Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

A solution of \(\mathrm{Ca}\) in liquid ammonia when evaporated to dryness gives as residue of (a) \(\left[\mathrm{Ca}\left(\mathrm{NH}_{3}\right) \mathrm{x}\right]^{2+}\) (b) \(\mathrm{Ca}\left(\mathrm{NH}_{3}\right)_{6}\) (c) pure Ca (d) \(\left[\mathrm{Ca}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\)

Step-by-Step Solution

Answer

The residue is pure Ca.

Step 1: Understanding the Problem

We have a solution of calcium (Ca) in liquid ammonia. The problem asks what the residue is when this solution is evaporated to dryness. To solve this, we need to understand how calcium behaves in liquid ammonia.

Step 2: Examining Calcium in Liquid Ammonia

Calcium reacts with liquid ammonia ( ext{NH}_3) to form a deep blue solution due to the formation of solvated electrons. These electrons are free and do not strongly bond to the calcium.

Step 3: Evaporation of Ammonia

When the ammonia is evaporated, it leaves no carrier for the solvated electrons. Calcium, being a metal, will remain in its elemental form because the ammonia that could coordinate does not form a stable complex after evaporation.

Step 4: Conclusion of the Process

Calcium does not form a stable compound that remains as a residue when ammonia evaporates. Thus, the residue after the evaporation of ammonia is pure calcium (option c).

Key Concepts

Chemical Reactions

In chemistry, a chemical reaction is a process where reactants are transformed into products. This involves breaking and forming bonds between atoms. Chemical reactions can occur in various phases, including solid, liquid, and gas. In the case of calcium reacting with liquid ammonia, the interaction involves the dissolving of the metal in the solvent, which leads to the formation of an interesting product known as solvated electrons. However, it's essential to note that no permanent compound is formed when calcium dissolves in ammonia. Key characteristics of chemical reactions:

• Involves breaking and forming of chemical bonds
• Energy change in the form of heat, light, or electricity
• Formation of new substances

These basic principles help us understand various phenomena like the behavior of calcium in liquid ammonia, which does not create a stable compound after ammonia evaporates.

Solvated Electrons

A solvated electron is an electron that is detached from its atom and is free in a solution. In the deep blue solution of calcium in liquid ammonia, these electrons play a crucial role. These electrons are not strongly bonded to the calcium ions and exist freely within the solution. Solvated electrons:

• Contribute to the deep blue color observed in the solution
• Are crucial in the study of metallic solutions like metals dissolved in liquid ammonia
• Show a unique form of charge carrier

When ammonia is evaporated, solvated electrons have no medium to exist in and therefore disappear, leaving behind the elemental form of the metal.

Calcium in Liquid Ammonia

Calcium is a metallic element that exhibits interesting behavior when dissolved in liquid ammonia. When calcium is added to liquid ammonia, it dissolves to form a dark blue solution, indicative of solvated electrons. This unique interaction doesn't result in permanent chemical compounds. Upon evaporation of the ammonia:

• The dark blue color fades as ammonia, the solvent, is removed
• The solvated electrons cease to exist due to the absence of a medium
• Calcium, lacking coordination with ammonia, is left as pure calcium

Thus, after evaporation, the residue consists solely of pure calcium, as no lasting compound forms. This demonstrates the temporary nature of the calcium and ammonia reaction.