Ligand substitution and the chelate effectAQA A-Level Chemistry: Subtopic test

1. 1
   Aqueous copper(II) sulfate contains the pale blue complex ion [Cu(H₂O)₆]²⁺. When an excess of concentrated ammonia solution is added the colour changes to deep blue, but the product is [Cu(NH₃)₄(H₂O)₂]²⁺ rather than [Cu(NH₃)₆]²⁺.

   (a)

   Which equation represents the reaction that forms the deep blue complex ion?

   [1 mark]

   • A[Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 6H₂O
   • B[Cu(H₂O)₆]²⁺ + 6NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 6H₂O
   • C[Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
   • D[Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]⁴⁺ + 4H₂O

   (b)

   What type of reaction is the change from [Cu(H₂O)₆]²⁺ to [Cu(NH₃)₄(H₂O)₂]²⁺?

   [1 mark]

   • ALigand substitution
   • BOxidation of copper
   • CPrecipitation
   • DCondensation

   (c)

   Explain why the co-ordination number of copper does not change when ammonia replaces water in this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student investigates reactions of aqueous metal ions with concentrated hydrochloric acid. Aqueous copper(II) sulfate contains [Cu(H₂O)₆]²⁺ and aqueous iron(III) nitrate contains [Fe(H₂O)₆]³⁺. Each reacts with the acid to form a yellow-green solution of [CuCl₄]²⁻ and a yellow solution of [FeCl₄]⁻ respectively.

   (a)

   Which equation represents the reaction of the iron(III) complex with chloride ions?

   [1 mark]

   • A[Fe(H₂O)₆]³⁺ + 6Cl⁻ ⇌ [FeCl₆]³⁻ + 6H₂O
   • B[Fe(H₂O)₆]³⁺ + 4Cl⁻ ⇌ [FeCl₄]³⁻ + 6H₂O
   • C[Fe(H₂O)₆]³⁺ + 4Cl⁻ ⇌ [FeCl₄]⁻ + 4H₂O
   • D[Fe(H₂O)₆]³⁺ + 4Cl⁻ ⇌ [FeCl₄]⁻ + 6H₂O

   (b)

   Which statement explains why the co-ordination number changes from 6 to 4 in these reactions?

   [1 mark]

   • ACl⁻ is smaller than H₂O so fewer ligands are needed
   • BCl⁻ is larger than H₂O so only four can fit around the central ion
   • CCl⁻ is a bidentate ligand so each bonds twice
   • DCl⁻ is uncharged so it is repelled less

   (c)

   Write an equation for the formation of [CuCl₄]²⁻ from [Cu(H₂O)₆]²⁺ and state the change in co-ordination number of copper.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Nickel(II) ions in aqueous solution form [Ni(H₂O)₆]²⁺, and adding excess ammonia solution gives [Ni(NH₃)₆]²⁺. When ethane-1,2-diamine, H₂NCH₂CH₂NH₂ (en), is added, the ammonia ligands are replaced: [Ni(NH₃)₆]²⁺ + 3en → [Ni(en)₃]²⁺ + 6NH₃. This reaction goes essentially to completion at room temperature.

   (a)

   Explain why en is described as a bidentate ligand and state the co-ordination number of nickel in [Ni(en)₃]²⁺.

   [3 marks]

   (b)

   Explain, in terms of entropy and enthalpy change, why the reaction goes essentially to completion.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The blood protein haemoglobin contains haem, an iron(II) complex in which the Fe²⁺ ion is bonded to a multidentate ligand. Oxygen forms a co-ordinate bond to the Fe²⁺ ion so that it can be carried from the lungs to the tissues. Carbon monoxide is a toxic gas. Separately, the hexadentate ligand EDTA⁴⁻ is used in some medical treatments to remove toxic metal ions M²⁺ from the body by the reaction [M(H₂O)₆]²⁺ + EDTA⁴⁻ → [M(EDTA)]²⁻ + 6H₂O.

   (a)

   Explain how haemoglobin transports oxygen, and why carbon monoxide is toxic.

   [6 marks]

   (b)

   For the reaction of [Cu(H₂O)₆]²⁺ with EDTA⁴⁻ at 298 K, ΔH = −34 kJ mol⁻¹ and ΔS = +245 J K⁻¹ mol⁻¹. Calculate ΔG and explain, in terms of entropy and enthalpy, why EDTA⁴⁻ replaces the water ligands.

   [6 marks]

   Total for question 4: 12 marks

End of questions