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CHEMISTRY, M8 2021 VCE 16 MC

Which one of the following statements about IR spectroscopy is correct?

  1. IR radiation changes the spin state of electrons.
  2. Bond wave number is influenced only by bond strength.
  3. An IR spectrum can be used to determine the purity of a sample.
  4. In an IR spectrum, high transmittance corresponds to high absorption.
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\(C\)

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→ Every pure compound has a different fingerprint region on the Infrared spectrum. 

→ Hence the fingerprint region of the sample can be compared against the fingerprint region of the pure substance to determine the purity of the sample.

\(\Rightarrow C\)

♦♦♦ Mean mark 15%.

CHEMISTRY, M8 2023 VCE 7-2*

The infrared (IR) spectrum of the molecule 3-methyl-2-butanone is shown below.
 

Explain why different frequencies of infrared radiation can be absorbed by the same molecule as shown in the spectrum above.   (3 marks)

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→ As infrared radiation is passed through the molecules, the different bonds within the molecule vibrate at specific wavelengths leading to the absorption of the infrared radiation.

→ In this way, different frequencies of infrared radiation can be absorbed by a molecule as bonds differ in electronegativity, dipole strengths and in the masses of atoms at the end of bonds.  

→ For example, the double bond between the oxygen atom and carbon atom in the given molecule has a greater dipole than the carbon-hydrogen bonds. This causes a transmittance at 1450 whereas \(\ce{C-H}\) bonds have a transmittance at 3000.

→ An oxygen atom has a higher molecular mass than hydrogen atoms and this also leads to different frequencies of infrared radiation being absorbed in the one molecule.

(Students could have also discussed the strength of bonds, bond length or molecular vibrations)

Show Worked Solution

→ As infrared radiation is passed through the molecules, the different bonds within the molecule vibrate at specific wavelengths leading to the absorption of the infrared radiation.

→ In this way, different frequencies of infrared radiation can be absorbed by a molecule as bonds differ in electronegativity, dipole strengths and in the masses of atoms at the end of bonds.  

→ For example, the double bond between the oxygen atom and carbon atom in the given molecule has a greater dipole than the carbon-hydrogen bonds. This causes a transmittance at 1450 whereas \(\ce{C-H}\) bonds have a transmittance at 3000.

→ An oxygen atom has a higher molecular mass than hydrogen atoms and this also leads to different frequencies of infrared radiation being absorbed in the one molecule.

(Students could have also discussed the strength of bonds, bond length or molecular vibrations)

♦♦ Mean mark 30%.
COMMENT: A deep understanding of the principles behind analytical techniques required here.