Mr Cole Chemistry

The Chelate Effect

  • Questions
  • Answers
  1. 1,2-diaminoethane is added to a solution containing chromium (II) ions.
    1. Write an equation for the ligand substitution reaction that occurs.
      [Cr(H2O)6]2+ + 3 H2NCH2CH2NH2 –> [Cr(H2NCH2CH2NH2)3]2+ + 6H2O
    2. Draw the complex that forms in the reaction.
    3. Explain why this reaction is likely to be thermodynamically feasible.
      This reaction is not feasible because six monodentate ligands are being replaced with three bidentate ligands. The coordination number does not change (it remains at 6). This means that the number of bonds being made and broken are equal, which means that the enthalpy change of the reaction is going to be very low. This means that the entropy change of the reaction is going to determine Gibbs Free Energy. The entropy change of the reaction is positive, because the number of independently moving molecules has increased.
  2. EDTA4- is added to a solution of copper (II) ions. Following this, ammonia is added to the solution.
    1. Write an equation for the ligand substitution reaction that occurs when EDTA– is added.
      [Cu(H2O)6]2+ + EDTA– –> [CrEDTA]4- + 6H2O
    2. Explain what is meant by the term multidentate.
      A ligand that makes many coordinate bonds with the central ion.
    3. Explain why no reaction occurs when the ammonia is added.
      A ligand substitution reaction would be replacing one multidentate ligand with six monodentate ligands. This would result in a positive Gibbs Free Energy as enthalpy change would be very small but entropy would be decreasing.
  1. 1,2-diaminoethane is added to a solution containing chromium (II) ions.
    1. Write an equation for the ligand substitution reaction that occurs.
    2. Draw the complex that forms in the reaction.
    3. Explain why this reaction is likely to be thermodynamically feasible.
  2. EDTA4- is added to a solution of copper (II) ions. Following this, ammonia is added to the solution.
    1. Write an equation for the ligand substitution reaction that occurs when EDTA– is added.
    2. Explain what is meant by the term multidentate.
    3. Explain why no reaction occurs when the ammonia is added.
Return to Topic