Amino acidsAQA A-Level Chemistry: Subtopic test

1. 1
   Glycine, H₂N–CH₂–COOH, is the simplest amino acid. In the solid state and in neutral aqueous solution it exists mainly as a zwitterion.

   (a)

   Which structure shows glycine as a zwitterion?

   [1 mark]

   • A⁺H₃N–CH₂–COO⁻
   • BH₂N–CH₂–COOH
   • C⁺H₃N–CH₂–COOH
   • DH₂N–CH₂–COO⁻

   (b)

   Which group in glycine accepts a proton, so that glycine can act as a base?

   [1 mark]

   • A–CH₂–
   • B–COOH
   • C–NH₂
   • DC=O

   (c)

   Explain how glycine forms a zwitterion.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student dissolves alanine, CH₃CH(NH₂)COOH, in solutions of different pH: dilute hydrochloric acid, water and aqueous sodium hydroxide.

   (a)

   Which species is formed when alanine dissolves in dilute hydrochloric acid?

   [1 mark]

   • ACH₃CH(NH₂)COO⁻
   • BCH₃CH(⁺NH₃)COOH
   • CCH₃CH(⁺NH₃)COO⁻
   • DCH₃CH(NH₂)COOH

   (b)

   Which species is formed when alanine dissolves in aqueous sodium hydroxide?

   [1 mark]

   • ACH₃CH(⁺NH₃)COOH
   • BCH₃CH(⁺NH₃)COO⁻
   • CCH₃CH(NH₂)COOH
   • DCH₃CH(NH₂)COO⁻

   (c)

   Write an equation for the reaction of alanine with aqueous sodium hydroxide, showing the structure of the organic product.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Glycine, H₂NCH₂COOH, is a white crystalline solid that melts above 200 °C. Propanoic acid, CH₃CH₂COOH, has almost the same relative molecular mass but is a liquid at room temperature.

   (a)

   Explain why glycine, Mr = 75.0, is a solid at room temperature but propanoic acid, Mr = 74.0, is a liquid.

   [3 marks]

   (b)

   A 1.50 g sample of glycine is dissolved in water and neutralised with 0.200 mol dm⁻³ aqueous sodium hydroxide. Calculate the volume of sodium hydroxide solution needed, in cm³. Use Ar: H = 1.0, C = 12.0, N = 14.0, O = 16.0.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist is studying a 2-amino acid of general formula H₂N–CHR–COOH, where R is an unreactive alkyl group.

   (a)

   Explain why the amino acid shows both acidic and basic properties, and give the structure of the species present when it is dissolved in dilute acid, in water and in aqueous alkali.

   [6 marks]

   (b)

   A 0.300 g sample of the amino acid is neutralised by 22.50 cm³ of 0.150 mol dm⁻³ sodium hydroxide. Calculate the relative formula mass of the amino acid, deduce the identity of R and name the amino acid. Use Ar: H = 1.0, C = 12.0, N = 14.0, O = 16.0.

   [6 marks]

   Total for question 4: 12 marks

End of questions