Expt 064 -- Copper Ammine Complex Formation

Aqueous copper(II) ion is mixed with solutions of ammonia of varying concentration. The formation of copper(II) hydroxide and the copper ammine complex is observed.

Background

Copper(II) undergoes two reactions studied here:

1. Cu²⁺(aq) + 2 OH^-(aq) --> Cu(OH)₂(s)

   Cu(OH)₂(s) is pale blue.

2. Cu²⁺(aq) + 4 NH₃(aq) --> Cu(NH₃)₄²⁺(aq)

   Cu(NH₃)₄²⁺(aq) is a deep blue in color.

Safety

NaOH and HCl are caustic and corrosive. Ammonia, copper chloride, NaOH, and HCl are toxic. Wear goggles and apron. The room must be well ventilated. Wash hands before leaving the laboratory.

Procedure

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.
3. 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.

   Well:	1	2	3	4	5	6	7	8
   CuCl2	4	4	4	4	4	4	4	4
   NH3	0	1	2	3	4	5	6	7
   H2O	8	7	6	5	4	3	2	1

4. Stir each well with a toothpick. (Use the same toothpick for all wells; transfer from well to well is minimal for this experiment.)

   !!!Click here to See Movie.

5. Allow the mixture to settle. Note and record observations for reaction in each well.

   !!!Click here to See Picture.

6. Remove liquid on top of the precipitate. Test 2 of these wells for further reaction by adding the reagents below.
7. Add 10 drops of 0.2 M HCl to well 6.
8. Add 10 drops of 0.2 M NaOH to well 8.

   Well:	1	2	3	4	5	6	7	8
   NaOH								10
   HCl						10

9. Note and record any evidence for reaction.

   !!!Click here to See Picture.

Handout

Name ___________________________ Class ________

Teacher__________________________

SmallScale 64 Copper Ammine Complex Formation

Makeup students should record the data from the pictures on the CD-ROM.

Carefully record the colors and the presence of any precipitate.

Well	Color	Precipitate(?)
1
2
3
4
5
6
7
8
6+HCl
8+NaOH

Write a brief report accounting for the observations made for these reaction mixtures.

Curriculum-

Use when discussing equilibrium, Le Chatelier's principle, precipitation, complex ion formation, transition elements, or coupled equilibria.

Safety-

NaOH and HCl are caustic and corrosive. Ammonia, copper chloride, NaOH, and HCl are toxic. Wear goggles and apron. The room must be well ventilated. Wash hands before leaving the laboratory.

Time-

Teacher Preparation: 15 minutes

Class Time: 40 minutes

Materials-

• 2 mL of 0.1 M copper(II) chloride solution (dissolve 1.70 g of CuCl₂•2H₂O in enough water to make 100 mL of solution.)
• 2 mL of 0.5 M ammonium hydroxide solution (add 17 mL 3 M NH₃ to 60 mL of distilled water, and add enough distilled water to bring the final volume to 100 mL.)
• 1 mL of 0.2 M hydrochloric acid solution (add 7 mL of 3 M HCl to 60 mL of distilled water, and add enough distilled water to bring the final volume to 100 mL.)
• 1 mL of 0.2 M sodium hydroxide solution (add 7 mL of 3 M NaOH to enough distilled water to bring the final volume to 100 mL.)
• 12-well strip
• distilled water
• toothpicks
• cotton swab

Disposal-

After neutralization, these chemicals may be disposed of at the sink using large volumes of water.

Data Analysis-

When ammonia is added, both OH^- and NH₃ are provided. At first the OH^- reacts with Cu²⁺ to produce Cu(OH)₂. As more NH₃ is added, the concentration of NH₃ made available increases much faster than the [OH^-]. (NH₃ is a weak base.) Once the [NH₃] becomes high enough, the Cu(NH₃)₄²⁺ complex forms at the expense of Cu(OH)₂.

Literature Data-

K_sp Cu(OH)₂ = 1.3 x 10^-20

K_stab Cu(NH₃)₄²⁺ = 1.2 x 10¹²

Reference-

An early version of this experiment was developed by John M. Boblick, Poolesville, MD at the 1987 Dreyfus Woodrow Wilson summer program.

Key Words 1-

precipitation, solubility, complex ion, ligand

Elements-

Cu N