Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

Ammonia forms the complex ion \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\) with copper ions in the alkaline solutions, but not in acidic solutions. What is the reason for it?

Step-by-Step Solution

Answer

Ammonia forms a complex with copper only in alkaline conditions because, in acidic solutions, ammonia is converted to ammonium and cannot act as a ligand.

Step 1: Understand the Complex Ion Formation

The complex ion \([\mathrm{Cu}(\mathrm{NH}_{3})_{4}]^{2+}\) is formed when copper ions \(\mathrm{Cu}^{2+}\) coordinate with ammonia molecules \(\mathrm{NH}_{3}\). This occurs in alkaline solutions where ammonia is present and can interact with copper ions to form the complex.

Step 2: Analyze the Role of Ammonia

Ammonia acts as a ligand, available to bind with \(\mathrm{Cu}^{2+}\) ions only when it's in a free form, which is the case in alkaline solutions. In alkaline conditions, ammonia doesn't readily react with other ions present, maintaining its availability to form complexes.

Step 3: Consider the Conditions in Acidic Solutions

In acidic solutions, ammonia molecules (\(\mathrm{NH}_{3}\)) react with \(\mathrm{H}^{+}\) ions to form ammonium ions (\(\mathrm{NH}_{4}^{+}\)). This reduces the concentration of free ammonia molecules that are necessary to coordinate with \(\mathrm{Cu}^{2+}\), making complex formation less favorable.

Step 4: Summarize the Impact of Acidic Conditions

Since ammonia gets protonated to form \(\mathrm{NH}_{4}^{+}\) in acidic solutions, it is not available to act as a ligand for copper ions. Thus, the necessary conditions for the formation of \([\mathrm{Cu}(\mathrm{NH}_{3})_{4}]^{2+}\) are not met in an acidic environment.

Key Concepts

Copper-Ammonia Complex

Copper ions, when surrounded by ammonia molecules, can form a distinctive complex ion known as \([\mathrm{Cu}(\mathrm{NH}_{3})_{4}]^{2+}\). This complex is notable for its stability and deep blue color. The process occurs when copper(II) ions \(\mathrm{Cu}^{2+}\) coordinate with ammonia \(\mathrm{NH}_{3}\), a type of weak base. Formation of this complex is common in alkaline solutions because the environment supports the presence of non-protonated ammonia molecules. In the \(\text{[Cu(NH}_3\text{)}_4\text{]}^{2+}\) complex, four ammonia molecules occupy the coordination sites around a central copper ion. This arrangement not only stabilizes the copper ion in solution but also alters its color and chemical properties.

• Alkaline condition needed for formation.
• Visible color change to blue indicates the complex formation.
• Structure involves ammonia acting as a ligand around copper ions.

Role of Ammonia as Ligand

Ammonia plays a crucial role as a ligand due to its lone pair of electrons, which makes it capable of forming bonds with metal ions like copper. In the complex \[\text{[Cu(NH}_3\text{)}_4\text{]}^{2+}\], each ammonia molecule donates a lone pair to the copper ion, forming a coordinate covalent bond. This transformation is possible only when ammonia exists in its free form, which primarily happens in basic, or alkaline, solutions.
In acidic solutions, ammonia undergoes protonation becoming ammonium ions (\(\mathrm{NH}_{4}^{+}\)), thereby losing its ability to act as a ligand. Therefore, the essential interaction between ammonia and copper ions for complex formation is inhibited under such conditions. The role of ammonia is pivotal since the formation of the complex dramatically changes the solubility and stability parameters of the copper solution, enhancing our understanding of coordination chemistry.

• Ammonia ligands contribute electrons to form stable complexes.
• Lone pair donation is key in the ligand process.
• Ammonium formation in acidic solutions prevents ligand action.

Effect of pH on Complex Formation

The formation of complex ions like \(\mathrm{[Cu(NH}_{3}\mathrm{)}_{4}]^{2+}\) heavily depends on the pH of the solution. In alkaline solutions, ammonia is present as a free molecule that can easily coordinate with copper ions to form the complex. There are minimal hydrogen ions \(\mathrm{H}^{+}\) present to react with ammonia, thanks to the higher pH, and thus ammonia remains available to act as a ligand.
In contrast, in acidic solutions, the abundance of hydrogen ions leads to the conversion of ammonia into ammonium ions \(\mathrm{NH}_{4}^{+}\). This protonation reaction removes the ammonia from the ligand pool necessary for complex formation. Therefore, understanding the effect of pH is crucial to controlling and predicting the behavior of such metal complexes in different environments.

• Alkaline pH supports complex formation.
• Acidic pH results in ammonia protonation, inhibiting complex formation.
• pH level determines the availability of ammonia as a ligand.