Test-tube reactions for functional groupsAQA A-Level Chemistry: Subtopic test

1. 1
   A technician has four unlabelled bottles. Each contains one of hex-1-ene, hexan-1-ol, hexanal and hexanoic acid. She labels them W, X, Y and Z and plans simple test-tube reactions to identify each liquid.

   (a)

   Which reagent would immediately change colour on shaking with hex-1-ene at room temperature?

   [1 mark]

   • AAcidified potassium dichromate(VI)
   • BTollens' reagent
   • CAqueous sodium carbonate
   • DBromine water

   (b)

   Which test would give a positive result with hexanoic acid but with none of the other three liquids?

   [1 mark]

   • ABromine water is decolourised
   • BAqueous sodium carbonate gives effervescence
   • CFehling's solution gives a red precipitate
   • DWarm acidified potassium dichromate(VI) turns green

   (c)

   Write an equation for the reaction of hexanoic acid with aqueous sodium carbonate.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student is given an organic liquid that might be a halogenoalkane. She warms it with aqueous sodium hydroxide, cools the mixture, acidifies it with dilute nitric acid and then adds aqueous silver nitrate. A cream precipitate forms.

   (a)

   Which halogen does the halogenoalkane contain?

   [1 mark]

   • AChlorine
   • BIodine
   • CBromine
   • DFluorine

   (b)

   Why is the mixture acidified with dilute nitric acid before the silver nitrate is added?

   [1 mark]

   • ATo remove excess hydroxide ions, which would otherwise form a precipitate with silver ions
   • BTo increase the rate of hydrolysis of the halogenoalkane
   • CTo oxidise the halide ions to halogens
   • DTo convert the alcohol product into an alkene

   (c)

   Name the type of reaction between the halogenoalkane and aqueous sodium hydroxide, and explain how this reaction leads to the cream precipitate.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An analyst has an unlabelled liquid with the molecular formula C₃H₆O. It could be propanal, propanone or prop-2-en-1-ol, CH₂=CHCH₂OH.

   (a)

   Describe the observations when each of the three compounds is warmed separately with acidified potassium dichromate(VI) and with Tollens' reagent.

   [3 marks]

   (b)

   The unknown liquid turns acidified dichromate(VI) green but gives no silver mirror with Tollens' reagent. Deduce its identity, explain your reasoning, and describe a further test that would confirm it.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A research chemist uses test-tube reactions to identify the functional groups in organic compounds obtained from different sources.

   (a)

   Describe a sequence of test-tube reactions to identify four unlabelled liquids: but-1-ene, butan-2-ol, butanal and butanoic acid. State the reagent and observation for each test, and explain why one test alone is not enough to separate two of the liquids.

   [6 marks]

   (b)

   Lactic acid, CH₃CH(OH)COOH, is tested with aqueous sodium carbonate, with warm acidified potassium dichromate(VI) and with Tollens' reagent. Predict the result of each test and explain what each result shows about the structure.

   [6 marks]

   Total for question 4: 12 marks

End of questions