An equilibrium involving copper(II) ions
Students add ammonia to a solution of copper(II)sulfate, observe the colour changestaking 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.
Copper(II) sulfate solution, 1.0 M (HARMFUL), about 3 cm3 (Note 1)
Ammonia solution, 1.0 M, about 10 cm3
Dilute sulfuric acid, 1.0 M (IRRITANT), about 10 cm3
Refer to Health & Safety and Technical notes section below for additional information.
Each student or working group will require:
Dropping pipettes, 2
Health & Safety and Technical notes
Copper(II) sulfate solution, CuSO4(aq), (HARMFUL) - see CLEAPSS Hazcard and CLEAPSS Recipe Book
Ammonia solution, NH3(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
Dilute sulfuric acid, H2SO4(aq) (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
1 The copper(II) sulfate solution is most conveniently supplied in a bottle fitted with teat pipette.
a Put 10 drops of copper(II) sulfate solution into each of two test-tubes.
b 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?
c Add more drops of ammonia solution. What happens? Continue until you have a clear blue solution.
d Divide the solution from c into two test-tubes. Add dilute sulfuric acid drop-by-drop to one of the solutions from c. 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?
e Can you repeat the whole process by adding ammonia again to the acidified solution?
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 hexaaquacopper(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 tetraamminediaquacopper(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 hexaaquacopper(II) ions in the process.
Health & Safety checked August 2008
The University of Sydney - this describes how to obtain a solid sample of a salt containing the dark blue tetraamminediaquacopper(II) ions.
Page last updated on 12 July 2012