Spectroscopy Overview Quiz
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Questions and Answers

What does spectroscopy study?

  • The behavior of gases under pressure
  • The interaction of matter with light (correct)
  • The study of chemical reactions
  • The interaction of sound with matter

What unit measures frequency?

  • Meter (m)
  • Hertz (Hz) (correct)
  • Watt (W)
  • Newton (N)

What occurs when electromagnetic radiation passes through a prism?

  • It creates an interference pattern
  • It decreases in speed
  • It splits into different colors (correct)
  • It increases in energy

What is the effect of an excited atom or molecule in an emission spectrum?

<p>It emits energy in the form of light (C)</p> Signup and view all the answers

How is an absorption spectrum obtained?

<p>By absorbing electromagnetic radiation at specific wavelengths (D)</p> Signup and view all the answers

What is the relationship between energy, frequency, and wavelength expressed mathematically?

<p>E = hν = hc/λ (C)</p> Signup and view all the answers

Which of the following is another name for UV-visible spectroscopy?

<p>Electronic spectroscopy (D)</p> Signup and view all the answers

What does wave number measure?

<p>Number of waves per unit distance (A)</p> Signup and view all the answers

What happens to the energy levels of a molecule when it absorbs UV-visible radiation?

<p>Vibrational energy increases before electronic energy. (D)</p> Signup and view all the answers

Which region of the UV spectrum is defined as being below 200 nm?

<p>Far UV Region (B)</p> Signup and view all the answers

Which of the following electronic transitions requires the largest amount of energy?

<p>σ → σ* transition (A)</p> Signup and view all the answers

What type of energy does Et represent in the total energy equation E = Et + Er + Ev?

<p>Electronic transition energy (B)</p> Signup and view all the answers

What is the common solvent used for preparing samples to be analyzed in UV-visible spectroscopy?

<p>Ethyl alcohol (B)</p> Signup and view all the answers

Which electronic transition would alkenes typically undergo when exposed to UV-visible radiation?

<p>π → π* transition (A)</p> Signup and view all the answers

Which type of energy is always lower than electronic energy in a molecule?

<p>Vibrational energy (D)</p> Signup and view all the answers

Why is Far UV spectroscopy studied under vacuum conditions?

<p>To prevent scattering of UV light (C)</p> Signup and view all the answers

Which type of electronic transition typically requires the least energy?

<p>n → π* transition (C)</p> Signup and view all the answers

What is the primary function of a chromophore in a molecule?

<p>Imparting color (C)</p> Signup and view all the answers

What effect occurs when the absorption maxima of a compound shifts to a longer wavelength?

<p>Bathochromic Shift (A)</p> Signup and view all the answers

Which functional groups can undergo n → π* transitions?

<p>C=O and C≡N (A)</p> Signup and view all the answers

How do auxochromes alter the properties of chromophores?

<p>By modifying the wavelength or intensity of absorption (C)</p> Signup and view all the answers

Which of the following statements regarding π → σ* transitions is correct?

<p>These transitions are forbidden and theoretically possible. (C)</p> Signup and view all the answers

Which statement best describes hypsochromic shift?

<p>Shifting absorption to shorter wavelengths (A)</p> Signup and view all the answers

Which of the following compounds shows an n → σ* transition?

<p>C=O (C)</p> Signup and view all the answers

What occurs during a bathochromic shift?

<p>The absorption maxima moves to a longer wavelength. (C)</p> Signup and view all the answers

What is an example of a hyperchromic effect?

<p>Absorption intensity increases due to auxochrome. (B)</p> Signup and view all the answers

Which scenario best describes a hypsochromic shift?

<p>Absorption maxima shifts to a shorter wavelength. (B)</p> Signup and view all the answers

What does Beer’s Law state about absorbance?

<p>Absorbance is directly proportional to the concentration of the solution. (D)</p> Signup and view all the answers

In a hypochromic effect, what happens to the absorption intensity?

<p>It decreases. (B)</p> Signup and view all the answers

Which statement correctly explains the relationship between path length and absorbance according to Beer’s law?

<p>Absorbance increases with increasing path length. (A)</p> Signup and view all the answers

Which factor does NOT contribute to a hypsochromic shift?

<p>Addition of electron-donating groups. (D)</p> Signup and view all the answers

What is a common cause for the hyperchromic effect in a compound?

<p>Introduction of auxochrome. (C)</p> Signup and view all the answers

Flashcards

Spectroscopy

The study of how light interacts with matter.

Photon

A packet of energy that makes up electromagnetic radiation. It has an oscillating electric and magnetic field.

Frequency

The number of times the electric field in a photon oscillates per second.

Wavelength

The distance between two crests or troughs of a wave.

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Spectrum

A graph showing the intensity of absorbed or emitted radiation by a sample against the frequency or wavelength.

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Emission spectrum

The spectrum produced by a substance when it emits light after being excited.

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Absorption spectrum

The spectrum produced when a substance absorbs light at specific wavelengths.

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UV-visible spectroscopy

The study of how molecules absorb light in the ultraviolet and visible regions.

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Electronic Transition

The process of a molecule gaining energy and its electrons moving from a lower energy level to a higher energy level. This occurs when a molecule absorbs ultraviolet or visible light.

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σ → σ* transition

A type of electronic transition where an electron in a sigma bonding orbital is excited to a sigma anti-bonding orbital. This requires a lot of energy.

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π → π* transition

A type of electronic transition where an electron in a pi bonding orbital is excited to a pi anti-bonding orbital. This transition is common in molecules with double or triple bonds.

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n → σ* transition

A type of electronic transition where a non-bonding electron is excited to a sigma anti-bonding orbital.

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n → π* transition

A type of electronic transition where a non-bonding electron is excited to a pi anti-bonding orbital.

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σ → π* transition

A type of electronic transition where a sigma bonding electron is excited to a pi anti-bonding orbital.

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π → σ* transition

A type of electronic transition where a pi bonding electron is excited to a sigma anti-bonding orbital.

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UV-Vis Spectroscopy

The study of how molecules absorb ultraviolet (UV) and visible (Vis) light based on electronic transitions.

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Chromophore

The part of a molecule responsible for absorbing UV-visible light and giving the molecule color. This is usually a functional group with double or triple bonds capable of undergoing n → π* or π → π* transitions.

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Auxochrome

A functional group attached to a chromophore that changes how a chromophore absorbs UV-visible light. It can shift the wavelength at which the molecule absorbs light or change the intensity of absorption.

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Bathochromic Shift (Red Shift)

A shift in the maximum absorbance wavelength (λmax) toward a longer wavelength. The compound absorbs light at a redder color. This can happen due to the presence of an auxochrome or a change in the solvent.

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Hypsochromic Shift (Blue Shift)

A shift in the maximum absorbance wavelength (λmax) toward a shorter wavelength. The compound absorbs light at a bluer color. This can be caused by factors like introducing a substituent or changing the solvent.

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Hyperchromic Effect

An increase in the intensity of absorbance. The molecule absorbs more light at its maximum absorbance wavelength.

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Hypochromic Effect

A decrease in the intensity of absorbance. The molecule absorbs less light at its maximum absorbance wavelength.

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Beer-Lambert Law

The relationship between the absorbance of a solution and the concentration of the analyte, as well as the path length of the light beam through the solution.

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Lambert Law

A statement that the absorbance of a solution is directly proportional to the path length of the light beam through the solution. This means that the longer the light path through the solution, the more light will be absorbed.

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Beer's Law

A statement that the absorbance of a solution is directly proportional to the concentration of the analyte. This means that the higher the concentration of the analyte, the more light will be absorbed.

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Study Notes

Spectroscopy Overview

  • Spectroscopy is the study of the interaction between matter and electromagnetic radiation.
  • It's a branch of science that analyses the way matter interacts with light.
  • This involves looking at a spectrum, which is like a rainbow of colors.
  • Different colors in a spectrum relate to different wavelengths and energies of light.

Electromagnetic Radiation

  • Electromagnetic radiation is composed of discrete packets of energy called photons.
  • A photon is essentially an oscillating electric and magnetic field that's perpendicular to each other.

Frequency and Wavelength

  • Frequency (ν) is the number of times the electrical field of radiation oscillates in one second, measured in Hertz (Hz).
  • Wavelength (λ) is the distance between two consecutive points in the same phase of a wave, such as two crests, and is measured in meters or nanometers.
  • The speed of light (c) is related to wavelength and frequency by the equation: c = νλ.

Electromagnetic Spectrum

  • The electromagnetic spectrum shows the range of wavelengths and frequencies of electromagnetic radiation.
  • It encompasses various types of radiation, from radio waves to gamma rays.
  • These have different energy levels. The higher the frequency, the greater the energy.

Types of Spectra

  • Emission spectra display the wavelengths of light emitted by atoms or molecules.
  • Absorption spectra show the wavelengths absorbed by atoms or molecules.

UV-Visible Spectroscopy

  • UV-visible spectroscopy analyzes the absorption of UV or visible light.
  • Used to measure the energy level shifts within electrons, especially valence electrons.
  • The transition of electrons between energy levels absorption results in the spectrum.
  • A key spectral region is the near UV region from 200–400nm. The far UV region below 200 nm requires specific conditions and is analyzed under vacuum.
  • Common solvents are used to dissolve the sample for analysis (ethyl alcohol or hexane).

Theory of UV-Visible Spectroscopy

  • Molecules have three main energy types: electronic, rotational, and vibrational.
  • The total energy (E) of a molecule is the sum of these energies: E = Ee + Er + Ev.
  • Electronic transitions within molecules are responsible for UV-Visible absorption.

Electronic Transitions

  • Different electronic transitions (σ to σ*, n to σ*, n to π*, π to π*) in molecules result in distinct absorption.
  • σ → σ* transition involves an electron moving from a bonding orbital to an antibonding orbital, which requires high energy (e.g. CH4 absorbs around 125 nm).
  • π → π* transition occurs in multiple bonds and molecules (e.g. alkenes between 170 to 205 nm).
  • n → σ* transition involves lone pair electrons (e.g. between 250-280 nm )
  • n → π* transition involves a lone pair electron transitioning to an anti-bonding orbital (e.g. around 300 nm).
  • σ → π* and π → σ* transitions are forbidden electronic transitions.

Chromophores and Auxochromes

  • Chromophores are the parts of molecules responsible for color.
  • Auxochromes are functional groups that, when attached to a chromophore, alter its absorption properties (wavelength and intensity).

Absorption and Intensity Shifts

  • Bathochromic Shift (red shift): a shift in absorption to longer wavelengths.
  • Hypsochromic Shift (blue shift): a shift in absorption to shorter wavelengths.
  • Hyperchromic Effect: an increase in absorption intensity.
  • Hypochromic Effect: a decrease in absorption intensity..

Beer-Lambert Law

  • The Beer-Lambert Law relates the absorbance of a solution to the concentration of the absorbing substance, path length of the light through the solution, and molar absorptivity.
  • A = εcl (Absorbance = molar absorptivity × concentration × path length)
  • It describes how absorbance (A) of radiation decreases through an absorbing medium directly proportional to its length and concentration.
  • Limitations include deviations at high concentrations or impure samples

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Spectroscopy Lecture Notes PDF

Description

Test your knowledge on the fundamentals of spectroscopy with this quiz. Explore topics such as electromagnetic radiation, absorption and emission spectra, and the relationship between energy, frequency, and wavelength. Perfect for students looking to solidify their understanding of spectroscopic principles.

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