Chemistry Class 12 | Chapter 12 | Topic 3b | UV-VIS Spectroscopy | in urdu | tutoria.pk

Summary

This script provides an in-depth explanation of ultraviolet (UV) and visible spectroscopy, focusing on their interaction with matter. It discusses how electromagnetic radiation, particularly UV and visible light, can excite electrons in molecules, leading to electronic transitions from ground to excited states. The transcript covers different types of transitions (sigma to sigma star, pi to pi star, etc.) and their energy requirements, explaining their role in spectroscopy. Examples like formaldehyde and methylene blue are used to illustrate the concepts. Additionally, the script touches on color absorption in the visible spectrum and the associated wavelength ranges for different colors.

Takeaways

• 😀 UV and visible spectroscopy involves the interaction of electromagnetic radiation with matter, particularly focusing on electronic transitions.
• 😀 Absorption of UV and visible radiation by molecules leads to the excitation of electrons from lower energy states to higher energy states.
• 😀 The script discusses the concept of electronic transitions, including sigma to sigma*, pi to pi*, and n to sigma* transitions.
• 😀 UV radiation has higher energy compared to visible radiation, making it responsible for more energetic transitions.
• 😀 The UV-visible spectrum ranges from 200 to 800 nanometers, with UV covering the 200-400 nm range and visible light from 400 to 750 nm.
• 😀 Different types of transitions in molecules are associated with varying levels of energy absorption, with sigma to sigma* transitions requiring the highest energy.
• 😀 The transition from n to pi* involves lower energy, and pi to pi* transitions typically occur in the visible region.
• 😀 Molecules with double bonds often undergo pi to pi* transitions, while lone pairs of electrons are involved in n to pi* transitions.
• 😀 The absorption of light in compounds like methylene blue leads to the observed color, as the compound absorbs shorter wavelength light and reflects longer wavelengths.
• 😀 The visible spectrum of light is associated with specific wavelengths for different colors, such as red (620-700 nm), green (500-570 nm), and blue (450-495 nm).

Q & A

What is the primary focus of UV-visible spectroscopy discussed in the transcript?

The primary focus of the UV-visible spectroscopy discussed in the transcript is how ultraviolet and visible radiation interact with matter, particularly in terms of electronic transitions in molecules and their applications in spectroscopy.

What types of electronic transitions are discussed in the transcript?

The transcript discusses several types of electronic transitions, including sigma to sigma-star, pi to pi-star, and n to sigma-star transitions, which occur when electrons in molecules absorb UV or visible radiation and transition to higher energy states.

How does the absorption of UV-visible radiation lead to electronic transitions?

When a molecule absorbs UV or visible radiation, it gains energy that allows its electrons to move from lower energy occupied molecular orbitals to higher energy unoccupied orbitals, resulting in electronic transitions.

How is formaldehyde used as an example in the explanation of electronic transitions?

Formaldehyde is used as an example to illustrate how different types of electrons (bonding, non-bonding, and pi electrons) in a molecule undergo electronic transitions. The script explains how sigma, pi, and n electrons transition to higher energy states when absorbing radiation.

What role do bonding and non-bonding electrons play in electronic transitions?

Bonding electrons and non-bonding (lone pair) electrons both play a role in electronic transitions. Bonding electrons are involved in sigma to sigma-star transitions, while non-bonding electrons participate in n to sigma-star transitions.

How does the energy of UV radiation compare to visible radiation in terms of electronic transitions?

UV radiation has higher energy compared to visible radiation, which is why UV radiation can cause higher energy electronic transitions in molecules. Visible radiation typically causes lower energy transitions.

What is the relationship between the wavelength of radiation and the energy of transitions?

Shorter wavelengths of radiation, such as UV light, carry higher energy and cause transitions from lower to higher energy states in molecules. Longer wavelengths, such as visible light, carry less energy and cause less energetic transitions.

Why do molecules like methylene blue appear blue in color?

Methylene blue appears blue because it absorbs light in the red-orange-yellow region of the spectrum (higher wavelengths) and reflects light in the blue region (lower wavelengths), which is why the color blue is observed.

How are the colors of visible radiation related to their wavelengths?

The colors of visible radiation are associated with specific wavelength ranges. For example, red light corresponds to wavelengths between 620 and 700 nm, and violet light corresponds to wavelengths below 400 nm. Each color absorbs specific wavelengths of light, which contributes to the perceived color.

How does the absorption of UV-visible radiation affect the color perception of a compound?

The absorption of UV-visible radiation affects color perception by determining which wavelengths are absorbed and which are reflected. If a compound absorbs wavelengths from the red, orange, or yellow parts of the spectrum, it will reflect the complementary color (such as blue) to the observer.