Problem 37 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not correctly matched. List I List II (a) \(\mathrm{Cu}_{2} \mathrm{O}\) Red (b) \(\mathrm{Cu}_{2} \mathrm{I}_{2}\) Blue (c) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) Blue (d) \(\mathrm{CuO}\) Black

Step-by-Step Solution

Answer

(b) \( \mathrm{Cu}_{2} \mathrm{I}_{2} \) is incorrectly matched.

Step 1: Review Each Item in List II

Examine the compounds against their described colors in List II: - (a) \( \mathrm{Cu}_{2} \mathrm{O} \): Red- (b) \( \mathrm{Cu}_{2} \mathrm{I}_{2} \): Blue- (c) \( \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \): Blue- (d) \( \mathrm{CuO} \): Black

Step 2: Confirm Correct Color Matches

Compare each color description with known chemical properties:- \( \mathrm{Cu}_{2} \mathrm{O} \) is correctly matched as red.- \( \mathrm{CuO} \) is known to be black, so (d) is correct.- \( \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \) is generally known for being blue, correct.- Check \( \mathrm{Cu}_{2} \mathrm{I}_{2} \).

Step 3: Identify Incorrect Match

After referencing the colors:- (a) \( \mathrm{Cu}_{2} \mathrm{O} \) is indeed red- (c) and (d) correspond to their colors as blue and black respectively.- (b) \( \mathrm{Cu}_{2} \mathrm{I}_{2} \) should be white, not blue.

Key Concepts

Copper Compounds

Copper compounds are well-known for their diverse and distinctive colors. This unique characteristic is due to the electronic transitions between the d-orbitals of copper ions, affected by their oxidation states and surrounding ligands. Let's explore some common copper compounds and their colors:

• Cuprous Oxide (\( \mathrm{Cu}_{2} \mathrm{O} \): This compound is red, which results from the copper(I) ion's electronic transitions in low-symmetry fields, making it easily recognizable.
• Cuprous Iodide (\( \mathrm{Cu}_{2} \mathrm{I}_{2} \): Despite being listed in the exercise as blue, cuprous iodide is actually white. It does not have d-d transitions visible in the light spectrum, making it colorless or white to our eyes.
• Copper(II) Oxide (\( \mathrm{CuO} \): Appearing black in color, this compound contains copper in the +2 oxidation state (copper(II)), known for absorbing most colors except black.

Understanding these compounds' colors helps in identifying and classifying them based on their chemical properties and oxidation states. Remember that these visual clues can efficiently aid in predicting chemical behavior in reactions.

Iron Complexes

Iron complexes are notable for their wide range of colors, which are primarily influenced by the ligands attached to the iron and the metal's oxidation state. A common example, featured in our exercise, is Prussian Blue.Prussian Blue is a complex compound with the formula \( \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \), known for its deep blue color. This vivid color arises from the charge transfer when an electron moves from one iron atom to another, through the cyanide ligands, cycling between iron(II) and iron(III) oxidation states. This is called intervalence charge transfer.Other common iron complexes and their characteristic colors:

• Iron(III) Chloride: Typically brownish-yellow, due to ligand-to-metal charge transfer.
• Hematite (\( \mathrm{Fe}_{2}\mathrm{O}_{3} \): Usually red, because of its iron(III) composition.

The color variations in iron complexes are crucial for applications ranging from pigments to biological systems, contributing to their functionality by means of visible light absorption.

Color Matching in Chemistry

Color matching in chemistry involves correlating a compound's observed color with its chemical makeup. This process assists in deducing structural and compositional information about substances.In our exercise, color mismatches can provide hints for corrections.

For instance, \( \mathrm{Cu}_{2} \mathrm{I}_{2} \) was incorrectly matched as blue, leading us to reassess our chemical knowledge because it is, in fact, white.
The ability to accurately match colors to compounds relies on understanding:

• Electronic Transitions: The movement of electrons between energy levels, which often results in the absorption and reflection of specific light wavelengths.
• Ligand Field Theory: Explains how the arrangement and types of ligands influence the color of metal complexes.
• Oxidation States: These can significantly alter a compound's electronic structure and, consequently, its resultant color.

Facade apparent colors in lab observations call for correct amendments in educational exercise, fostering a deeper chemistry proficiency among students. Understanding and identifying these discrepancies improves conceptual clarity and practical chemistry skills.