A Sequence of Chemical Reactions (2024)

• Objectives
• Reactions
• Procedure

Objectives

• Illustrate variety of substances of which an element can be part:

  metal --> blue solution --> blue solid --> black solid --> blue solution (again) --> metal (again).

• Conservation of mass and of moles:
  • We should recover as much copper as we started with.
  • Same amount of copper present at every stage: same number of moles.
• Experience in standard chemical techniques: filtration and quantitative transfers.

Reactions

1. Copper metal "dissolves" in nitric acid (HNO₃). Actually, the nitrate ion oxidizes the copper metal to copper (II) ion while itself being transformed to NO₂ gas in the process; the copper (II) ion then binds to six water molecules. The physical change you should observe is the copper-colored metal vanishing as the solution turns blue (from [Cu(H₂O)₆]²⁺, the hexaaquacopper ion) and a brown gas (NO₂) is evolved.

   Cu (s) + 4 H₃O⁺ (aq) + 2 NO₃^- (aq) --> [Cu(H₂O)₆]²⁺ (aq) + 2 NO₂ (g)

   
   
2. Hydroxide ion (OH-) binds to the copper (II) ion even more strongly than does water. As a result, hydroxide ion can displace water from the copper (II) ion, yielding copper hydroxide, Cu(OH)₂, a blue precipitate.

   [Cu(H₂O)₆]²⁺ (aq) + 2 OH^- --> Cu(OH)₂ (s) + 6 H₂O (l)

   
   
3. Heating copper hydroxide produces copper oxide, CuO, a black solid.

   Cu(OH)₂ (s) --> CuO (s) + H₂O (l)

   
   
4. Copper oxide dissolves in acid, regenerating the copper (II) ion, which once again binds to water.

   CuO (s) + 2 H₃O⁺ (aq) + 3 H₂O (l) --> [Cu(H₂O)₆]²⁺ (aq)

   
   
5. Finally, zinc metal reduces the hydrated copper (II) ion back to metallic copper while itself turning being oxidized to zinc (II) ions. We have seen this reaction before in the copper chloride lab).

   [Cu(H₂O)₆]²⁺ (aq) + Zn (s) --> Cu (s) + Zn²⁺ (aq) + 6 H₂O (aq)

   
   At the same time, some of the zinc metal, which is present in excess, reduces hydronium ions to H₂.

   Zn (s) + 2 H₃O⁺ (aq) --> Zn²⁺ (aq) + H₂ (g) + 2 H₂O (l)

   
   

Procedure

1. Transform Cu(s) to [Cu(H₂O)₆]²⁺(aq)
   • Obtain a piece of copper wire and weigh it to the nearest 0.01 g. The pieces of wire are closer to 0.50 g than 0.35 g. That's OK: use the pieces that we provide.
   • Use about 4-5 ml of concentrated HNO₃ solution.
   • Be careful with the nitric acid: like other strong acids, it will sting if you get it on your skin and can damage clothing; unlike most other acids, it will also stain the affected area yellow.
   • If some copper remains undissolved by the time the production of gas is finished, then put the beaker on the hot plates in the hoods to hasten the reaction.
   • It is important to carry out this step in the fume hood because the brown NO₂ gas is an irritant. Keep the mixtures in the hood until after you add the 10 ml of distilled water after completely dissolving the copper.
2. Transform [Cu(H₂O)₆]²⁺ (aq) to Cu(OH)₂(s)
   • Be careful in handling NaOH, for it is a strong base which will sting if it contacts the skin. Add the NaOH solution dropwise to the copper solution.
   • After a blue precipitate is formed, periodically test the acidity of the solution by dipping your stirring rod into the solution and touching it to red litmus paper. Try not to transfer the blue precipitate onto the litmus paper: that will result in some loss of copper, and a possibly false blue on the litmus paper. The solution starts out acidic because of excess nitric acid from the previous step, so the first OH- added goes into neutralizing the acid; once the acid is neutralized, the next OH^- added goes to forming the blue Cu(OH)₂ precipitate. Only after that is finished does added OH^- hang around idle, and only at that time will it turn red litmus paper blue. We want to make sure all the copper present is turned to Cu(OH)₂, so we add OH^- until the solution turns the litmus paper blue.
3. Transform Cu(OH)₂(s) to CuO(s)
   • Add water to the reaction mixture obtained in the previous step, and add one or two boiling stones as well.
   • Heat the contents of the beaker, but do not boil. Boiling makes the black CuO so fine that the filtration step is excessively long. Heat the beaker until all the blue Cu(OH)₂ disappears and is replaced by black CuO.
   • Filter and wash the CuO as described in the procedure (part C). Keep the solid on the filter paper, and discard the filtrate.
4. Transform CuO(s) back to [Cu(H₂O)₆]²⁺(aq)
   • Dissolve the CuO on the filter paper as described in the procedure (part D).
   • Sulfuric acid solution is corrosive and will sting skin with which it comes into contact.
5. Transform [Cu(H₂O)₆]²⁺(aq) back to Cu(s)
   • Add about 1 g Zn to the blue solution obtained in the previous step, and after the solution has lost all its blue color, you may need to add some sulfuric acid to react any excess Zn.
   • Wash the copper metal three times with distilled water and transfer it to an evaporating dish as described in the procedure (part E), and then wash it three times with 5-ml portions of isopropanol. Washing with isopropanol will reduce the time needed for the drying step.
   • Dry the copper over a beaker of boiling water as described in the procedure (E). Weigh the dry copper and record the mass. Compute the percent of copper recovered.