Ligand exchange and the chelate effectEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student adds dilute aqueous ammonia, first a few drops and then in excess, to a pale blue aqueous solution of copper(II) sulfate. The solution contains the hexaaquacopper(II) ion, [Cu(H₂O)₆]²⁺.

   (a)

   What is observed when only a few drops of ammonia solution have been added?

   [1 mark]

   • AA deep blue solution forms
   • BA pale blue precipitate forms
   • CA green precipitate forms
   • DThere is no visible change

   (b)

   What is the formula of the species responsible for the deep blue colour when ammonia is in excess?

   [1 mark]

   • A[Cu(NH₃)₆]²⁺
   • B[Cu(NH₃)₂(H₂O)₄]²⁺
   • C[Cu(NH₃)₄(H₂O)₂]²⁺
   • DCu(OH)₂(H₂O)₄

   (c)

   Write an equation for the formation of the pale blue precipitate and state the role of ammonia in this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student dissolves cobalt(II) chloride in water to give a pink solution containing [Co(H₂O)₆]²⁺. She then adds concentrated hydrochloric acid, and the solution turns blue. When water is added to the blue solution, the pink colour returns.

   (a)

   What are the colour and shape of the complex ion formed when concentrated hydrochloric acid is added?

   [1 mark]

   • ABlue, tetrahedral
   • BBlue, octahedral
   • CPink, tetrahedral
   • DPink, octahedral

   (b)

   Why does the coordination number change from 6 to 4 when chloride ions replace water ligands?

   [1 mark]

   • AChloride ions are bidentate, so each occupies two coordination sites
   • BChloride ions are smaller than water molecules, so fewer are needed
   • CChloride ions are uncharged, so they bond more weakly
   • DChloride ions are larger than water molecules, so only four can fit around the Co²⁺ ion

   (c)

   Explain why the colour of the blue solution changes to pink when water is added.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student studies two reactions of aqueous copper(II) sulfate. In the first, concentrated hydrochloric acid is added to the solution. In the second, she prepares crystals of tetraamminecopper(II) sulfate monohydrate, [Cu(NH₃)₄]SO₄·H₂O, by adding excess concentrated ammonia solution to copper(II) sulfate solution and then adding ethanol.

   (a)

   Describe what happens when concentrated hydrochloric acid is added to aqueous copper(II) sulfate. Include an equation and the change in shape of the complex ion.

   [3 marks]

   (b)

   In the second reaction, the crystals are washed with a little ethanol and then dried between filter papers rather than being heated in an oven. Explain why ethanol is added to the reaction mixture, why the crystals are washed with ethanol, and why they are not heated.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Aqueous [Ni(NH₃)₆]²⁺ reacts with 1,2-diaminoethane (en, H₂NCH₂CH₂NH₂) in a ligand exchange reaction: [Ni(NH₃)₆]²⁺ + 3en → [Ni(en)₃]²⁺ + 6NH₃. At 298 K the enthalpy change is −12 kJ mol⁻¹ and the entropy change of the system, ΔS_system, is +145 J K⁻¹ mol⁻¹.

   (a)

   Calculate ΔS_total for this reaction at 298 K, and explain why the reaction takes place in terms of the particles involved and the bonds formed.

   [6 marks]

   (b)

   EDTA⁴⁻ is a hexadentate ligand. It reacts with [Ni(H₂O)₆]²⁺ in the reaction [Ni(H₂O)₆]²⁺ + EDTA⁴⁻ → [Ni(EDTA)]²⁻ + 6H₂O. Explain why this reaction goes essentially to completion, whereas the reaction [Ni(H₂O)₆]²⁺ + 6NH₃ → [Ni(NH₃)₆]²⁺ + 6H₂O gives a complex of much lower stability.

   [6 marks]

   Total for question 4: 12 marks

End of questions