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

The blue color is due to \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\), so the answer is (a).

Step 1: Identify the Reaction Involved

When \(\mathrm{NH}_{4} \mathrm{OH}\) is added to \(\mathrm{CuSO}_{4}\), an important chemical reaction occurs between ammonia, \(\mathrm{NH}_{3}\), and copper ions, \(\mathrm{Cu}^{2+}\), to form a complex ion.

Step 2: Determine the Complex Ion

When ammonia is added to the aqueous \(\mathrm{CuSO}_{4}\) solution, \(\mathrm{Cu}^{2+}\) ions react with \(\mathrm{NH}_{3}\) molecules to form the complex ion \[\left[ \mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\]. This complex is responsible for the intense blue color.

Step 3: Determine the Correct Option

Match the complex ion responsible for the blue color to the given options. Option (a) \[\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\]\ is the correct ion that causes the blue color.

Key Concepts

Complex Ion Formation

Complex ion formation is a fascinating area of chemistry where metal ions bond with molecules or ions, known as ligands, to form complex ions. This process is essential in understanding many chemical reactions in coordination chemistry. When a metal ion, such as copper (\(\text{Cu}^{2+}\)), encounters ammonia (\(\text{NH}_3\)) in a solution, the ammonia molecules act as ligands. They donate electron pairs to the metal ion, forming a coordinate covalent bond.

• This interaction leads to the creation of a new species, the complex ion.
• Complex ions often exhibit distinct colors, resulting from electronic transitions within the partly filled d-orbitals of metal ions.

The unique properties of complex ions, including their color and stability, make them significant in various chemical processes. Understanding these ions helps in recognizing the behavior of transition metals in different environments.

Ammonia Complexes

Ammonia complexes are significant examples of complex ions where ammonia, a small molecule with a lone pair of electrons on the nitrogen atom, acts as a ligand. It can easily coordinate with transition metal ions due to its ability to donate an electron pair. Ammonia complexes are particularly well-known because they demonstrate spectacular changes in the properties of solutions, such as color:

• Ammonia ligands replace water molecules around a metal ion, altering the electronic properties of the metal center.
• This substitution often results in the appearance of vibrant colors, exemplifying the dramatic effect ligands can have on complex ions.

Ammonia complexes illustrate the dynamic nature of coordination chemistry and the role of ligand exchange in influencing metal ion characteristics.

Copper-Ammonia Complex

The copper-ammonia complex is a classic case illustrating the formation and properties of complex ions. When copper sulfate (\(\text{CuSO}_4\)) dissolves in water, it produces \(\text{Cu}^{2+}\) ions. Upon the addition of ammonia, these \(\text{Cu}^{2+}\) ions interact with ammonia molecules to form the copper-ammonia complex ion, \(\left[ \text{Cu}\left(\text{NH}_3\right)_4\right]^{2+}\).This complex ion is responsible for an intense blue color due to the ligand ammonia altering the copper ion's electronic structure:

• The \(\text{d}\)-orbital splitting and electron transitions within these orbitals cause the characteristic blue hue.
• This reaction showcases how ligand binding can impact the visual and chemical behavior of metal ions.

Studying copper-ammonia complexes aids in understanding the broader principles of coordination chemistry, particularly the influence of ligands on metal ions and their respective properties.

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and making of chemical bonds, often resulting in new products with distinct properties. The reaction between ammonia and copper sulfate is a classic example showcasing complex formation:

• In this case, \(\text{NH}_3\) molecules in solution react with \(\text{Cu}^{2+}\) ions.
• This results in the formation of the complex ion \(\left[ \text{Cu}\left(\text{NH}_3\right)_4\right]^{2+}\), responsible for the characteristic blue color seen in the solution.

Such chemical transformations are important for identifying changes in the coordination environment around metal ions.Understanding these reactions helps in comprehending the diverse nature of chemistry and how substances interact at the molecular level to form products with unique qualities. Through this lens, the dynamic nature of chemistry and its application in real-world scenarios becomes clearer.