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

The solution is due to solvated electrons: (a) e (NH3)xy^-.

Step 1: Understand the Reaction

When metallic sodium is dissolved in liquid ammonia, the reaction involves the formation of a solvated electron. This results in the deep blue colored solution observed.

Step 2: Identify Possible Outcomes

The exercise provides four options. The correct solution involves understanding which one corresponds to the effect metallic sodium has in liquid ammonia. Option (a) suggests a solvated electron is involved in the coloration.

Step 3: Recall Chemical Processes

Recall that in the dissolution of metallic sodium in liquid ammonia, the sodium donates an electron which becomes solvated in the ammonia. This is because the electron can move around freely, creating a color change due to its interaction with light.

Step 4: Evaluate Each Option

- Option (a) refers to a solvated electron formation, which aligns with the deep blue color observed. - Option (b) is less likely because solvated sodium atoms do not result in such a color. - Option (c) is unlikely as a simple pairing of ions doesn't lead to the specific deep-blue color effect. - Option (d) involves different compounds and does not produce a blue solution.

Step 5: Select the Correct Answer

The chemical process of metal electrons being solvated leads to the conclusion that the deep blue solution is due to the presence of solvated electrons. Therefore, the correct option is (a).

Key Concepts

Metallic Sodium Dissolution

When metallic sodium is introduced into liquid ammonia, it undergoes a unique process known as dissolution. Dissolution in this context refers to the sodium metal interacting with the ammonia solvent, resulting in separation of the sodium atoms into individual ions. Unlike the simple physical dissolution like sugar in water, here it involves a more intricate chemical process.

• The sodium atom loses an electron and forms a positively charged ion ( ext{Na}^+ ).
• This free electron doesn't just vanish; instead, it gets solvated by the ammonia molecules around it.

This leads to something quite special: the formation of free-moving, solvated electrons. These electrons are key to the fascinating properties exhibited by the solution, including its notable deep blue color.

Liquid Ammonia Reaction

The reaction of metallic sodium in liquid ammonia is intriguing because liquid ammonia itself is a peculiar solvent compared to water or other common solvents. In this environment, sodium donates an electron which is then surrounded by ammonia molecules, resulting in what's known as a solvated electron. This solvated electron is stable in liquid ammonia, due to ammonia's capability to stabilize anions and make the electron less reactive.
These conditions create a unique environment:

• Ammonia molecules act as a cage around the free electron, stabilizing it.
• The solvated electron behaves as if it is in a ‘sea’ of ammonia molecules.

The physical environment and chemical interactions in liquid ammonia therefore allow the sustained presence of the deep blue color associated with solvated electrons.

Deep Blue Color Chemical Reaction

The deep blue color of the sodium and liquid ammonia solution is a visual cue of the underlying chemistry at work. This color is directly attributed to the presence of solvated electrons. These electrons interact with light in a way that causes the solution to take on a characteristic color. The deep blue hue stems from the absorption of certain wavelengths of light by the solvated electrons:

• Solvated electrons absorb lower energy visible light.
• This results in the emission or transmission of light at the blue end of the spectrum, thus giving the solution its distinct color.

In essence, the interaction between the light and solvated electrons plays a fundamental role in the appearance of the solution. Understanding this phenomenon gives insight into the dynamic interaction of electronic structures within various chemical environments.