Bronsted-Lowry acids and basesAQA A-Level Chemistry: Subtopic test

1. 1
   Ammonia gas dissolves in water to form a solution that turns red litmus blue. The equilibrium in the solution can be written as NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.

   (a)

   Which species acts as a Brønsted–Lowry acid in the forward reaction?

   [1 mark]

   • ANH₃
   • BNH₄⁺
   • CH₂O
   • DOH⁻

   (b)

   Which species is the conjugate acid of ammonia in this equilibrium?

   [1 mark]

   • ANH₄⁺
   • BOH⁻
   • CH₂O
   • DNH₂⁻

   (c)

   Explain, in terms of proton transfer, why ammonia acts as a base in water.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Sodium hydrogencarbonate is added to water. The hydrogencarbonate ion, HCO₃⁻, reacts with acids and with alkalis according to the equations: (1) HCO₃⁻ + H₃O⁺ → H₂CO₃ + H₂O and (2) HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O.

   (a)

   What is the role of the hydrogencarbonate ion in reaction (2)?

   [1 mark]

   • AA base, because it accepts a proton from OH⁻
   • BA base, because it donates a proton to OH⁻
   • CNeither, because it is a spectator ion
   • DAn acid, because it donates a proton to OH⁻

   (b)

   Which species is the conjugate base of H₂CO₃ in the reverse of reaction (1)?

   [1 mark]

   • ACO₃²⁻
   • BHCO₃⁻
   • CH₃O⁺
   • DH₂O

   (c)

   Use the two equations to explain why the hydrogencarbonate ion can act as both an acid and a base.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Two acid–base reactions are studied. Reaction 1: hydrogen chloride gas and ammonia gas meet in a dry tube and form a white smoke of solid ammonium chloride, HCl(g) + NH₃(g) → NH₄Cl(s). Reaction 2: in the nitrating mixture used to make nitrobenzene, concentrated nitric acid is mixed with concentrated sulfuric acid, HNO₃ + H₂SO₄ ⇌ H₂NO₃⁺ + HSO₄⁻.

   (a)

   Explain, in terms of proton transfer, why Reaction 1 is an acid–base reaction, identifying the acid and the base.

   [3 marks]

   (b)

   In Reaction 2, identify the Brønsted–Lowry acid and base in the forward reaction and the two conjugate acid–base pairs.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher writes three equilibria on the board for a class. (1) CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺. (2) CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻, which is the reaction of the ions in sodium ethanoate solution. (3) NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.

   (a)

   Use equations (1) and (3) to explain what is meant by a Brønsted–Lowry acid and base, and why acid–base equilibria involve the transfer of protons in both directions.

   [6 marks]

   (b)

   Use equations (1) and (2) to explain why the ethanoate ion is the conjugate base of ethanoic acid. Evaluate the statement 'a substance must contain hydroxide ions to be a base'.

   [6 marks]

   Total for question 4: 12 marks

End of questions