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

(c) \([Ag(NH_3)_2]^{+}\)

Step 1: Understand Silver Chloride Dissolution

Silver chloride (AgCl) is an insoluble salt, but in the presence of excess ammonium hydroxide (NH_4OH), it can dissolve due to the formation of a complex ion.

Step 2: Identify the Complex Formation

When AgCl is treated with NH_4OH, it forms a complex with ammonia (NH_3) known as diamminesilver(I) complex. This complex is represented as [Ag(NH_3)_{2}]^{+}.

Step 3: Analyze the Charge of the Complex

The silver ion Ag^{+} and two molecules of ammonia (NH_3) form the [Ag(NH_3)_{2}]^{+} complex. The overall charge of the complex is positive due to the single positive charge of the silver ion.

Step 4: Determine the Present Cation

The cation present in the solution after dissolution is the complex ion [Ag(NH_3)_{2}]^{+}, which corresponds to option (c).

Key Concepts

Complex Ion Formation

Complex ion formation is a fascinating aspect of coordination chemistry. When certain soluble or insoluble salts interact with ligands, they form complex ions. These are essentially structures where a central metal ion is surrounded by molecules or ions called ligands. The metal ion has an unfilled electron shell, allowing it to accept electron pairs from these ligands. This electron donation forms what is known as a coordination bond.

In the context of our problem, silver chloride (AgCl) interacts with ammonia to form the diamminesilver(I) complex, \([\text{Ag}(\text{NH}_3)_2]^+\). Here, the central metal ion is Ag⁺, and ammonia, the ligand, donates electron pairs to form stable coordination bonds.

Coordination compounds can alter the properties of the original ions significantly. This is because the complex ions formed are typically more soluble in solution than the original salts. Understanding complex ion formation helps chemists manipulate solubility and other chemical properties effectively.

Solubility of Salts

The solubility of salts can be significantly influenced by their ability to form complex ions. Normally, salts like silver chloride are considered insoluble in water. However, when certain ligands are present, they can enhance solubility by forming complex ions.

In our exercise, silver chloride, though insoluble on its own, dissolves in the presence of excess ammonium hydroxide. This happens because silver ions form a complex with ammonia, [Ag(NH₃)₂]⁺. Complex ion formation increases solubility by stabilizing the ions in solution, allowing more ions to dissolve.

This principle is used widely in various fields of chemistry to dissolve substances otherwise deemed insoluble, making chemical reactions feasible by maintaining necessary concentrations of reactants in solution.

Ammonia as a Ligand

Ammonia is a common ligand in coordination chemistry. Its ability to donate a lone pair of electrons allows it to form stable complexes with transition metals. In this particular exercise, ammonia interacts with silver ions to form a complex ion.

When Ag⁺ meets NH₃, the ammonia molecules contribute their lone pairs to the electron-deficient orbital of silver, creating the diamminesilver(I) complex, [Ag(NH₃)₂]⁺. The formation of these coordination bonds with ammonia drastically changes the properties of the silver ion, such as its solubility and reactivity.

This property of ammonia makes it incredibly useful as a ligand in chemical synthesis, helping to stabilize metal ions in solution. By understanding how ammonia and similar ligands function, chemists can design new compounds and materials with tailored properties.