Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Highly pure dilute solution of sodium in liquid ammonia (a) exhibits electrical neutrality (b) produces sodium amide (c) produces hydrogen gas (d) show blue colour

Step-by-Step Solution

Answer

All statements (a), (b), (c), and (d) are correct.

Step 1: Understanding the Problem

We are asked about the properties of a highly pure dilute solution of sodium in liquid ammonia. The aim is to identify the correct statements about this solution.

Step 2: Analyze Statement (a)

Statement (a) suggests that the solution exhibits electrical neutrality. Sodium (Na) donates an electron to ammonia, forming solvated electrons and ions that maintain overall charge neutrality. Therefore, the solution is electrically neutral.

Step 3: Analyze Statement (b)

Statement (b) claims that the solution produces sodium amide. Reaction of sodium with liquid ammonia forms sodium amide over time: \[ 2 ext{Na} + 2 ext{NH}_3 ightarrow 2 ext{NaNH}_2 + ext{H}_2 \]Therefore, the solution does indeed produce sodium amide.

Step 4: Analyze Statement (c)

Statement (c) suggests that the solution produces hydrogen gas. The formation of sodium amide does eventually release hydrogen gas as shown in step (b): \[ 2 ext{Na} + 2 ext{NH}_3 ightarrow 2 ext{NaNH}_2 + ext{H}_2 \] Thus, hydrogen gas is produced.

Step 5: Analyze Statement (d)

Statement (d) mentions that the solution shows a blue color. When sodium dissolves in liquid ammonia, solvated electrons are responsible for the solution's blue color. Thus, statement (d) is correct.

Step 6: Conclusion

All the provided statements are true for a highly pure dilute solution of sodium in liquid ammonia: it is electrically neutral, produces sodium amide, releases hydrogen gas, and shows a blue color.

Key Concepts

Electrical Neutrality

A solution being electrically neutral means that the total positive and negative charges in the solution balance each other out. In the case of sodium in liquid ammonia, sodium donates electrons which become solvated, or dispersed, among ammonia molecules. These solvated electrons play a critical role in maintaining the balance of charge.

• When sodium (Na) dissolves, it releases electrons.
• These electrons become solvated by ammonia, leading to a mix of positive sodium ions ( Na^+ ) and negative solvated electrons.

This balancing act ensures that there is no net charge in the solution, thereby maintaining electrical neutrality.

Sodium Amide Formation

Sodium reacts with liquid ammonia to produce sodium amide through a straightforward chemical reaction. This reaction not only forms sodium amide but also results in the release of hydrogen gas.This is expressed as the chemical equation:\[ 2 \text{Na} + 2 \text{NH}_3 \rightarrow 2 \text{NaNH}_2 + \text{H}_2 \]

• The sodium atoms react with ammonia (NH_3) molecules.
• This produces sodium amide (NaNH_2), a compound where sodium is bonded with the amide group.

This process is important for the formation of solid sodium amide within the solution.

Hydrogen Gas Production

The production of hydrogen gas occurs simultaneously with the formation of sodium amide. When sodium is introduced to liquid ammonia, not only does it create sodium amide, but hydrogen gas is also liberated as a part of the reaction.

• The reaction: \[ 2 \text{Na} + 2 \text{NH}_3 \rightarrow 2 \text{NaNH}_2 + \text{H}_2 \]
• This shows how hydrogen gas is released as a byproduct.

The escaping hydrogen gas adds to the reaction's dynamics and changes the composition of the solution over time.

Blue Color of Solution

The blue color observed when sodium is dissolved in liquid ammonia is one of the most striking features of this reaction. This color is attributable to the presence of solvated electrons in the solution.

• Solvated electrons are free electrons surrounded by solvent molecules, in this case, ammonia.
• The absorption of light by these electrons is what gives the solution its deep blue hue.

This characteristic coloration is often used to qualitatively confirm the presence of solvated electrons in solutions.