Problem 46 - A Complete Resource Book in Chemistry for JEE Main

Question

When excess of dilute \(\mathrm{NH}_{4} \mathrm{OH}\) is added to an aqueous solution of \(\mathrm{CuSO}_{4}\) an intense blue colour is obtained. This blue colour is due to the presence of (a) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\) (b) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\) (c) \(\mathrm{CuSO}_{4}\) (d) \(\mathrm{Cu}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

(a) The blue color is due to the [Cu(NH3)4]^{2+} complex ion.

Step 1: Reactants Identification

Identify the chemical species involved in the reaction. We have aqueous copper sulfate (CuSO4) and ammonium hydroxide (NH4OH). Upon adding excess NH4OH, the transition metal copper forms a complex.

Step 2: Complex Formation

Understand that copper can form coordination complexes. When NH4OH is added to CuSO4, the copper ions react with ammonia (NH3) from the dissociation of NH4OH creating a complex with intense blue color.

Step 3: Write Chemical Equation

Write the balanced chemical equation for the complex formation:\[\text{Cu}^{2+} + 4 \text{NH}_3 \rightleftharpoons [\text{Cu(NH}_3{)}_4]^{2+}\]This represents the formation of tetraamminecopper(II) ion.

Step 4: Identify Color Source

The intense blue color is due to the [Cu(NH3)4]^{2+} complex ion which is known for its blue color in solution.

Step 5: Choose Correct Option

From the given options, the intense blue color is caused by the [Cu(NH3)4]^{2+} complex, matching option (a).

Key Concepts

Complex Ion Formation

Complex ion formation involves the combination of a central metal ion with ligands, which are molecules or ions that can donate a pair of electrons to form a coordinate covalent bond. This process is significant in chemistry, particularly when dealing with transition metals. Upon the reaction of aqueous copper sulfate (\( ext{CuSO}_4 \)) with ammonium hydroxide (\( ext{NH}_4 ext{OH} \)), copper ions (\( ext{Cu}^{2+} \)) interact with ammonia (\( ext{NH}_3 \)) molecules, resulting in a coordination complex.

In forming these complex ions, the metal serves as the Lewis acid, meaning it accepts electron pairs. The ligands, on the other hand, act as Lewis bases by donating electron pairs. This interaction leads to the formation of a stable complex ion. These interactions are essential not only in explaining the vivid colors associated with metal solutions but also in various biological and industrial processes.

Some important points about complex ions include:

• They can involve different geometries depending on the number and type of ligands.
• The strength and color of these complexes can change based on the ligands involved.

Understanding the formation of complex ions allows us to predict and explain certain reactions, such as the vibrant colors seen in aqueous solutions involving transition metals.

Tetraamminecopper(II) Ion

The tetraamminecopper(II) ion, represented as \( [\text{Cu(NH}_3)_4]^{2+} \), is formed when copper ions (\( ext{Cu}^{2+} \)) are bound to four ammonia (\( ext{NH}_3 \)) molecules. This specific ion is responsible for the striking deep blue coloration seen in an aqueous solution.

The creation of \( [\text{Cu(NH}_3)_4]^{2+} \) occurs when excess ammonia is introduced to copper sulfate, prompting a series of reactions:

• The copper ion in the solution reacts with ammonia, which acts as a ligand by donating electron pairs.
• Four ammonia molecules surround the copper ion, leading to a stable four-coordinate complex.
• This complex is what gives the solution its intense blue hue, distinguishing it from other potential reactions.

This particular compound showcases the intriguing and often beautiful outcomes of ligand-metal interactions, demonstrating a key aspect of transition metal chemistry that enhances our understanding of chemical behavior and aesthetic properties of compounds.

Transition Metal Chemistry

Transition metal chemistry is a fascinating area of study due to the unique properties of transition metals in forming complex ions and coordination compounds. Transition metals, such as copper in the \( ext{CuSO}_4 \) solution, often have multiple oxidation states and the ability to form various complex geometries.

One of the distinctive features of transition metals is their ability to produce colored compounds. This is due to specific electron transfers between d-orbitals when ligands form coordinate bonds with the metal ions. In the case of tetramminecopper(II) ion formation, the complexing process introduces a new arrangement of electrons, leading to its observed color.

Key characteristics of transition metal chemistry include:

• Variable oxidation states allowing diverse chemical behavior.
• Formation of colored ions due to electron d-d transitions.
• Ability to form stable complexes, which have significant roles in catalysis and biological systems.

These metals, through their versatile chemistry, play crucial roles not just in laboratories, but in real-world applications such as in catalysis, materials science, and even medicine. Understanding these principles is essential for exploring and utilizing the broad potential of transition metal chemistry.