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Questions and Answers
What does spectroscopy study?
What does spectroscopy study?
What unit measures frequency?
What unit measures frequency?
What occurs when electromagnetic radiation passes through a prism?
What occurs when electromagnetic radiation passes through a prism?
What is the effect of an excited atom or molecule in an emission spectrum?
What is the effect of an excited atom or molecule in an emission spectrum?
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How is an absorption spectrum obtained?
How is an absorption spectrum obtained?
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What is the relationship between energy, frequency, and wavelength expressed mathematically?
What is the relationship between energy, frequency, and wavelength expressed mathematically?
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Which of the following is another name for UV-visible spectroscopy?
Which of the following is another name for UV-visible spectroscopy?
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What does wave number measure?
What does wave number measure?
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What happens to the energy levels of a molecule when it absorbs UV-visible radiation?
What happens to the energy levels of a molecule when it absorbs UV-visible radiation?
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Which region of the UV spectrum is defined as being below 200 nm?
Which region of the UV spectrum is defined as being below 200 nm?
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Which of the following electronic transitions requires the largest amount of energy?
Which of the following electronic transitions requires the largest amount of energy?
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What type of energy does Et represent in the total energy equation E = Et + Er + Ev?
What type of energy does Et represent in the total energy equation E = Et + Er + Ev?
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What is the common solvent used for preparing samples to be analyzed in UV-visible spectroscopy?
What is the common solvent used for preparing samples to be analyzed in UV-visible spectroscopy?
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Which electronic transition would alkenes typically undergo when exposed to UV-visible radiation?
Which electronic transition would alkenes typically undergo when exposed to UV-visible radiation?
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Which type of energy is always lower than electronic energy in a molecule?
Which type of energy is always lower than electronic energy in a molecule?
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Why is Far UV spectroscopy studied under vacuum conditions?
Why is Far UV spectroscopy studied under vacuum conditions?
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Which type of electronic transition typically requires the least energy?
Which type of electronic transition typically requires the least energy?
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What is the primary function of a chromophore in a molecule?
What is the primary function of a chromophore in a molecule?
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What effect occurs when the absorption maxima of a compound shifts to a longer wavelength?
What effect occurs when the absorption maxima of a compound shifts to a longer wavelength?
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Which functional groups can undergo n → π* transitions?
Which functional groups can undergo n → π* transitions?
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How do auxochromes alter the properties of chromophores?
How do auxochromes alter the properties of chromophores?
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Which of the following statements regarding π → σ* transitions is correct?
Which of the following statements regarding π → σ* transitions is correct?
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Which statement best describes hypsochromic shift?
Which statement best describes hypsochromic shift?
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Which of the following compounds shows an n → σ* transition?
Which of the following compounds shows an n → σ* transition?
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What occurs during a bathochromic shift?
What occurs during a bathochromic shift?
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What is an example of a hyperchromic effect?
What is an example of a hyperchromic effect?
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Which scenario best describes a hypsochromic shift?
Which scenario best describes a hypsochromic shift?
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What does Beer’s Law state about absorbance?
What does Beer’s Law state about absorbance?
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In a hypochromic effect, what happens to the absorption intensity?
In a hypochromic effect, what happens to the absorption intensity?
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Which statement correctly explains the relationship between path length and absorbance according to Beer’s law?
Which statement correctly explains the relationship between path length and absorbance according to Beer’s law?
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Which factor does NOT contribute to a hypsochromic shift?
Which factor does NOT contribute to a hypsochromic shift?
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What is a common cause for the hyperchromic effect in a compound?
What is a common cause for the hyperchromic effect in a compound?
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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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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.
