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) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{6}\right]^{+}\) (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 is [Ag(NH_3)_2]^+.

Step 1: Understand the Reaction

Silver chloride (AgCl) can dissolve in an excess of NH_4OH due to the formation of a complex ion. The reaction involves ammonia molecules coordinating with silver ions to form a soluble complex.

Step 2: Identify the Complex Formation

In the presence of excess ammonia (NH_3), the silver ion (Ag^+) forms a coordination complex. The common complex formed is [Ag(NH_3)_2]^+, where two ammonia molecules are bonded to one silver ion.

Step 3: Choose the Correct Option

From the options given, you need to identify which is the formed cation in the solution. Option (c), [Ag(NH_3)_2]^+, matches the complex formed and is the correct solution.

Key Concepts

Complex Ion Formation

In coordination chemistry, complex ion formation occurs when a central metal ion is surrounded by molecules or ions called ligands. These ligands, often neutral molecules like water, ammonia, or ions like chloride, coordinate to the central metal ion through a donor atom such as nitrogen, oxygen, or halides.

Key characteristics of complex ion formation include:

• Coordination Bonds: The bonds formed between the metal and the ligands are known as coordination bonds, a type of covalent bond where both electrons come from the ligand.


• Stability Factors: The stability of a complex ion often depends on the metal ion's oxidation state, the type of ligands, and the overall charge of the complex.


• Geometry: The spatial arrangement of the ligands around the metal center can result in various geometric shapes like linear, tetrahedral, and octahedral structures.

The process of forming these complex ions helps certain compounds dissolve in solvents they would not normally dissolve in, as seen with silver chloride in ammonia. This occurs because the newly formed complex ion is more soluble than the original compound.

Coordination Complex

A coordination complex is the entire assembly of a metal ion bonded with surrounding ligands. These complexes are characterized by:

Distinguishing Features:

• Central Metal Ion: Usually a transition metal, capable of accepting electrons from ligands.


• Coordination Number: It refers to the number of ligand donor atoms bonded to the central metal ion. Common coordination numbers are 2, 4, and 6.


• Ligands: These are ions or molecules with electron pairs that can be donated to the metal ion, forming a coordination bond.

Coordination complexes are essential in many biological systems, industrial catalysis, and materials science. They exhibit a variety of properties, including differing colors and catalytic behaviors, based on the metal and ligands involved.

Applications and Examples:

• In biochemistry, hemoglobin is a coordination complex with iron as the central metal ion which binds oxygen molecules.


• In industrial processes, coordination complexes often serve as catalysts, facilitating chemical reactions.

The precise configuration of these complexes influences their chemical reactivity and interaction with other molecules.

Silver-Ammonia Complex

The silver-ammonia complex is a prominent example of complex ion chemistry. In this scenario, when silver chloride ( AgCl ) encounters an excess of ammonia ( NH_{3} ), it forms a soluble complex ion known as [Ag(NH_{3})_{2}]^{+} .

Formation Process:

• When silver chloride is mixed with ammonia, it is initially insoluble in water.


• Excess ammonia provides lone pairs of electrons to form coordination bonds with the Ag^{+} ion.


• This interaction creates a stable [Ag(NH_{3})_{2}]^{+} complex that is soluble in the solution.

The nature of this complex allows it to dissolve, illustrated in many laboratory and industrial processes, where otherwise insoluble compounds need to be utilized. Understanding the formation and structure of such a complex is fundamental in fields involving metal recovery and purification.

The behavior of the silver-ammonia complex also demonstrates the versatility of transition metals and their ability to form variable coordination compounds, which have numerous applications in various scientific fields.