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Infrared spectroscopy Knowledge-Based Practice

Chemistry · Applied Chemistry · Chemistry · Analytical Techniques

5 Chemistry practice questions on Infrared spectroscopy. Questions that assess core concepts and understanding.

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Questions

  1. 1.

    Which of the following spectral features allows an aldehyde to be distinguished from an aliphatic ketone in an infrared spectrum when both display a strong carbonyl absorption near 1715 cm−1^{-1}?

    • A) A broad, intense absorption band centered between 3200 cm−1^{-1} and 3600 cm−1^{-1}
    • B) A sharp absorption band of medium intensity appearing near 2250 cm−1^{-1}
    • C) A distinct pair of sharp absorption bands located between 1600 cm−1^{-1} and 1680 cm−1^{-1}
    • D) A pair of weak to moderate absorption bands appearing near 2720 cm−1^{-1} and 2820 cm−1^{-1}
  2. 2.

    Which of the following describes the effect of intermolecular hydrogen bonding on the O–H stretching absorption band in an infrared spectrum?

    • A) It shifts the band to higher wavenumbers and results in a sharp, narrow peak.
    • B) It renders the vibration infrared-inactive by eliminating the change in dipole moment.
    • C) It shifts the band to lower wavenumbers and causes significant band broadening.
    • D) It splits the band into an intense, symmetric triplet without altering the mean wavenumber.
  3. 3.

    In infrared spectroscopy, the spectral region below approximately 1500 cm−1^{-1} is designated as the fingerprint region. Which of the following statements correctly describes the analytical significance of this region?

    • A) It displays simple, easily assigned stretching bands that directly identify the major functional groups present in an unknown sample.
    • B) It contains complex patterns of bending and skeletal vibrations unique to a specific compound, allowing definitive identification by spectral comparison.
    • C) It exhibits absorptions resulting primarily from electronic transitions between bonding and antibonding molecular orbitals.
    • D) It is typically devoid of molecular absorption peaks, serving as an unattenuated baseline used to calibrate detector sensitivity.
  4. 4.

    According to the Hooke's law approximation for molecular vibrations, which of the following changes will lead to an increase in the vibrational stretching frequency of a covalent bond?

    • A) Increasing the reduced mass of the bonded atoms while maintaining the same force constant
    • B) Decreasing the force constant of the chemical bond while keeping atomic masses constant
    • C) Substituting one of the participating atoms with its heavier isotope
    • D) Increasing the bond order from a single bond to a double or triple bond
  5. 5.

    Which of the following conditions must be fulfilled for a molecular vibration to absorb infrared radiation and appear as an active band in an infrared spectrum?

    • A) The vibration must promote a valence electron from a bonding orbital to an antibonding orbital.
    • B) The vibration must induce a reorientation of nuclear magnetic spin states within an external magnetic field.
    • C) The vibration must cause a change exclusively in the molecular polarizability without altering charge separation.
    • D) The vibration must produce a net change in the electric dipole moment of the molecule.

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