Copper-Ammonia Complex

Category: 
Demonstration
Topics: 
Acid-Base Equilibria
Acids and Bases
Complex Ions
Equilibrium
Solutions
States of Matter

When concentrated ammonia solution (ammonium hydroxide) is added to a clear, light blue, aqueous solution of copper(II) chloride, a powdery, light blue precipitate of copper(II) hydroxide forms. Further addition of ammonia causes the copper ion to go back into solution as a deep blue ammonia complex. The addition of 12M sulfuric acid reverses the changes through the copper hydroxide precipitate back to clear, light blue color of the original solution. 

Curriculum Notes 

This demo is a good illustration of Le Chatelier's principle and of complex ion formation. Allow about 10 minutes for this demo.

Two days of lead time is required for this project.

Discussion 

  • The copper ion in the aqueous solution of exists predominantly as [Cu(H2O)6]2+. This complex ion imparts a characteristic pale blue color to the solution. Since ammonia is a weak base, when it is added, hydroxide ion forms:
    • NH3(aq) + H2O(l) <==> NH4+(aq) + OH-(aq); pKb = 9.25 (1)
  • The hydroxide ion reacts with the hexaaquacopper(II) ion to form the insoluble compound, copper(II) hydroxide dihydrate:
    • [Cu(H2O)6]2+(aq) + 2 OH-(aq) <==> Cu(OH)2 * 2 H2O(s) + 2 H2O(l)
  • In the typical intro Gen Chem class it is probably sufficient to represent the system thus:
    • Cu2+(aq) + 2 OH-(aq) <==> Cu(OH)2 (s) ; Ksp = 2.2 X 10-20 (2)
  • Continued addition of ammonia results the formation of a soluble deep blue complex copper ammonia ion:
    • [Cu(H2O)6]2+(aq) + 4 NH3 (aq) <==> [Cu(NH3)4(H2O)2]2+(aq) + 4 H2O(l); Kc = 1.2 X 1013 (3)
  • In a typical intro Gen Chem class this might be represented thus:
    • Cu2+(aq) + 4 NH3 (aq) <==> Cu(NH3)42+(aq)(l)

Materials 

  • 500 mL three-necked round-bottom flask containing 200 mL of 0.1M copper(II) chloride dihydrate in water and a magnetic stir bar, 24/40 vertical ground glass joints.
  • vented addition funnel w/stopper containing 150 ml of 12M sulfuric acid acid, 24/40 ground glass joints.
  • addition funnel containing 150 ml of concentrated ammonia solution (ammonium hydroxide), 24/40 ground glass joints.
  • ring stand and clamps to secure apparatus.
  • magnetic stir plate.
  • white background (optional)

Procedure 

  • turn on stir plate
  • slowly add ammonia until a pale blue precipitate forms
  • add more ammonia until a deep blue solution forms
  • add sulfuric acid until the pale blue precipitate reforms
  • continue to add sulfuric acid until the original clear, pale blue solution reforms

Safety Precautions 

  • The ammonia solution gives off ammonia gas vapor. This demo should only be performed in a well-ventilated room. Avoid breathing ammonia vapors.
  • This reaction uses hazardous substances - see below.
  • 12 M Sulfuric Acid
    • Inhalation: Inhalation produces damaging effects on the mucous membranes and upper respiratory tract. Symptoms may include irritation of the nose and throat, and labored breathing. May cause lung edema, a medical emergency.
    • Ingestion: Corrosive. Swallowing can cause severe burns of the mouth, throat, and stomach, leading to death. Can cause sore throat, vomiting, diarrhea. Circulatory collapse with clammy skin, weak and rapid pulse, shallow respirations, and scanty urine may follow ingestion or skin contact. Circulatory shock is often the immediate cause of death.
    • Skin Contact: Corrosive. Symptoms of redness, pain, and severe burn can occur. Circulatory collapse with clammy skin, weak and rapid pulse, shallow respirations, and scanty urine may follow skin contact or ingestion. Circulatory shock is often the immediate cause of death.
    • Eye Contact: Corrosive. Contact can cause blurred vision, redness, pain and severe tissue burns. Can cause blindness.
  • Concentrated Ammonia solution (Ammonium Hydroxide):
    • Inhalation: Vapors and mists cause irritation to the respiratory tract. Higher concentrations can cause burns, pulmonary edema and death. Brief exposure to 5000 ppm can be fatal.
    • Ingestion: Toxic! May cause corrosion to the esophagus and stomach with perforation and peritonitis. Symptoms may include pain in the mouth, chest, and abdomen, with coughing, vomiting and collapse. Ingestion of as little as 3-4 mL may be fatal.
    • Skin Contact:Causes irritation and burns to the skin.
    • Eye Contact: Vapors cause irritation. Splashes cause severe pain, eye damage, and permanent blindness.
  • Copper Chloride Solution
    • Ingestion: Induce vomiting immediately as directed by medical personnel. Never give anything by mouth to an unconscious person. Call a physician immediately.
    • Skin Contact: In case of contact, wipe off excess material from skin then immediately flush skin with plenty of water for at least 15 minutes. Remove contaminated clothing and shoes. Wash clothing before reuse. Call a physician.
    • Eye Contact: Immediately flush eyes with plenty of water for at least 15 minutes, lifting lower and upper eyelids occasionally. Get medical attention immediately.