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

(b) \([\mathrm{Ag(NH}_3)_2]^+\) is formed, making \(\mathrm{AgCl}\) soluble.

Step 1: Understanding the Problem

We need to find out which compound is formed when \(\mathrm{AgCl}\) dissolves in \(\mathrm{NH}_{4} \mathrm{OH}\). This occurs because \(\mathrm{AgCl}\) reacts with \(\mathrm{NH}_{4} \mathrm{OH}\) to form a complex or another compound.

Step 2: Identify Possible Reactions

When \(\mathrm{AgCl}\) is mixed with \(\mathrm{NH}_{4} \mathrm{OH}\), \(\mathrm{NH}_{3}\) from the \(\mathrm{NH}_{4} \mathrm{OH}\) can complex with \(\mathrm{Ag}^{+}\) ion to form a diamine complex, \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\).

Step 3: Evaluate the Stability of Products

\(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\) is a stable complex ion that forms when \(\mathrm{AgCl}\) dissolves in \(\mathrm{NH}_{4} \mathrm{OH}\) due to the reaction of \(\mathrm{Ag}^{+}\) and \(\mathrm{NH}_{3}\). This makes the \(\mathrm{AgCl}\) soluble.

Step 4: Choose the Correct Option

From the options given, (b) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\) matches the identified process in Step 3. This is the reason for \(\mathrm{AgCl}\)'s solubility in \(\mathrm{NH}_{4} \mathrm{OH}\).

Key Concepts

AgCl solubility

The solubility of silver chloride (\(\mathrm{AgCl}\)) in water is very low. Silver chloride is known as a sparsely soluble salt. However, its solubility can increase under certain conditions. When \(\mathrm{AgCl}\) is placed in an aqueous solution that contains \(\mathrm{NH}_4\mathrm{OH}\) (ammonium hydroxide), it becomes more soluble. This happens because \(\mathrm{AgCl}\) can form a complex with ammonia (\(\mathrm{NH}_3\)) which is derived from ammonium hydroxide. In this interaction, \(\mathrm{NH}_3\) forms a coordination complex with the silver ions from \(\mathrm{AgCl}\).
As a result, \(\mathrm{AgCl}\) dissolves in the solution.

• This process is typical for metal salts that are known to form coordination complexes with ligands.
• By forming a complex, the concentration of free \(\mathrm{Ag}^+\) ions in solution decreases, which leads to the dissolution of more \(\mathrm{AgCl}\).

complex ion formation

Complex ion formation is a chemical process where transition metals bond with other molecules or ions, called ligands. When \(\mathrm{AgCl}\) dissolves in \(\mathrm{NH}_4\mathrm{OH}\), it results in the formation of a complex ion, \([\mathrm{Ag}(\mathrm{NH}_3)_2]^+\).
Silver ions \(\mathrm{Ag}^+\) act as the central metal ion, and the ammonia molecules \(\mathrm{NH}_3\) serve as the ligands. Each ammonia molecule donates a pair of electrons to the silver ion, forming a stable bond.

• This association increases the solubility of \(\mathrm{AgCl}\) because the formation of the complex reduces the concentration of \(\mathrm{Ag}^+\) ions in solution, driving further dissolution of the solid.
• Complex ions are often more soluble than their constituent salts.
• This principle is key in modulating the solubility of many ionic compounds through changes in chemical environment.

Silver amine complex

The silver amine complex, \([\mathrm{Ag}(\mathrm{NH}_3)_2]^+\), is a typical example of how ligands can drastically alter the behavior and properties of a metal ion.
In this complex, each silver ion is surrounded by two ammonia molecules. The formation of this complex is due to the strong attractions between the silver ion and the lone pairs of electrons available on the \(\mathrm{NH}_3\) molecules.

• These interactions are known as coordinate covalent bonds, where the same atomic entity (\(\mathrm{NH}_3\)) supplies both electrons for the bonding.
• This stable arrangement explains why \(\mathrm{AgCl}\) becomes soluble in the presence of \(\mathrm{NH}_4\mathrm{OH}\).
• The silver amine complexes are very stable and are useful in various chemical processes including analytical chemistry to detect silver ions and in photography due to their light-sensitive nature.