Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

The pair of which salts is expected to have the same colour in their freshly prepared aqueous solutions. (a) \(\mathrm{VOCl}_{2}, \mathrm{CuCl}_{2}\) (b) \(\mathrm{CuCl}_{2}^{2}, \mathrm{FeCl}_{2}\) (c) \(\mathrm{FeCl}_{2}, \mathrm{VOCl}_{2}\) (d) \(\mathrm{MnCl}_{2}, \mathrm{FeCl}_{2}^{2}\)

Step-by-Step Solution

Answer

(a) \( \mathrm{VOCl}_{2} \) and \( \mathrm{CuCl}_{2} \) both have blue solutions.

Step 1: Identify the Central Ion of Each Salt

First, identify the metal ion in each of the given salts. This will help us compare the characteristics of their aqueous solutions. The salts in the pairs given are: - \( \mathrm{VOCl}_{2} \) with a central ion of \( \mathrm{V}^{2+} \)- \( \mathrm{CuCl}_{2} \) with a central ion of \( \mathrm{Cu}^{2+} \)- \( \mathrm{FeCl}_{2} \) with a central ion of \( \mathrm{Fe}^{2+} \)- \( \mathrm{MnCl}_{2} \) with a central ion of \( \mathrm{Mn}^{2+} \)

Step 2: Understand Color of Aqueous Solutions

The color in aqueous solutions is primarily due to the d-electrons in the transition metal ions. Specifically, it’s the d-d transition that often results in color. The presence of water as a ligand in the d-subshell transitions results in characteristic colors:- \( \mathrm{V}^{2+} \) is typically blue color.- \( \mathrm{Cu}^{2+} \) is typically blue as well.- \( \mathrm{Fe}^{2+} \) often forms a pale green solution.- \( \mathrm{Mn}^{2+} \) is usually faint pink.

Step 3: Compare Colors of Each Pair of Salts

Now we will compare the colors of the salts in each pair:- Pair (a): \( \mathrm{VOCl}_{2} \) and \( \mathrm{CuCl}_{2} \) both produce blue aqueous solutions.- Pair (b): \( \mathrm{CuCl}_{2}^{2} \) and \( \mathrm{FeCl}_{2} \) show different colors, blue and pale green respectively.- Pair (c): \( \mathrm{FeCl}_{2} \) and \( \mathrm{VOCl}_{2} \) show different colors, pale green and blue respectively.- Pair (d): \( \mathrm{MnCl}_{2} \) and \( \mathrm{FeCl}_{2}^{2} \) show different colors, faint pink and pale green respectively.

Step 4: Determine the Correct Pair

From the comparison, it's clear that the pair consisting of \( \mathrm{VOCl}_{2} \) and \( \mathrm{CuCl}_{2} \) (option a) are expected to have the same (blue) color in their freshly prepared aqueous solutions.

Key Concepts

Transition Metal Chemistry

Transition metals are elements found in the d-block of the periodic table. They are well-known for their ability to form colorful compounds due to their d-orbitals. These metals include elements like iron (Fe), copper (Cu), manganese (Mn), and vanadium (V), each having different electron arrangements.
This unique characteristic allows them to bond with various ligands, leading to distinct colors in compounds or solutions.
One major reason behind the colors is the complex electronic structures of these ions, particularly the partially filled d-orbitals.
In simple terms:

• Transition metals can exhibit different oxidation states, meaning they can donate different numbers of electrons.
• The variable oxidation states lead to various electronic configurations, contributing to different colors.

Understanding these properties is key to predicting the colors of transition metal solutions.

Aqueous Solutions

An aqueous solution is simply when a substance is dissolved in water. For transition metals, when they dissolve, the water acts as a ligand, surrounding the metal ion and forming what is called a coordination complex.
This complexation can significantly affect the color of the solution because it alters the electronic configuration of the central metal ion.
Each central metal ion interacts differently with water, leading to unique colors for each metal ion in solution. For example:

• Copper (II) Chloride: Dissolves to form a blue solution.
• Iron (II) Chloride: Produces a pale green solution when dissolved.
• Vanadium (II) Chloride: Results in a blue solution.
• Manganese (II) Chloride: Yields a faint pink solution.

The specific geometric arrangement of water molecules around the metal ion in an aqueous solution can shift absorbance and emissions of light, giving rise to visible color.

d-d Transitions

The beautiful colors in transition metal aqueous solutions are often due to a phenomenon called d-d transitions. These transitions occur within the d-orbitals of the metal ions when they absorb specific wavelengths of light.
If we consider the molecular orbital theory, transition metals have five d-orbitals that may have different energy levels when surrounded by ligands like water.

• D-d transitions happen when an electron in a lower energy d-orbital absorbs light and jumps to a higher energy d-orbital.
• The particular wavelength of light (color) absorbed depends on the difference in energy between these d-orbitals.

Since different transition metals and their complexes have unique arrangements of d-orbitals, they absorb various portions of the visible light spectrum, resulting in diverse colors. This makes understanding these transitions crucial for predicting the appearance of transition metal compounds.