Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{AgCl}\) precipitate dissolves in \(\mathrm{NH}_{4} \mathrm{OH}\) due to the formation of (a) \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}\) (b) \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{OH}\) (c) \(\mathrm{Ag}\left(\mathrm{NH}_{4}\right)_{2} \mathrm{Cl}\) (d) \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

The correct answer is (a) \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\mathrm{Cl}\).

Step 1: Understand Precipitation Reaction

Silver chloride (\(\mathrm{AgCl}\)) is an insoluble salt that forms a white precipitate when silver ions come into contact with chloride ions in a solution.

Step 2: Examining Ammonia Reaction with Silver Ions

When \(\mathrm{NH}_{4}\mathrm{OH}\) is added, it acts as a source of \(\mathrm{NH}_{3}\) (ammonia). The \(\mathrm{NH}_{3}\) can react with \(\mathrm{Ag}^{+}\) ions to form a complex ion.

Step 3: Formation of Complex Ion

\(\mathrm{Ag}^{+}\) ions form a complex with ammonia, resulting in \([\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}]^+\). This species is soluble in water, which helps dissolve the previously insoluble \(\mathrm{AgCl}\) precipitate.

Step 4: Determine the Correct Option

The complex ion formed is \([\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}]^+\) which combines with chloride ions in solution to form \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\mathrm{Cl}\) which matches option (a).

Key Concepts

Complex Ion Formation

In the world of coordination chemistry, complex ion formation is a fascinating phenomenon. At its core, a complex ion is a charged species consisting of a central metal ion surrounded by molecules or ions, called ligands. These ligands are typically negatively charged or neutral molecules that have lone pairs of electrons that can be shared with the metal ion through coordinate bonds.
Consider silver ions (g^+]) in solution. When ammonia (H_3]) is introduced, it acts as a ligand. When silver ion and ammonia meet, they form a complex ion with the ammonia serving as a ligand: [[g(NH_3)_2]]^+]. This reaction involves the donation of nitrogen's lone pair of electrons to the silver ion, creating a stable coordination complex. It transforms the behavior of the metal ion, often enhancing solubility or altering its reactivity.
Because these complex ions can change the solubility and color of the solution, understanding their formation is crucial in analyzing chemical reactions, especially in solutions.

• Key players: Metal ion and ligands
• The result is a soluble complex ion
• Ammonia provides electron pairs for bonding

Solubility of Precipitates

Solubility often dictates the visibility of a chemical reaction via the formation or dissolution of precipitates in a solution. Precipitates are solid particles that form when the product of a reaction is insoluble in the solution.
Silver chloride (gCl]), for example, is known for its limited solubility in water, readily forming a white precipitate in the presence of a silver ion (g^+]) and a chloride ion (l^-]). This process can be reversed with the intervention of complex ion formation. By adding a compound like ammonia (H_3]), silver ions are trapped in complex ions ([g(NH_3)_2]]^+]), enhancing the solubility of the previously insoluble gCl] precipitate.
Solubility changes caused by complex ion formation are often utilized in industrial and laboratory settings to dissolve substances that otherwise would not dissolve easily.

• Formation of precipitates indicates low solubility
• Complex ions can increase the solubility of these precipitates
• Used for purification and separation processes

Silver-Ammonia Complex

The silver-ammonia complex is a key player in understanding reactions involving silver salts and ammonia. When ammonia is added to a solution containing silver ions, silver forms a soluble complex [g(NH_3)_2]]^+].
This transformation relies on ammonia serving as a ligand to bind with silver ions through coordinate bonds, resulting in a significant change in properties. Initially, silver chloride (gCl]) forms a white precipitate, but upon the addition of ammonia, the insoluble gCl] dissolves as [g(NH_3)_2]]^+] complex ions form. In essence, the silver ion's affinity for ammonia makes this process feasible by enhancing solubility and facilitating the dissolution of the precipitate.
The formation of this silver-ammonia complex is not only academically intriguing but also practical for applications such as analytical detection and purification of silver ions.

• Ammonia as a ligand transforms solubility properties
• Re-ligated silver ion with ammonia yields a soluble complex
• Commonly leveraged in chemical analysis