x}_{y}^{-}\) (b) solvated atomic sodium, \(\mathrm{Na}\left(\mathrm{NH}_{3}\right) \mathrm{y}\) (c) \(\left[\mathrm{Na}^{+}+\mathrm{Na}^{-}\right]\) (d) \(\mathrm{NaNH}_{2}+\mathrm{H}_{2}\)

Step-by-Step Solution

Answer

(a) solvated electron, e \(\left(\mathrm{NH}_{3}\right) \mathrm{x}_{y}^{-}\)

Step 1: Understanding the Reaction

Sodium metal dissolves in liquid ammonia, which is a process where sodium atoms lose electrons.

Step 2: Formation of Solvated Electrons

The free electrons from sodium interact with ammonia molecules. This results in forming solvated electrons.

Step 3: Color Explanation

The deep blue color is characteristic of these solvated electrons, which become dispersed uniformly in the ammonia solution.

Key Concepts

Sodium in Liquid Ammonia

When you dissolve metallic sodium in liquid ammonia, a fascinating transformation occurs. At this molecular level, sodium acts much like you might when peeling off old wallpaper—it sheds some of its parts. Specifically, when sodium dissolves in liquid ammonia, each sodium atom releases an electron, floating freely in the solution.

Liquid ammonia is rather like the sodium's favorite swimming pool, providing an excellent medium for this to occur. This process is unique and unusual because most metals do not behave this way.

• Liquid ammonia acts as a great solvent for the reaction.
• Sodium loses electrons easily at room temperature in this medium.

This dissolution of sodium does not involve a bubbling, fizzing reaction like dissolving a sugar cube in water. Instead, it is a rather smooth and quiet transition, where the results manifest visibly through the distinctive deep blue coloration.

Deep Blue Solution

Once sodium loses its electrons, these electrons do not wander aimlessly in the solvent. They pair up with ammonia molecules, a combination that gives rise to a deep blue solution.

This vivid blue is not just for looks. It's a signature of the presence of something called solvated electrons, which are tightly enmeshed with the ammonia molecules.

Here's how this beautiful shade emerges:

• The electrons interact with ammonia, stabilizing in the solution.
• As they become encapsulated by ammonia molecules, they alter the solution's optical properties, producing the deep blue appearance.

This color is quite rare and special. It opens up the solution to various investigations and applications in chemistry, serving as a handy marker for scientists.

Electron Solvation Process

The very heart of this transformative reaction lies in what chemists call the electron solvation process. Once sodium electrons are free, they cozy up with ammonia molecules rather than floating freely in oblivion. This is the solvation process.

In a solvation shell, ammonia molecules surround each electron, stabilizing it.

Here's why this is interesting:

• The solvation shell ensures the free electron becomes stable enough to remain in solution without reacting prematurely with water or oxygen.
• This phenomenon allows for intriguing chemical behaviors, making it an object of intense study.

During this process, the electron's presence is key to maintaining the deep blue color, marking a fine balance between instability and stability—a delicate dance facilitated by ammonia that makes advanced chemistry explorations possible.