Problem 19 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When concentrated \(\mathrm{HCl}\) is added to an aqueous solution of \(\mathrm{CoCl}_{2}\), its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction? (a) \(\left[\mathrm{CoCl}_{4}\right]^{2-}\) (b) \(\left[\mathrm{CoCl}_{6}\right]^{3-}\) (c) \(\left[\mathrm{CoCl}_{6}\right]^{4-}\) (d) \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\)

Step-by-Step Solution

Answer

(a) \( \left[\text{CoCl}_4\right]^{2-} \)

Step 1: Understand the Chemical Reaction

When concentrated HCl is added to an aqueous \( \text{CoCl}_2 \) solution, Cl- ions become abundant. These ions react with the Co(II) ions in the solution and facilitate a change in the coordination sphere of the cobalt complexes.

Step 2: Identify the Initial Complex

In an aqueous solution of \( \text{CoCl}_2 \), the initial complex ion is \( \left[\text{Co}\left(\text{H}_2\text{O}\right)_6\right]^{2+} \), which is responsible for the reddish-pink color.

Step 3: Determine the Change Due to Concentrated HCl

The addition of concentrated HCl increases the concentration of Cl- ions, which leads to the removal of water ligands and the coordination of chloride ions to the cobalt center. This changes the coordination complex from \( \left[\text{Co}\left(\text{H}_2\text{O}\right)_6\right]^{2+} \) to one containing chloride ligands.

Step 4: Identify the New Complex

The change in color from reddish-pink to deep blue indicates the formation of the complex ion \( \left[\text{CoCl}_4\right]^{2-} \). This tetrahedral complex is known for exhibiting a deep blue color.

Step 5: Select the Correct Option

Based on the color change and the known properties of the complexes, the complex ion \( \left[\text{CoCl}_4\right]^{2-} \) (Option a) is responsible for the deep blue color observed after the addition of concentrated HCl.

Key Concepts

Complex Ions

Complex ions are fascinating entities formed when metal cations bond with surrounding molecules or ions, called ligands. These ligands donate electron pairs to the metal, creating a coordinate bond. This unique formation allows metals to exhibit properties that are essential in both chemical and biological systems. For cobalt in an aqueous solution of \(\mathrm{CoCl}_2\), the initial complex formed is \([\mathrm{Co} (\mathrm{H}_2\mathrm{O})_6]^{2+}\). Here, six water molecules act as ligands, each donating a pair of electrons to the cobalt ion.

• Complex ions often display interesting visual properties, such as distinctive colors.
• The type and number of ligands affect not just the color but also the structural geometry of the complex.
• Cobalt complexes are known for their vibrant color changes when different ligands are introduced.

Understanding complex ions is essential for grasping the behavior of transition metals in various environments, including catalysis and material science.

Ligand Exchange

Ligand exchange is a process where ligands in a complex ion are replaced by different ones, changing the properties of the complex. This phenomenon is observed when concentrated \(\mathrm{HCl}\) is added to the \(\mathrm{CoCl}_2\) solution, initiating a ligand exchange reaction. The chloride ions introduced by the \(\mathrm{HCl}\) solution replace some or all of the water ligands initially present.

• In this reaction, water molecules in \([\mathrm{Co} (\mathrm{H}_2\mathrm{O})_6]^{2+}\) are replaced by chloride ions.
• This leads to the formation of a new complex, \([\mathrm{CoCl}_4]^{2-}\), which changes the observable color of the solution.
• The mechanism of ligand exchange depends on many factors, including temperature, concentration, and the nature of the ligands involved.

Ligand exchange is crucial in coordination chemistry due to its impact on the stability and reactivity of metal complexes.

Color Change in Solutions

The color change in a solution is an enchanting indicator of a chemical transformation, often signaling a ligand exchange or other structural rearrangements within a complex. When \(\mathrm{HCl}\) is added to the \(\mathrm{CoCl}_2\) solution, the observed change from reddish-pink to deep blue can be attributed to the formation of a different complex ion.

• The original pink color arises from \([\mathrm{Co} (\mathrm{H}_2\mathrm{O})_6]^{2+}\), where water ligands contribute to the emission of light at specific wavelengths.
• Upon ligand exchange, the tetrahedral \([\mathrm{CoCl}_4]^{2-}\) complex forms, reflecting and absorbing light differently, resulting in the deep blue hue.
• Color changes can also reflect changes in the electronic configuration of the metal center.

Recognizing color changes and understanding their causes is an integral skill in chemistry, aiding in the identification of chemical species and their transformations.