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 \([\mathrm{Ca}(\mathrm{NH}_3)_6]^{2+}\), option (d).

Step 1: Understanding the Reaction

Calcium (\(\mathrm{Ca}\)) is dissolved in liquid ammonia (\(\mathrm{NH}_3\)), forming a solution that likely involves complex formation between calcium and ammonia. Liquid ammonia can solvate metal cations well, so it's common to form amine complexes.

Step 2: Recognizing the Product of Evaporation

When the ammonia is evaporated from the solution, the residue left behind will be a complex between calcium and ammonia. Since \(\mathrm{Ca}^{2+}\) can form complexes with six molecules of ammonia, the most stable complex is typically \([\mathrm{Ca}(\mathrm{NH}_3)_6]^{2+}\).

Step 3: Evaluating Options

From the given options, only \([\mathrm{Ca}(\mathrm{NH}_3)_6]^{2+}\) (option (d)) represents a stable amine complex of calcium with ammonia that can form after evaporation. The other options do not accurately describe the residue expected from this process.

Step 4: Conclusion of the Evaluation

After assessing the behavior of calcium in liquid ammonia and the process of evaporation, the correct option that describes the residue is \([\mathrm{Ca}(\mathrm{NH}_3)_6]^{2+}\).

Key Concepts

Metal-Ammonia Complexes

In coordination chemistry, metal-ammonia complexes arise when metal ions, such as calcium, interact with ammonia molecules. Ammonia, being a neutral molecule, serves as a good ligand due to its lone pair of electrons, which it can donate to the metal ion. This ligand-to-metal donation creates a stable complex.

Within liquid ammonia solutions, metals can become solvated by ammonia molecules. This situation tends to mimic how metals interact with water to form hydrated ions. The interaction is similar because ammonia, like water, has a lone pair of electrons, which can coordinate with many types of metal ions.

Metal-ammonia complexes often display characteristic colors and properties linked to the metal ion's oxidation state and its coordination number. For calcium, which typically forms a +2 charged ion, the coordination with ammonia usually results in a complex such as \([\text{Ca}(\text{NH}_3)_6]^{2+}\). This particular coordination commonly involves six ammonia molecules surrounding a single calcium ion, given the steric and electronic compatibility.

This type of complex formation is crucial for understanding various catalytic processes and chemical reactions in solution chemistry. They serve as basic models to understand much larger and complex molecular interactions.

Solution Chemistry

Solution chemistry focuses on the dispersion of substances and their interactions within a dissolving medium. This medium is often a solvent like water or, as in the case of our example, liquid ammonia.

Many metal ions dissolve well in liquid ammonia, forming unique solutions that have distinct properties. These solutions occur through interactions between the solutes, which are substances dissolved in the solvent, and the solvent molecules themselves. In the scenario with calcium in ammonia, the ionic character of calcium leads to its readiness to interact with ammonia molecules.

The behavior of these solutions can vary significantly based on factors like concentration, temperature, and the nature of the solute and solvent. Liquid ammonia as a solvent is particularly interesting because it can stabilize a wide range of metal cations, helping to form stable complexes. This property showcases the versatile nature of solution chemistry when different solvents other than water are used.

These solutions can exhibit various unique properties, such as specific conductivity and distinctive physical appearances. This makes them fascinating subjects of study within both academic and industrial chemistry, providing insights into solubility dynamics and coordination interactions.

Calcium Amine Complexes

Calcium amine complexes are specific types of coordination compounds where calcium ions are coordinated by amine groups, like those found in ammonia molecules. In the realm of coordination chemistry, these complexes are notable for their specific binding arrangements and the stability they provide to the calcium ion.

Calcium, as a Group 2 metal, typically forms complexes where it maintains a +2 charge. When interacting with ammonia, it can form a complex such as \([\text{Ca}(\text{NH}_3)_6]^{2+}\), which is characterized by being surrounded by six ammonia ligands. This number corresponds to the maximum coordination number for calcium with ammonia in such a setting, dictated by spatial and electronic considerations.

The evaporation of ammonia from a solution containing calcium results in such complexes being left as a residue. This process highlights the stable nature of calcium-ammonia interactions in coordination chemistry, where the ammonia ligands effectively coordinate to the metal, preventing free metal ions from remaining.

Studying these complexes can have practical implications, ranging from understanding solubility and reaction pathways to practical applications like catalysis or material formation. These diverse uses underscore the importance of calcium amine complexes in scientific exploration and technological innovation.