Try this class practical to explore an equilibrium involving copper(II) ions

In this experiment, students add ammonia to a solution of copper(II) sulfate, observe the colour changes taking place, and then reverse the reaction by the addition of sulfuric acid.

This experiment is best carried out by students working individually. It takes about 15–20 minutes.

Equipment

Apparatus

  • Eye protection
  • Test tubes, x3
  • Test tube rack
  • Test tube holder
  • Dropping pipettes, x2

Chemicals

  • Copper(II) sulfate solution, 1.0 M (HARMFUL), about 3 cm3 (see note 3 below)
  • Ammonia solution, 1.0 M, about 10 cm3
  • Dilute sulfuric acid, 1.0 M (IRRITANT), about 10 cm3

Health, safety and technical notes

  1. Read our standard health and safety guidance.
  2. Wear eye protection throughout.
  3. Copper(II) sulfate solution, CuSO4(aq), (HARMFUL) – see CLEAPSS Hazcard HC027c and CLEAPSS Recipe Book RB031. The copper(II) sulfate solution is most conveniently supplied in a bottle fitted with teat pipette.
  4. Ammonia solution, NH3(aq) – see CLEAPSS Hazcard HC006 and CLEAPSS Recipe Book RB006.
  5. Dilute sulfuric acid, H2SO4(aq) (IRRITANT) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098.

Procedure

  1. While wearing eye protection, put 10 drops of copper(II) sulfate solution into each of two test tubes.
  2. Add ammonia solution drop-by-drop to the first test tube. Shake the tube gently from side to side after adding each drop. What happens as you add a few drops of the solution?
  3. Add more drops of ammonia solution. What happens? Continue until you have a clear blue solution.
  4. Divide the solution from step 3 into two test tubes. Add dilute sulfuric acid drop-by-drop to one of the solutions from step 3. Shake the tube gently from side to side after adding each drop. Do you get back to where you started – compare the three test tubes?
  5. Can you repeat the whole process by adding ammonia again to the acidified solution?

Teaching notes

If this experiment is being carried out with pre-A-level students, the reactions occurring can simply be explained by reference to the addition of an alkali (containing hydroxide ions) being added to a solution of a copper compound, producing copper(II) hydroxide initially and later a complex compound of ammonia. The reversal of the process is easy to explain since sulfuric acid is capable of neutralising the alkaline ammonia and causing the reaction to reverse back to the start:

CuSO4(aq) (pale blue solution) + 2NH3(aq) + 2H2O(l) → Cu(OH)2(s) + (NH4)2SO4(aq) (pale blue precipitate)

Cu(OH)2(s) (pale blue precipitate) + ammonia → complex copper compound (dark blue solution)

A rather more advanced treatment in terms of complexes and ligand exchange would involve the following explanation:

  1. Ammonia is a weak base and forms a few ammonium and hydroxide ions in solution:

    NH3(g) + H2O(l) ⇌ NH4+(aq) + OH(aq)
  2. The hexa-aqua-copper(II) ions react with hydroxide ions to form a precipitate. This involves deprotonation of two of the water ligand molecules:

    [Cu(H2O)6]2+(aq)(pale blue) + 2OH(aq) → [Cu(H2O)4(OH)2](s)(pale blue precipitate) + 2H2O(l)
  3. The copper(II) hydroxide precipitate reacts with ammonia molecules to form tetra-amine-di-aqua-copper(II) ions This involves ligand exchange:

    [Cu(H2O)4(OH)2](s)(pale blue precipitate) + 4NH3(aq) ⇌ [Cu(NH3)4(H2O)2)]2+(aq)(dark blue solution) + 2OH(aq) + 2H2O(l)
  4. Thus the overall reaction, combining 2 with 3, gives:

    [Cu(H2O)6]2+(aq) + 4NH3(aq) ⇌ [Cu(NH3)4(H2O)2)]2+(aq) + 4H2O(l)
  5. Addition of dilute sulfuric acid introduces H+ ions, which react with NH3 molecules to form NH4+ ions, and this draws the equilibrium in 4 back to the left-hand side, regenerating the hexa-aqua-copper(II) ions in the process.