Infrared spectroscopyEdexcel A-Level Chemistry: Subtopic test

1. 1
   An organic compound P has the molecular formula C₃H₆O. A student records its infrared spectrum and finds a strong absorption at 1715 cm⁻¹ and no absorption between 3200 and 3600 cm⁻¹.

   (a)

   Which bond is responsible for the absorption at 1715 cm⁻¹?

   [1 mark]

   • AC=C
   • BC=O
   • CO–H
   • DN–H

   (b)

   Which statement about P is correct?

   [1 mark]

   • AP is an alcohol
   • BP is a carboxylic acid
   • CP is an alkene
   • DP could be an aldehyde or a ketone

   (c)

   Explain why the infrared spectrum cannot distinguish between propanal and propanone, and describe a chemical test that does.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist is given two colourless liquids, ethanol and ethanoic acid, whose labels have fallen off. She records an infrared spectrum of each.

   (a)

   Which absorption range is due to the very broad O–H stretching vibration in a carboxylic acid?

   [1 mark]

   • A2500–3300 cm⁻¹
   • B1680–1750 cm⁻¹
   • C1620–1669 cm⁻¹
   • D3300–3500 cm⁻¹

   (b)

   Which compound would show an absorption at 3200–3600 cm⁻¹ but no absorption at 1680–1750 cm⁻¹?

   [1 mark]

   • AEthanoic acid
   • BEthanal
   • CEthanol
   • DPropanone

   (c)

   Describe how the two spectra differ, so that the chemist can identify each liquid.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student heats propan-1-ol with acidified potassium dichromate(VI) and distils off the product as soon as it forms. The infrared spectrum of the distillate shows absorptions at 2980 cm⁻¹ and 1725 cm⁻¹ only, apart from the fingerprint region.

   (a)

   Identify the bonds responsible for the two absorptions, and state what the absence of any absorption at 3200–3600 cm⁻¹ shows about the distillate.

   [3 marks]

   (b)

   The student repeats the experiment but heats the mixture under reflux with excess acidified potassium dichromate(VI). Describe how the infrared spectrum of the product would differ from the first spectrum, name the product and write an equation using [O] for its formation from the first product.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A quality-control laboratory in a flavourings factory uses infrared spectroscopy to check the identity of organic liquids delivered by suppliers before they are used. Absorption ranges from the Edexcel data booklet may be used.

   (a)

   Propanoic acid, CH₃CH₂COOH, and 1-hydroxypropanone, CH₃COCH₂OH, are isomers with molecular formula C₃H₆O₂. Explain how their infrared spectra are similar and how they differ, and how a spectrum could be used to decide which isomer a sample is.

   [6 marks]

   (b)

   A delivery labelled propylamine, CH₃CH₂CH₂NH₂, gives an infrared spectrum with absorptions at 3350 cm⁻¹, 2960 cm⁻¹ and 1720 cm⁻¹, and no broad absorption between 2500 and 3300 cm⁻¹. Evaluate whether the delivery is pure propylamine. Include an explanation of why the absorption at 3350 cm⁻¹ alone cannot show that no alcohol is present.

   [6 marks]

   Total for question 4: 12 marks

End of questions