Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

During the deepening of the colour of a dye, the absorption shifts towards red. This shifting is called (a) bathochromic shift (b) hypochromic shift (c) hyperchromic shift (d) auxochromic shift

Step-by-Step Solution

Answer

The correct answer is (a) bathochromic shift.

Step 1: Understanding the Terms

First, we need to recognize the terms presented in the options: (a) Bathochromic shift refers to a change in absorption maximum of a molecule toward longer wavelengths, often interpreted as a shift towards the red end of the spectrum. (b) Hypochromic shift usually involves a decrease in absorption intensity without necessarily specifying wavelength shift. (c) Hyperchromic shift indicates an increase in absorption intensity, not necessarily related to wavelength shift. (d) Auxochromic shift is not a standard term related to wavelength shift specifically.

Step 2: Connection with Color Deepening

In the exercise, deepening of color refers to the absorption shifting towards the red part of the spectrum. This requires a direct shift from the original absorption towards longer wavelengths.

Step 3: Recognizing the Correct Shift

Knowing that a bathochromic shift is associated with a shift to longer wavelengths, which includes red, it logically fits the description of the deepening of dye color towards the red.

Key Concepts

Spectroscopy

Spectroscopy is a powerful tool used to study how molecules interact with light. It allows scientists to analyze the structure and behavior of molecules by observing how they absorb, emit, or scatter light at different wavelengths. The essence of spectroscopy involves breaking down light into its component colors (or wavelengths) and measuring how a substance affects light under other factors. This helps to gain insights into the properties and compositions of substances. In simpler terms:

• Measurement: Analyzing wavelengths of light absorbed or emitted.
• Applications: Used in chemistry, physics, and even in medical diagnostics.
• Techniques: Includes infrared (IR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, and more.

Understanding spectroscopy helps in identifying 'wavelength shifts', crucial to comprehending concepts like bathochromic shifts.

Chromophore

A chromophore is a specific part of a molecule that is responsible for its color. It is the region of the molecule where the absorption of a specific range of light occurs, resulting in color. When a chromophore absorbs light at a certain wavelength, electrons within this part of the molecule are excited to a higher energy level. This process typically happens within visible light ranges and impacts how the molecule is perceived in terms of color. Key points about chromophores include:

• Function: They dictate the color observed in dyes and pigments.
• Interaction: Variations in energy and wavelength absorption result in visible colors.
• Importance: Integral in understanding color theory and designing dyes.

By understanding chromophores, we can better grasp how shifts in wavelength can influence color perception, such as in a bathochromic shift.

Wavelength Shift

A wavelength shift refers to the change in the position of an absorption peak when light interacts with a substance, shifting either to longer or shorter wavelengths. This term is commonly used in spectroscopy to describe movements within the light spectrum when molecules absorb light differently due to changes in the molecular environment. These shifts are categorized into two main types:

• Bathochromic Shift: Movement towards longer wavelengths, often towards the red end of the spectrum.
• Hypsochromic Shift: Movement towards shorter wavelengths, usually towards the blue end.

Understanding wavelength shifts like bathochromic shifts is essential in fields like chemistry where identifying substances and reactions often revolves around these changes in wavelength absorption.

Color Theory

Color theory is a fundamental concept in both science and art. It involves understanding how colors mix, match, and contrast with one another, and how they are perceived by the human eye. In the context of physics and chemistry, color theory helps in explaining how light absorption, reflection, and transmission result in the colors we see. Some critical aspects of color theory include:

• Color Perception: Colors are perceived based on light absorption and emission by chromophores.
• Primary Colors: Red, blue, and yellow, which mix to produce other colors.
• Spectral Influence: Changes in light wavelength absorption, including bathochromic shifts, affect color intensity and tone.

By integrating knowledge of color theory with spectroscopy and chromophores, one can understand how color changes occur at a molecular level, aiding in the design of different colored dyes and materials.