Base properties of aminesAQA A-Level Chemistry: Subtopic test

1. 1
   A technician measures the pH of three aqueous solutions, each of concentration 0.100 mol dm⁻³, at 298 K. The solution of methylamine has a pH of 11.8, the solution of ammonia has a pH of 11.1 and the solution of phenylamine has a pH of 8.8.

   (a)

   Which of the three compounds is the strongest base?

   [1 mark]

   • APhenylamine
   • BAmmonia
   • CMethylamine
   • DThey are equally strong

   (b)

   Which statement explains why amines act as bases?

   [1 mark]

   • AThe N–H bond breaks to release H⁺
   • BThe nitrogen atom has a lone pair of electrons that can accept a proton
   • CThe nitrogen atom has delocalised electrons that can accept a proton
   • DThe amine releases OH⁻ directly from its own structure

   (c)

   Write an equation for the reaction of phenylamine with water and explain why phenylamine is described as a weak base.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician has separate 0.100 mol dm⁻³ solutions of ethylamine, CH₃CH₂NH₂, and phenylamine, C₆H₅NH₂, and plans to titrate samples of each against hydrochloric acid.

   (a)

   Which is the correct order of increasing base strength?

   [1 mark]

   • AEthylamine < ammonia < phenylamine
   • BAmmonia < ethylamine < phenylamine
   • CEthylamine < phenylamine < ammonia
   • DPhenylamine < ammonia < ethylamine

   (b)

   What is formed when ethylamine reacts with dilute hydrochloric acid?

   [1 mark]

   • AEthylammonium chloride, CH₃CH₂NH₃⁺Cl⁻
   • BChloroethane, CH₃CH₂Cl
   • CEthanamide, CH₃CONH₂
   • DEthanol, CH₃CH₂OH

   (c)

   A 25.0 cm³ sample of 0.100 mol dm⁻³ ethylamine reacts with 0.125 mol dm⁻³ hydrochloric acid. Calculate the volume of acid needed to neutralise it.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student measures the pH of a 0.100 mol dm⁻³ solution of ammonia at 298 K and finds it is 11.1. At this temperature the ionic product of water, Kw, is 1.00 × 10⁻¹⁴ mol² dm⁻⁶.

   (a)

   Write an equation for the reaction of ammonia with water, and calculate the concentration of hydroxide ions in the solution.

   [3 marks]

   (b)

   Calculate the percentage of ammonia molecules that have reacted, and use your answer to explain why ammonia is a weak base. Compare it with 0.100 mol dm⁻³ sodium hydroxide, which has a pH of 13.0.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician has 0.100 mol dm⁻³ aqueous solutions of ethylamine (pH 11.9), ammonia (pH 11.1) and phenylamine (pH 8.8) at 298 K. When phenylamine is shaken with water, a cloudy mixture of tiny droplets forms because phenylamine is only slightly soluble. Adding dilute hydrochloric acid makes the mixture clear, and adding excess sodium hydroxide solution then makes it cloudy again.

   (a)

   Explain the differences in base strength between ethylamine, ammonia and phenylamine in terms of the availability of the lone pair on nitrogen.

   [6 marks]

   (b)

   Explain the observations when hydrochloric acid and then sodium hydroxide are added to the mixture of phenylamine and water.

   [6 marks]

   Total for question 4: 12 marks

End of questions