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 correct answer is option (d): \\( \left[\mathrm{Ca}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+} \\).

Step 1: Understand the Reaction

Calcium (Ca) in liquid ammonia dissolves to form an ammoniated electron solution. In this reaction, Calcium typically forms a coordination complex with ammonia molecules.

Step 2: Identify Possible Products

When calcium is dissolved in liquid ammonia, the possible products upon evaporation might include calcium coordinated with ammonia molecules, or a reduction product like pure calcium.

Step 3: Consider Coordination Complex Formation

Calcium has a high tendency to form complexes with ligands like NH3. Upon evaporation of the dissolved calcium in liquid ammonia, it is expected to form a coordination complex rather than remaining as pure metal.

Step 4: Predict the Residue

Given the options: the residue upon evaporation would likely be a coordination complex of calcium with ammonia molecules. Options (a) and (d) suggest a complex with positive charge, which is consistent with typical behavior when metals dissolve in ammonia to form cationic complexes.

Step 5: Select the Appropriate Option

Among the options provided, \( \left[\mathrm{Ca}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+} \) is the appropriate representation of a stable coordination complex form. This complex represents calcium surrounded by 6 ammonia molecules and holds a 2+ charge, indicating oxidation as usually seen in metal-ammonia solutions.

Key Concepts

Ammoniated Electron Solutions

Ammoniated electron solutions are fascinating mixtures created when metals dissolve in liquid ammonia. These solutions are unique because they contain solvated electrons, free electrons that are not bound to any particular atom or ion. When a metal such as calcium (Ca) is placed in liquid ammonia, a deep blue solution is often observed. This is due to the presence of these free electrons. Solvated electrons can move freely, giving the solution its characteristic metallic properties.

These solutions are interesting for their:

• Ability to conduct electricity: Due to the presence of free, solvated electrons.
• Color changes: They exhibit a typical deep blue color.
• Stability: These solutions can remain stable at low temperatures but may change when the solvent evaporates.

The process in which the electron sits amidst the ammonia molecules leads to the formation of metal ion complexes as the ammonia coordinates with the metal, stabilizing the ionic state commonly seen in coordination chemistry.

Calcium Ammonia Complex

When calcium is dissolved in liquid ammonia, it forms a coordination complex with ammonia molecules. This solution involves calcium ions surrounded by ammonia, creating a stable ionic structure. During evaporation, the solution precipitates into a residue that is made up of a coordination complex rather than reverting to pure calcium.

The calcium ammonia complex is important due to its:

• Structure: Calcium ions typically bind with six ammonia molecules, creating an octahedral coordination complex.
• Charge: The resulting complex often carries a 2+ charge, as calcium loses two electrons during the formation of the complex.
• Role in coordination chemistry: It highlights how metals form stable bonds with nitrogen atoms in ammonia.

This phenomenon underscores the theme of coordination chemistry, where transition metals like calcium interact closely with ligands like ammonia, maintaining coordination and stability in the solution.

Coordination Chemistry

Coordination chemistry is the study of compounds that involve a central metal atom or ion bonded to surrounding molecules or ions, called ligands. This area of chemistry explains how entities like calcium form stable complexes with molecules such as ammonia due to their electron pairing and sharing properties.

Key features of coordination chemistry include:

• Ligand bonding: Molecules like ammonia act as ligands that donate electron pairs to the metal ion.
• Complex formation: These ligand bonds lead to the formation of a coordination sphere around the metal ion.
• Predictability of structures: Typical geometric arrangements like octahedral or tetrahedral can be predicted based on the number of ligands.

Understanding coordination chemistry is crucial when studying metal complexes in various environments. In this context, it allows us to grasp how calcium in liquid ammonia forms structured and charged complexes, enhancing our comprehension of both chemical reactions and the stability of such solutions.