Reactions of metal ions with sodium hydroxide and ammoniaEdexcel International A Level Chemistry: Subtopic test

1. 1
   A technician has four solutions, each containing one of the ions Cu²⁺(aq), Fe²⁺(aq), Fe³⁺(aq) or Zn²⁺(aq). She adds aqueous sodium hydroxide to a sample of each solution, first dropwise and then in excess.

   (a)

   Which ion gives a white precipitate with sodium hydroxide that dissolves in excess sodium hydroxide?

   [1 mark]

   • ACu²⁺
   • BFe³⁺
   • CZn²⁺
   • DFe²⁺

   (b)

   Which ion gives a green precipitate that turns brown at the surface on standing in air?

   [1 mark]

   • ACu²⁺
   • BZn²⁺
   • CFe³⁺
   • DFe²⁺

   (c)

   State what is seen when sodium hydroxide is added to the Cu²⁺(aq) solution and write an ionic equation for the reaction, using the simple formulae of the species.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Aqueous ammonia is added first dropwise and then in excess to separate solutions of Ni²⁺(aq), Co²⁺(aq) and Cu²⁺(aq). In each case a precipitate forms at first.

   (a)

   What is seen when excess aqueous ammonia is added to the precipitate formed from Ni²⁺(aq)?

   [1 mark]

   • AThe green precipitate dissolves to give a blue solution
   • BThe green precipitate remains unchanged
   • CThe green precipitate turns brown
   • DThe green precipitate dissolves to give a colourless solution

   (b)

   Which type of reaction occurs when the precipitate dissolves in excess ammonia?

   [1 mark]

   • ARedox
   • BLigand exchange
   • CNeutralisation by an acid
   • DThermal decomposition

   (c)

   Explain, with an equation, why a precipitate forms when a few drops of aqueous ammonia are added to Cu²⁺(aq).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Aqueous chromium(III) ions, Cr³⁺(aq), are green and aqueous zinc ions, Zn²⁺(aq), are colourless. A student adds sodium hydroxide solution to each, first dropwise and then in excess, and later adds dilute sulfuric acid to the precipitates.

   (a)

   Describe what is seen when sodium hydroxide solution is added, first dropwise and then in excess, to the Cr³⁺(aq) solution.

   [3 marks]

   (b)

   Explain what is meant by amphoteric, and use equations for chromium(III) hydroxide, [Cr(H₂O)₃(OH)₃], reacting with sulfuric acid and with excess sodium hydroxide to show that it is amphoteric.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician tests two unlabelled solutions, X and Y, each containing a single metal(II) ion from Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺. She adds aqueous sodium hydroxide and, separately, aqueous ammonia to each, first dropwise and then in excess. Solution X is green. With sodium hydroxide it gives a green precipitate that does not dissolve in excess. With ammonia it gives a green precipitate that dissolves in excess to give a blue solution. Solution Y is colourless. With sodium hydroxide it gives a white precipitate that dissolves in excess, and with ammonia it gives a white precipitate that dissolves in excess to give a colourless solution.

   (a)

   Deduce the metal ion in solution X, explaining all the observations, and write equations for the formation and the dissolving of the precipitate with ammonia.

   [6 marks]

   (b)

   Deduce the metal ion in solution Y, explaining why the other ions in the list are ruled out, and write equations for the reaction with excess sodium hydroxide and for the formation and the dissolving of the precipitate with ammonia.

   [6 marks]

   Total for question 4: 12 marks

End of questions