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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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→ 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.

♦♦♦ 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 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 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.

CHEMISTRY, M8 EQ-Bank 27

Explain how infrared spectroscopy is used in the analysis and identification of organic substances.   (3 marks)

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→ When analysing organic substances, IR radiation of specific wavelengths will be absorbed by certain atoms of the organic molecules. The absorption is dependent on the length of the compound’s atomic bonds.

→ By utilising the complete IR spectrum, the particular bonds in a molecule can be identified. The degree of the absorption can be used to calculate the relative number of each specific bond.

→ The resulting absorption data can be used to create an absorption spectrum of the organic material. This spectrum can then be compared to published data and used to identify the organic material.

Show Worked Solution

→ When analysing organic substances, IR radiation of specific wavelengths will be absorbed by certain atoms of the organic molecules. The absorption is dependent on the length of the compound’s atomic bonds.

→ By utilising the complete IR spectrum, the particular bonds in a molecule can be identified. The degree of the absorption can be used to calculate the relative number of each specific bond.

→ The resulting absorption data can be used to create an absorption spectrum of the organic material. This spectrum can then be compared to published data and used to identify the organic material.

CHEMISTRY, M8 EQ-Bank 25

The diagram shows the infrared spectrum of a compound.

 

The molecular weight of the compound analysed is approximately 60 g mol ¯1. Suggest TWO possible compounds that could fit this spectrum and justify your selection.   (4 marks)

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→ The absorbance shows an initial trough in the range 3100 – 3600 cm¯1. This absorbance maximises at around 3350 cm¯1. This data indicates the likely presence of a hydroxyl group within the compound.

→ The next trough occurs in the range 3100 – 3600 cm¯1. Its intensity trough is not broad. This data is consistent with the presence of a    bond.

→ There is little absorbance between 2500 – 1500 cm¯1. This data indicates the absence of any    group, discounting the possibility of the compound being carboxylic acid or a ketone.

→ An absorption peak between 1250 – 1050 cm¯1 indicates the presence of a    bond.

→ Propanol is a compound whose structure is consistent with the data above:

       

→ The molecular weight of propanol is 60.01 g mol ¯1 which is also supported by the data.

→ Since the spectrum of isomers is similar, the two possible compounds that fit this are propan-1-ol or propan-2-ol.

Show Worked Solution

→ The absorbance shows an initial trough in the range 3100 – 3600 cm¯1. This absorbance maximises at around 3350 cm¯1. This data indicates the likely presence of a hydroxyl group within the compound.

→ The next trough occurs in the range 3100 – 3600 cm¯1. Its intensity trough is not broad. This data is consistent with the presence of a    bond.

→ There is little absorbance between 2500 – 1500 cm¯1. This data indicates the absence of any    group, discounting the possibility of the compound being carboxylic acid or a ketone.

→ An absorption peak between 1250 – 1050 cm¯1 indicates the presence of a    bond.

→ Propanol is a compound whose structure is consistent with the data above:

       

→ The molecular weight of propanol is 60.01 g mol ¯1 which is also supported by the data.

→ Since the spectrum of isomers is similar, the two possible compounds that fit this are propan-1-ol or propan-2-ol.