Copper Ammine Complex Formation

Introduction

The reaction which a copper (II) ion undergoes within a solution of ammonia water (ammonium hydroxide) is dependent upon the concentration of the ammonia solution. The purpose of this experiment is to investigate the reactions between copper (II) ions and a varying concentration of the ammonia solution.

When ammonia is dissolved in water, the interaction of the ammonia molecules with the water molecules results in the production of ammonium ions and hydroxide ions. One possible reaction for the copper (II) ions is the combination with the hydroxide ions to form a slightly soluble copper (II) hydroxide, which can be identified by the formation of a cloudy, pale blue precipitate.

Another possible product of a reaction of the copper (II) ions, perhaps less easily recognized from the reactants in this system, is the formation of a copper (II) ion ammonia complex, usually identified by a characteristic deep blue color.

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1. Place 4 drops of 0.1 M copper chloride into each of the first 8 wells of a 12-well strip.
2. Add 1 drop of 0.5 M NH₃ to well 2, 2 drops to well 3, and so on until 7 drops are added to well 8. Add 8 drops of distilled water to well 1, 7 drops to well 2, and so on ending with 1 drop added to well 8.
3. Stir each well with a toothpick. (Use the same toothpick for all wells; transfer from well to well is minimal.)
4. Note and record observations for reaction in each well.
5. Add 10 drops of 0.2 M NaOH to well 8. Note any evidence for reaction.
6. Add 10 drops of 0.2 M HCl to well 6, Note any evidence for reaction.

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Describe the changes which took place when the
0.2 M hydrochloric acid was added to the selected well. Also describe the effect of adding 0.2 M sodium hydroxide to the well. Propose an explanation for each of these reactions.

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Safety

• Wear eye protection and aprons.
• NaOH is caustic and corrosive.
• HCl is caustic and corrosive.

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• The student will observe two distinctively different results in the set of 8 wells in which the reactions occur. At the lower concentrations of ammonium hydroxide a pale blue precipitate will be seen.
• This precipitate is composed of copper(II) hydroxide.
  • Cu²⁺(aq) + 2OH^-(aq) --> Cu(OH)₂(s)
• At the higher concentrations of ammonium hydroxide (wells 7 - 8), the characteristic deep blue color of the copper(II) ion ammonia complex is seen.
  • Cu²⁺(aq) + 4NH₃(aq) --> Cu(NH₃)₄²⁺(aq)
• The addition of 0.2 M hydrochloric acid to a well containing a low ammonia concentration will cause the precipitate to dissolve, while the addition of HCl to the higher concentrations of ammonia will cause the deep blue complex to decompose and the color to fade.
• Addition of 0.2 M sodium hydroxide causes the precipitate found at the lower concentrations of ammonia to dissolve and the blue complex to form. Sodium hydroxide solution, added to the wells containing blue complex, will cause the contents to become a lighter blue and to form the precipitate.
• The changes produced by the addition of the hydrochloric acid and sodium hydroxide solutions may be explained by applying Le Chatelier's principle to the equilibrium system involving the copper (II) ion and the ammonia solution.

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• 12-well strips
• 0.1 M copper(II) chloride solution (dissolve 1.7 g CuCl₂•2H₂O in water to make 100 mL of solution)
• 0.5 M ammonium hydroxide solution (add 3.3 mL concentrated ammonium hydroxide to 60 mL of distilled water, and add enough distilled water to bring the final volume to 100 mL.)
• 0.2 M hydrochloric acid solution (Add 1.7 mL of concentrated HCl to 60 mL of distilled water, and add enough distilled water to bring the final volume to 100 mL.)
• 0.2 M sodium hydroxide solution (add 0.80 g NaOH to enough distilled water to bring the final volume to 100 mL.)
• distilled water
• toothpicks
• cotton swabs

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The Woodrow Wilson microscale version of this lesson was developed by:

John M. Boblick
Science Resource Teacher
Poolesville Jr/Sr High School
Poolesville, MD 20837

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