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) \(\mathrm{CuCl}_{2}, \mathrm{FeCl}_{2}\) (c) \(\mathrm{FeCl}_{2}, \mathrm{VOCl}_{2}\) (d) \(\mathrm{MnCl}_{2}, \mathrm{FeCl}_{2}\)

Step-by-Step Solution

Answer

The pair
aVOCl_{2} and
aCuCl_{2} (option a) will have the same color.

Step 1: Understanding the Query

We need to determine which pair of salts forms aqueous solutions with the same color. Transition metal ions typically impart a characteristic color to their solutions, depending on the metal and its oxidation state.

Step 2: Determining Color of CuCl_{2} and FeCl_{2}

The solution of CuCl_{2}, which contains Cu^{2+} ions, is typically blue or green.
aFeCl_{2} contains
aFe^{2+} ions and usually forms a pale green solution.

Step 3: Analyzing
aVOCl_{2}

aVOCl_{2} contains
aVO^{2+} ions, which generally give a purple or blue-colored solution, resembling the expected color for
aCuCl_{2} aqueous solutions.

Step 4: Comparing
aFeCl_{2} Against Other Salts

Given the color of
aFeCl_{2} (pale green), compare it with others. Notably,
aMnCl_{2} forms a faint pink solution, different from
aFeCl_{2}.

Step 5: Conclusion Based on Observations

Comparing the solutions of
aVOCl_{2} and
aCuCl_{2} reveals they both can form solutions with a blue hue, unlike the other pairs examined.

Key Concepts

Aqueous Solution Coloring

In the world of chemistry, particularly when studying transition metals, one of the fascinating phenomena is the coloring of aqueous solutions. This coloration occurs because transition metal ions absorb visible light, promoting electrons to higher energy levels. The color we perceive is the complementary color of the absorbed light.
For instance, a solution of copper(II) chloride, \( \text{CuCl}_2 \), is typically blue or green because it absorbs light in the red to yellow part of the spectrum. Vanadium(II) chloride, \( \text{VOCl}_2 \), in aqueous solution, also shows a blue or purple color caused by similar absorption properties in slightly different spectral regions.
To distinguish these colors in solutions, it is essential to understand the behavior of electrons in various oxidation states and how they interact with light.

Oxidation States

The concept of oxidation states is crucial when analyzing the color of solutions containing transition metal ions. Oxidation states refer to the degree of oxidation of an element, represented by integers. These states profoundly affect the electronic structure and, consequently, the physical properties such as color.
For transition metals like copper and iron, their common oxidation states, \( \text{Cu}^{2+} \) and \( \text{Fe}^{2+} \), result in them having distinct colors in solution. Copper chloride \( \text{CuCl}_2 \) in the +2 oxidation state exhibits a characteristic blue-green color. On the other hand, iron chloride \( \text{FeCl}_2 \) with \( \text{Fe}^{2+} \) ions, appears pale green.

• The electron configuration of the metal ion is affected by its oxidation state, thus influencing the color.
• Different oxidation states can shift the wavelengths of absorbed light, leading to varied perceptions of color.

Understanding oxidation states is fundamental to predicting the expected colors of transition metal solutions.

Transition Metal Ions

Transition metal ions are unique due to their d-orbital electrons, which are primarily responsible for their distinct characteristics. Due to their partially filled d-subshells, transition metals display variable oxidation states and colorful compounds.
The ions in solution interact with surrounding water molecules via their electric fields. This interaction, known as complex formation, affects the energy levels of the d-orbitals. For example:

• \( \text{Cu}^{2+} \) ions in water lead to blue-colored solutions due to d-d electron transitions.
• \( \text{VO}^{2+} \) ions create blue or purple solutions, depending on conditions like pH.

These transition metal ions also tend to have magnetic and catalytic properties, contributing to their essential roles in diverse chemical reactions.

Coloration of Metal Salts

The coloration observed in metal salts dissolved in water is a direct consequence of the transition metal present and its associated ions. Different metal salts offer various hues, which arise from specific electron transitions within the metal's d-orbitals.
Examples of such coloration include:

• Cupric chloride \( \text{CuCl}_2 \), offering a vivid blue or green.
• Ferrous chloride \( \text{FeCl}_2 \) giving a subtle pale green.
• Manganese chloride \( \text{MnCl}_2 \) revealing a faint pink.

Such vivid colors not only enhance the identification of metal ions in laboratory settings but also assist in qualitative analysis procedures. Understanding these colors can help chemists anticipate chemical behavior and reactivity in transition metal compounds.