MHS Chemistry
The Copper Cycle

In this experiment you will start with a piece of copper metal, dissolve it in concentrated nitric acid, precipitate it as copper (II) hydroxide, dissolve it in hydrochloric acid, and reduce everything back to copper metal.  You will observe the appearance of each reactant and product.  Finally, you will carefully calculate the percent of copper recovered, keeping in mind what we have learned about significant digits.  (A possible variation is to start with an old penny, and determine the % mass that is copper.)

WARNING:  Concentrated nitric acid, concentrated hydrochloric acid, and 6 M NaOH are all corrosive to flesh and can cause severe burns or blindness.  Follow all safety rules from your instructor! WEAR GOGGLES AND APRONS!!!!

Before starting, make sure you have a copy of the Copper Cycle Observations form.  If you lose yours, click here to print a new one.

Procedure - First Day
 

  1. Get a piece of copper from your teacher and record its mass to the nearest milligram.  Aim for about 0.5 grams, but record the exact mass.

    2.  
  2. Place the copper into a clean 250 mL beaker and bring it to the fume hood.  Carefully add about 5 mL of concentrated nitric acid.  Leave it in the fume hood until the brown gas (dinitrogen tetroxide, N2O4) has dissipated and all the copper has dissolved.

    3. Cu(s) + NO3(aq)- + H(aq)+ ® Cu(aq)2+ + N2O4(g) + H2O(l)

    then        N2O4(g) ® 2NO2(g)
     

  3. When the copper has dissolved add distilled water, while stirring with a glass rod, until the beaker is about half full.  CAREFULLY bring the beaker (with the stirring rod in it) back to your lab bench.

    4.  
  4. While stirring gently and slowly - over ten seconds or so - add about 25 mL of 6 M NaOH.  You should get a sludgy precipitate of copper (II) hydroxide.

    5. Cu(aq)2+ + 2OH(aq)-® Cu(OH)2(s)
     
  5. With steady stirring, heat the beaker until the suspension begins to boil.  BE CAREFUL - keep stirring and heat gently!  You should get a dense black copper (II) oxide.  Allow it to cool with the stirring rod across the top of the beaker - not in it.  Allow it to settle for at least 10 minutes.

    6. Cu(OH)2(s)® CuO(s) + H2O(g)
     
  6. While it is cooling, heat about 200 mL of distilled water in another beaker.

    7.  
  7. Decant the solution with the copper (II) oxide and replace the liquid you pour off with hot water.  Stir and then allow it to settle again.  When the black copper (II) oxide has settled, decant and rinse a few more times.  Finally, add water until the beaker is about half full.
    8.  (If you are running out of time, decant the water from your settled precipitate, cover your beaker, and start here next time.)
     
  8. Add about 10 mL of concentrated hydrochloric acid (HCl) and stir gently until all the copper (II) oxide has dissolved.

    9. CuO(aq) + 2H(aq)+® Cu(aq)2+ + H2O(l)
Procedure - Second Day
  1. Add one or two small squares of aluminum foil (or a zinc strip) to the solution and stir.  The aluminum (or zinc) will dissolve, black specks will appear, and a gas will bubble away.  Continue adding aluminum one square at a time until the solution’s blue color cannot be seen.

    2. 3Cu(aq)2+ + 2Al(s)® 3Cu(s) + 2Al(aq)3+        OR        Cu(aq)2+ + Zn(s) ® Cu(s) + Zn(aq)2+
     
  2. Test for the presence of more copper (II) ions in the solution by putting about half a dropper-full of the solution into a small test tube.  Add a drop or two of concentrated ammonia solution to the test tube.  A deep blue color means there are more copper (II) ions in the solution and you should repeat step 9.  If the color is not blue then go on to the next step.

    3.  
  3. Mass a piece of filter paper.  Set up a funnel with the filter paper and carefully filter the suspension of copper metal.  Rinse the beaker with water several times to get all the copper.  Also, rinse the filter paper from the top down to wash away any impurities mixed in with the copper.

    4.  
  4. Allow the filter paper to dry to a constant mass.  Determine the mass of copper recovered.  Calculate the percentage of the original copper recovered.

    5.  
  5. Make sure everything is cleaned up.

    6.  
  6. Briefly explain why your % recovered was not exactly 100%.  Be specific.
Note
Your lab write up should be one sheet.  Make sure the title section is on the front, and that there is a clear and complete data section.  Show all calculations clearly as discussed in class, and make sure to include the analysis section (step 14).

[Copper Cycle score sheet][Copper Cycle analysis][MHS Chem page]