Problem 130 - A Complete Resource Book in Chemistry for JEE Main

Question

Highly pure dilute solution of sodium in liquid ammonia 1\. shows blue colour 2\. exhibits electrical conductivity 3\. produces sodium amide 4\. produces hydrogen gas (a) 1,2 (b) \(1,2,3\) (c) \(2,3,4\) (d) all of these

Step-by-Step Solution

Answer

All the properties (1, 2, 3, 4) are correct; the answer is (d) all of these.

Step 1: Understanding the Problem

The question is asking which properties a highly pure dilute solution of sodium in liquid ammonia exhibits. It provides a list of properties and we need to determine which ones are correct.

Step 2: Analyzing Property 1: Shows Blue Colour

A sodium solution in liquid ammonia does indeed show a characteristic blue color due to solvated electrons. This property is correct.

Step 3: Analyzing Property 2: Exhibits Electrical Conductivity

The solvated electrons in the sodium solution provide electrons that conduct electricity, indicating that this solution does indeed exhibit electrical conductivity.

Step 4: Analyzing Property 3: Produces Sodium Amide

After some time, the sodium in liquid ammonia reacts with ammonia to form sodium amide (NaNH_2), especially when the solution is not very dilute or when left for extended conditions. This property is correct.

Step 5: Analyzing Property 4: Produces Hydrogen Gas

The initial reaction between sodium and ammonia also eventually forms hydrogen gas as a byproduct along with sodium amide (2Na + 2NH_3 ightarrow 2NaNH_2 + H_2). This property also holds true.

Step 6: Conclusion

All four properties are valid for a highly pure dilute solution of sodium in liquid ammonia. Therefore, the correct answer is option (d), all of these.

Key Concepts

Sodium in Liquid Ammonia

When sodium is dissolved in liquid ammonia, it creates a unique and fascinating solution. This solution is distinct due to its striking blue color. The color results from the presence of solvated electrons in the solution. These free-moving electrons become part of the solution landscape and are not tightly bound to any atom. They interact with ammonia molecules in such a way that they absorb and release energy as visible blue light.

The resulting solution is quite rare in nature, mainly found in controlled laboratory settings. This blue, liquid ammonia solution also exhibits conductive properties, which we'll dive into further because of these free electrons. Overall, sodium in liquid ammonia offers a beautiful glimpse into unusual chemical behavior that is not often seen in more common solutions.

• Striking blue color due to solvated electrons
• Shows electrical conductivity
• Typically seen in lab settings

Solvated Electrons

Solvated electrons are the centerpiece of the fascinating properties observed in sodium dissolved in liquid ammonia. These electrons are unique because they aren't attached to a particular sodium or ammonia molecule but rather float freely in the solution. This freedom gives them the ability to move about, contributing significantly to the solution's overall properties.

It's these unattached electrons that are primarily responsible for the intense blue color observed—it forms because the solution absorbs certain wavelengths of light and reflects blue hues. Moreover, they act much like free electrons in metals, thus enabling the solution to conduct electricity effectively.

• Unattached, free-moving electrons in solution
• Key to blue color and electrical conductivity
• Behave like electrons in metal

Sodium Amide

Over time, or under certain conditions, sodium in liquid ammonia undergoes a chemical reaction leading to the formation of sodium amide. Sodium amide (\( \text{NaNH}_2 \)) is a product when sodium reacts with ammonia, a shift that usually occurs when the solution is less dilute or allowed to sit. This amide formation is part of the reason liquid ammonia solutions must be carefully managed and shielded from atmospheric moisture.

The reaction can be represented by the equation: \[ 2\text{Na} + 2\text{NH}_3 \rightarrow 2\text{NaNH}_2 + \text{H}_2 \] which also outlines the production of hydrogen gas. Handling these reactions requires careful lab conditions, given the flammable nature of hydrogen and the role of sodium amide as a strong base.

• Product of sodium reacting with ammonia
• Forms under less dilute or extended conditions
• Accompanied by hydrogen gas production

Reaction with Ammonia

The interaction of sodium with ammonia goes beyond just forming a solution; it triggers a series of chemical events. Initially, the sodium dissolves, creating a blue, conductive liquid. However, over time or under specific conditions, this setup progresses into more complex reactions.

As sodium continues to react with ammonia, it forms both sodium amide and hydrogen gas. Initially, solvated electrons result in a visible color and conductivity change. Over further reaction timescales, these electrons play a vital role in the formation of sodium amide and the liberation of hydrogen gas. This step is crucial, as sodium amide itself has various applications in industrial and synthetic chemistry.

• Initial formation of blue solution with solvated electrons
• Transformation into sodium amide and hydrogen gas
• Key reactions in controlled conditions