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? (a) in acidic solutions, hydration protects copper ions (b) the acidic solutions, protons coordinate with ammonia molecules forming \(\mathrm{NH}_{4}^{+}\)ions and \(\mathrm{NH}_{3}\) molecules are not available (c) in alkaline solutions, insoluble \(\mathrm{Cu}(\mathrm{OH})_{2}\) is precipitated which is soluble in excess of any alkali (d) copper hydroxide is an amphoteric substance

Step-by-Step Solution

Answer

(b) In acidic solutions, NH₄⁺ is formed, reducing available NH₃ for complex formation.

Step 1: Understanding the complex formation

Ammonia acts as a ligand and forms a complex ion with copper ions. This complex is stable in alkaline solutions.

Step 2: Role of acidic solutions

In acidic solutions, there is an excess of protons (H⁺ ions). Ammonia, a weak base, tends to react with these protons to form ammonium ions (NH₄⁺), thus reducing the number of free ammonia molecules available to complex with copper ions.

Step 3: Effect of pH on ammonia and copper ion interaction

In alkaline solutions, ammonia remains as NH₃ because there are fewer protons available to interact with, allowing NH₃ to complex with copper to form the \([\text{Cu(NH}_3\right)_4]^{2+}\) ion. In contrast, in acidic solutions, the ammonia is converted to NH₄⁺, preventing the formation of the complex.

Step 4: Analyzing the options

Option (b) describes the chemical interaction correctly: in acidic solutions, protons interact with ammonia to form NH₄⁺ ions, preventing NH₃ from forming complexes with copper ions.

Key Concepts

Complex Ion Formation

In coordination chemistry, **complex ion formation** refers to the process by which central metal ions become surrounded by molecules or ions known as ligands. These groups can donate lone electron pairs to the metal ion, forming a coordinate covalent bond. This specific interaction is crucial for the stability of the resulting complex ion.

Typically, when metal ions form complex ions, they are stabilized and become less reactive in their surrounding environment. The nature of the ligands, the central metal ion, and the specific conditions such as temperature and pH can affect the formation and stability of these complexes.

• Central metal ions like copper can bind to several ligands to form stable structures.
• The extent of complex ion formation can greatly depend on the properties of both the ligand and the ion involved.

Ammonia as Ligand

Ammonia (NH₃) is a well-known ligand capable of forming strong bonds with metal ions through its nitrogen atom. As a ligand, ammonia donates its lone pair of electrons to create stable complexes with various metal ions.

Unlike other common ligands, ammonia does not bring additional charge to the complex. This neutrality can be advantageous in many reactions, as it provides stability while also allowing specific reaction pathways to occur. Complexes involving ammonia are often found across a range of pH conditions, being influenced by the medium in which they are present:

• Ammonia is versatile and can form complexes with a wide range of metal ions.
• The nitrogen atom in ammonia serves as the electron donor to the complex.

Its neutrality and ability to stabilize metal ion charge make it a preferred ligand in various chemical processes.

Effect of pH on Complexes

The **effect of pH on complexes** is critical in determining the stability and formation of complex ions. pH levels influence the availability and state of ligands like ammonia. In the presence of different ionic concentrations, the pH can change the way these molecules behave.

When solutions are acidic, a high concentration of protons (H⁺) can lead to competitive binding. Ammonia in acidic solutions tends to convert into ammonium ions (NH₄⁺) due to these protons. This conversion is pivotal as it lowers the free NH₃ concentration available to bond with metal ions like copper. On the contrary, in alkaline solutions, protons are scarce, allowing ammonia to remain unprotonated and capable of forming complexes.

• High proton availability in acidic conditions drives ammonia conversion to NH₄⁺.
• In alkaline solutions, less competition leaves ammonia available for complexation.

This dynamic underscores the importance of pH in complex formation and illustrates how chemical equilibria can shift based on environmental conditions.

Copper-Ammonia Complex

The **copper-ammonia complex**, represented by \([ ext{Cu(NH}_3\right)_4]^{2+}\), is a remarkable example of how ammonia can act as a ligand to a metal ion. In this complex, four ammonia molecules coordinate with a copper ion to form a stable, deep blue-colored complex under alkaline conditions.

Copper ions naturally seek stable environments by forming complexes. Under acidic conditions, however, the formation of copper-ammonia complexes is hindered due to the reasons explained in the previous sections, primarily because of the conversion of ammonia to ammonium ions, which then cannot engage in forming the complex.

• Stable and forms a vivid blue complex in alkaline solutions.
• Not possible to form effectively in acidic solutions due to the absence of free ammonia.

This complex is used in various chemical applications due to its distinct color and stability, serving as a quintessential example of the delicate balance of chemistry and the critical role of the ligand.