Problem 36 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{AgCl}\) is soluble in \(\mathrm{NH}_{4} \mathrm{OH}\) solution. The solubility is due to the formation of (a) \(\mathrm{Ag}_{2} \mathrm{O}\) (b) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\) (c) \(\mathrm{AgOH}\) (d) \(\mathrm{NH}_{4} \mathrm{Cl}\)

Step-by-Step Solution

Answer

The solubility is due to the formation of \([\mathrm{Ag}(\mathrm{NH}_{3})_{2}]^{+}\).

Step 1: Understanding AgCl in NH4OH

When silver chloride (AgCl) is added to ammonium hydroxide (NH4OH), it dissolves due to a chemical reaction. This is an important example in inorganic chemistry, where a complex ion is formed.

Step 2: Predicting Complex Formation

The solubility phenomenon is attributed to the formation of a complex ion. Ammonia (NH_3) acts as a ligand that readily forms a coordination complex with silver ions (Ag^+).

Step 3: Identifying the Complex Ion

In this case, the complex ion formed when AgCl dissolves in the presence of NH_4OH is [Ag(NH_3)_2]^+. This complex makes AgCl soluble in ammonia solutions.

Key Concepts

Complex Ion Formation

When silver chloride (\(\mathrm{AgCl}\)) dissolves in ammonium hydroxide (\(\mathrm{NH_4OH}\)), this process highlights an essential principle in inorganic chemistry: complex ion formation. Complex ions occur when simple ions, like silver ions (\(\mathrm{Ag^+}\)), interact with other ions or molecules to form a stable structure. This phenomenon greatly enhances solubility in certain environments.
To understand why solubility occurs in this situation, consider the solubility of \(\mathrm{AgCl}\) in pure water. \(\mathrm{AgCl}\) is a sparingly soluble salt in water, meaning it doesn't dissolve well. However, when in the presence of \(\mathrm{NH_4OH}\), the silver ions (\(\mathrm{Ag^+}\)) can bind with ammonia molecules (\(\mathrm{NH_3}\)), leading to the formation of a soluble complex ion designated as \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\).
This soluble form allows more \(\mathrm{AgCl}\) to dissolve. The driving force for this reaction is the affinity that silver has for forming strong coordinate covalent bonds with ligands like ammonia. As these bonds form, the solubility of formerly insoluble salts can significantly increase.

Coordination Complexes in Inorganic Chemistry

Coordination complexes are indispensable substances in the field of inorganic chemistry, often involving the interaction between metal ions and various ligands. These complexes are aptly named because they consist of a central metal atom or ion bonded to surrounding molecules called ligands. The number of ligand molecules attached to the metal ion is called the coordination number.
In the case of the AgCl and \(\mathrm{NH_4OH}\) reaction, the silver ion (\(\mathrm{Ag^+}\)) acts as the central metal ion in the coordination complex. By forming a coordination complex like \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\), we observe how the presence of ammonia as a ligand modifies silver's chemical behavior, enabling it to dissolve better in solution.
Coordination complexes are not just limited to silver and ammonia; they occur widely across chemistry with various metals and ligand types. These complexes have vital applications including catalysis, material science, and even biological systems where metal-ligand interactions are crucial. Understanding coordination complexes helps clarify why certain reactions occur, as they account for unique features such as enhanced solubility, color change, or catalytic activity.

Ligands in Chemistry

In the study of chemistry, particularly within the domain of coordination complexes, ligands are central players. A ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. Ligands can be simple ions like chloride (Cl⁻), or more complex molecules like ammonia (\(\mathrm{NH_3}\)).
Ligands are characterized by their ability to donate electron pairs to the metal, forming coordinate covalent bonds. In the case of silver chloride (\(\mathrm{AgCl}\)) in ammonia, ammonia acts as the ligand. Ammonia donates its lone pair of electrons to the silver ion, forming the complex ion \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\).

• **Unidentate ligands**, like ammonia, bind through a single point of attachment.
• **Bidentate ligands** can attach at two points, providing more stability.
• **Polydentate ligands** engage at multiple sites, and chelation can occur, producing very stable complexes.

The role of ligands is critical, as they determine the structure, reactivity, and properties of the complex. They can alter a metal's oxidation state, influence its color, and ease its transition into dissolved forms from solids, as seen in the enhanced solubility of \(\mathrm{AgCl}\) in \(\mathrm{NH_4OH}\). Through understanding ligands, we gain insight into a wide range of chemical phenomena and applications, from industrial catalysis to biochemical interactions.