Reactions of transition metal ions with bases and ligandsEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student has four separate solutions containing the hexaaqua ions of chromium(III), iron(III), cobalt(II) and copper(II). She adds aqueous sodium hydroxide dropwise to a sample of each solution and then adds an excess.

   (a)

   Which of the four solutions gives a precipitate that dissolves in excess sodium hydroxide?

   [1 mark]

   • Airon(III)
   • Bcobalt(II)
   • Ccopper(II)
   • Dchromium(III)

   (b)

   What is seen when aqueous sodium hydroxide is added dropwise and then in excess to the iron(III) solution?

   [1 mark]

   • AA green precipitate that dissolves in excess to give a dark green solution
   • BAn orange-brown precipitate that does not dissolve in excess
   • CA pale blue precipitate that does not dissolve in excess
   • DA blue precipitate that turns to a deep blue solution in excess

   (c)

   Write the ionic equation for the formation of the precipitate when aqueous sodium hydroxide is added to the copper(II) solution, and state the colour of the precipitate.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student adds aqueous ammonia dropwise, and then in excess, to a pale blue solution of copper(II) sulfate containing [Cu(H₂O)₆]²⁺.

   (a)

   What is seen when excess aqueous ammonia is added to the precipitate that first forms?

   [1 mark]

   • AThe precipitate remains and the solution above it is colourless
   • BThe precipitate turns green and does not dissolve
   • CThe pale blue precipitate dissolves to give a deep blue solution
   • DThe precipitate dissolves to give a colourless solution

   (b)

   Which type of reaction converts the pale blue precipitate into the deep blue solution?

   [1 mark]

   • ALigand exchange
   • BRedox
   • CAmphoteric behaviour of the hydroxide
   • DPrecipitation

   (c)

   Write the ionic equation for the ligand exchange reaction that forms the deep blue complex ion from [Cu(H₂O)₆]²⁺ and ammonia, and state the coordination number of copper before and after the reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Hexaaqua complexes of copper(II) and nickel(II) take part in ligand exchange reactions. Concentrated hydrochloric acid is added to a solution of [Cu(H₂O)₆]²⁺, and, separately, excess ethane-1,2-diamine (en), NH₂CH₂CH₂NH₂, is added to a solution of [Ni(H₂O)₆]²⁺.

   (a)

   Write the equation for the reaction of [Cu(H₂O)₆]²⁺ with concentrated hydrochloric acid and state the colour change. Explain why the coordination number of copper changes.

   [3 marks]

   (b)

   Excess en reacts with [Ni(H₂O)₆]²⁺ to form [Ni(en)₃]²⁺, for which ΔH is approximately zero. Explain why the reaction is feasible and why the en complex is more stable than the hexaaqua complex.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician has labelled solutions of five metal ions, each present as the hexaaqua ion: Cr³⁺, Fe²⁺, Fe³⁺, Co²⁺ and Cu²⁺. She tests samples of each with aqueous sodium hydroxide and with aqueous ammonia, adding each reagent dropwise and then in excess.

   (a)

   Compare the reactions of chromium(III) and copper(II) ions with excess aqueous sodium hydroxide, and the reaction of copper(II) ions with excess aqueous ammonia. Include observations and equations, and explain the difference between amphoteric behaviour and ligand exchange.

   [6 marks]

   (b)

   Describe how aqueous sodium hydroxide and aqueous ammonia can be used to distinguish between samples of iron(II), iron(III) and cobalt(II) ions. Include the observations for each ion with excess of each reagent.

   [6 marks]

   Total for question 4: 12 marks

End of questions