Ligand substitution and the chelate effectAQA A-Level Chemistry: Mind map

Exam questions on Ligand substitution and the chelate effect

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⁺.
   Explain why the co-ordination number of copper does not change when ammonia replaces water in this reaction.2 marks
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.
   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
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.
   Explain why en is described as a bidentate ligand and state the co-ordination number of nickel in [Ni(en)₃]²⁺.3 marks