Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

Silver chloride dissolves in excess of \(\mathrm{NH}_{4} \mathrm{OH}\). The cation present in this solution is (a) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)\right]^{+}\) (b) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{4}\right]^{+}\) (c) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\) (d) \(\mathrm{Ag}^{+}\)

Step-by-Step Solution

Answer

The cation present in solution is \(\left[\text{Ag}\left(\text{NH}_3\right)_2\right]^+\) (option c).

Step 1: Identify Silver Chloride Reaction with NH4OH

Silver chloride (AgCl) is known to react with aqueous ammonium hydroxide (\(\text{NH}_4\text{OH}\)). Silver chloride is not very soluble in water, but when ammonia (NH3) is present, it can dissolve forming a complex ion. The first ion that can form is \(\left[\text{Ag}\left(\text{NH}_3\right)\right]^+\).

Step 2: Determine the Complex Formation

Ammonia acts as a ligand and complexes with the silver ion. The most stable complex ion formed is \(\left[\text{Ag}\left(\text{NH}_3\right)_2\right]^+\), which forms because ammonia is present in excess and complexes with Ag+ to form a diamine silver complex.

Step 3: Identify the Role of Excess NH4OH

In the presence of excess \(\text{NH}_3\), as supplied by \(\text{NH}_4\text{OH}\), silver ion prefers to form a stable complex with two NH3 ligands, rather than just one. Therefore, \(\left[\text{Ag}\left(\text{NH}_3\right)_2\right]^+\) is the ion typically formed in an ammonia-rich environment.

Step 4: Select the Correct Answer

Considering the formation of complexes with excess ammonia, the correct complex cation that is present is \(\left[\text{Ag}\left(\text{NH}_3\right)_2\right]^+\). Therefore, the answer is (c).

Key Concepts

Ligand

In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a complex. Ligands are fundamental as they possess lone pairs of electrons capable of being donated to a metal ion. This creates a coordinate bond.

Ligands can vary in terms of charge and size, and they can be neutral molecules like water or ammonia, or anions like chloride and hydroxide. The nature of the ligand influences the properties of the metal complex significantly, including its color, solubility, and stability. For instance, in the context of the silver ammonia complex, ammonia (\( ext{NH}_3\)) acts as a neutral ligand.

• They use their lone pairs of electrons to form stable bonds with metal ions.
• The presence of multiple ligands around a metal ion can stabilize the overall structure.

Understanding ligands is crucial to comprehend how metals like silver can change their properties when bonded in a complex.

Silver Ammonia Complex

The silver ammonia complex is a quintessential example of a metal-ligand interaction, where silver ions form complexes with ammonia molecules. When silver chloride (\( ext{AgCl}\)) dissolves in excess ammonium hydroxide (\( ext{NH}_4 ext{OH}\)), ammonia from \( ext{NH}_4 ext{OH}\) behaves like a ligand, forming a complex ion with silver ion.

The most stable silver ammonia complex formed is \( ext{[Ag(NH}_3)_2]^+\). This is known as the diamine silver complex. In this complex, two molecules of ammonia coordinate with the \(\text{Ag}^+\) ion.

• Initially, a simple complex such as \( ext{[Ag(NH}_3)]^+\) might form.
• However, in the presence of excess ammonia, the coordination number increases, leading to the preferred formation of \( ext{[Ag(NH}_3)_2]^+\).

This interaction demonstrates the flexibility and versatility of silver ions in forming stable and soluble complexes.

Coordination Chemistry

Coordination chemistry focuses on the study of complex ions or coordination complexes, which involve a central metal atom or ion bonded to surrounding molecules or ions called ligands. These interactions are crucial in many chemical reactions and biological processes.

Key components of coordination chemistry include the coordination sphere, coordination number, and the nature of the ligands. In the case of \( ext{[Ag(NH}_3)_2]^+\), silver acts as the central ion, while ammonia serves as the ligand, forming coordinate bonds.

Some noteworthy aspects of coordination chemistry include:

• Coordination number: This refers to the number of ligand atoms that are bonded directly to the central atom. For \( ext{[Ag(NH}_3)_2]^+\), the coordination number is 2 because two ammonia ligands are attached directly to the silver atom.
• Geometry: Coordination chemistry also studies the spatial arrangement of ligands around the central atom, influencing the geometry like linear, tetrahedral, or square planar structures.

Delving into coordination chemistry helps to illuminate numerous reactions, such as the formation of complex ions, that govern both industrial applications and biological functions.