Purpose:

Through a series of five chemical reactions (see figure below), you will convert solid copper back into solid copper!  While this may seem to be a pointless exercise, it will actually provide a thorough test of all the laboratory skills you have learned so far (weighing, filtering, transferring, attention to detail, and observations), as well as understanding chemical reactions (types, and balancing), and calculating theoretical yields from a sequential reaction:


Introduction:

Many compounds that do not occur naturally or are difficult to obtain may be synthesized in the laboratory.  A particular synthesis may proceed through several individual steps that involve the formation of many intermediate chemical compounds on the way to the final product.  Some of these steps do not produce 100% yields and so you obtain less product than you would have expected.  Chemists often devote a great deal of time and effort investigating the reaction conditions to improve the final yield.  However, there are even more subtle problems:  one's laboratory skills.  Even if the reaction can be coaxed to provide a 100% yield, if you drop part of it on the way to the balance, or do not fully dry it, or leave some of it on a piece of filter paper, you may significantly reduce your final yield.

Today's synthesis begins with an oxidation-reduction reaction where Cu (s) dissolves with nitric acid to form Cu2+(aq):

3 Cu(s)  +  8 HNO3(aq)     3 Cu(NO3)2(aq)  +  2 NO(g)  +  4 H2O(l)

3 Cu(s)  +  8 H+(aq)   +  8 NO3-(aq)     3 Cu 2+(aq)  +  6 NO3-(aq)   +  2 NO(g)  +  4 H2O(l)

3 Cu(s)  +  8 H+(aq)  +  2 NO3-(aq)     3 Cu 2+(aq)  +  2 NO(g)  +  4 H2O(l)

The three equations above, represent the balanced chemical equation or formula unit, the total ionic equation and net ionic equation for the oxidation of Cu(s) to Cu2+(aq) by the NO3-(aq) ion.  The solution goes through several color changes that represent different oxidation states of copper.  Finally, a blue aqueous solution of Cu2+ ions is produced.  At the same time the reduction of NO3-(aq) ion produced the colorless gas nitrogen monoxide, NO.  However, NO is extremely reactive and is converted to the orange-brown gas nitrogen dioxide, NO2 as soon as it comes in contact with the O2 in the air.  Nitrogen dioxide is the same gas you see over highly polluted urban areas.

The next step in the synthesis involves an acid-base reaction of Cu 2+(aq) to form Cu(OH)2 (s):

NaOH(aq) +  HNO3(aq)     NaNO3(aq) +  H2O(l)

Na+(aq)  +  OH -(aq)  +  H+(aq)  +  NO-3(aq)      Na +(aq)  +  NO3-(aq)   +  H2O(l)

OH-(aq)  +  H +(aq)       H2O(l)

In this series of reactions, a solution of sodium hydroxide neutralizes the acidic solution. After neutralization of H+(aq) ions by the base, excess sodium hydroxide increases the OH-(aq) concentration. The litmus paper tests for sufficient base. The excess OH-(aq) ions then react with the Cu2+(aq) ions to form the insoluble chalky, light blue hydroxide, copper (II) hydroxide, Cu(OH)2(s):

Cu(NO3)2(aq)  +  2 NaOH(aq)     Cu(OH)2(aq)  +  2 NaNO3(aq)

Cu2+(aq)  +  2 NO3-(aq)  +  2 Na +(aq)  +  2 OH -(aq)       Cu(OH)2(aq)  +   2 Na+(aq)  +  2 NO3-(aq)

Cu2+(aq)  +  2 OH-(aq)        Cu(OH)2(s)

The formation of solid Cu(OH)2(s) is a precipitation (metathesis) reaction.

The next step in the synthesis involves a decomposition reaction in which heat is used to decompose the the light blue copper (II) hydroxide into the black copper (II) oxide, CuO(s) :

Cu(OH)2(s)    CuO(s)  +  H2O(g)

The CuO(s) solid is then dissolved using sulfuric acid to form the hydrated copper (II) ion, Cu(H2O)42+(aq):

CuO(s)  +  H2SO4(aq)   +  3 H2O(l)    Cu(H2O)42+(aq)  +  SO42-(aq)

In this final step of the synthesis, the hydrated copper (II) ion, Cu(H2O)42+(aq) is reacted with zinc.  Since zinc is higher on the activity series than copper, the zinc will dissolve and displace the copper as a precipitate:

Cu(H2O)42+(aq)  +  Zn(s)    Cu(s)  +  Zn2+(aq)  +  4 H2O(l)


Procedure:

Dissolve the solid Copper:

Accurately weigh a clean, dry 100 mL beaker to 0.001g.  Add a spiral of copper wire to the beaker and determine it mass by difference.  Dissolve the copper by adding 5 mL of concentrated nitric acid, HNO3.  This operation must be carried out UNDER YOUR SNORKEL HOOD. The reaction, which should take about 10-15 minutes, converts copper metal to the water-soluble ionic compound  copper (II) nitrate, Cu(NO3)2.  It is very important to make sure all of the copper has dissolved.  You may not see any copper, but if there is still any orange-brown gas coming off, the reaction is not complete.

Precipitate of Cu(OH)2:

Add 15 mL of 6N sodium hydroxide, NaOH, to the copper (II) nitrate solution. Stir with a glass stirring rod and check with litmus paper to be sure that the solution is basic.  Use your glass stirring rod to place a drop of the solution onto a piece of red litmus paper.  If not basic, add 5 mL more of sodium hydroxide. This particular compound, copper (II) hydroxide, Cu(OH)2, which falls to the bottom (precipitates) is not very stable and may be readily converted to a more stable compound, copper (II) oxide, CuO, by heating.

Conversion of Cu(OH)2 to CuO:

Convert Cu(OH)2 to CuO by heating on a hot plate with plenty of STIRRING. The thick mass of CuO will spatter VERY READILY if overheated without adequate stirring. WATCH OUT. DON'T STOP STIRRING UNTIL AFTER HEAT HAS BEEN REMOVED. When all of the blue Cu(OH)2 has been converted to black CuO, filter the CuO through the Buchner funnel with suction (make sure you 'seat' the filter paper with distilled water). Transfer all of the CuO to the funnel with the aid of a well aimed stream of distilled water from the wash bottle. Wash the precipitate with distilled water to remove any soluble contaminants.  Transfer the filter with its contents to a 400 mL beaker. Raise the edge of the filter with the end of the scoopula so the entire cake of precipitate can be removed. Do not attempt to scrape the CuO off the filter paper.

Conversion of CuO to Cu(H2O)4 2+:

To the black CuO in the 400 mL beaker, add 20 mL of 3 M H2SO4  and swirl to dissolve completely (IMPORTANT: do not touch the filter paper with anything!)  When wet it tears very easily and disintegrates into white fibers that will throw off your mass. After all of the CuO has dissolved, use your stirring rod to remove the filter paper without ripping it.  Place it on the side of the beaker so the blue solution can drain.

Conversion of Cu(H2O)4 2+ back to Cu:

At this point you have a solution of Cu2+ ions in your 400 mL beaker.  Quantitatively transfer this solution to a 150 mL beaker using distilled water; you want to have a final volume of about 75 mL.  Add a piece of solid zinc to this solution.  You should notice an immediate reaction as the zinc displaces the copper from solution.  Just as you saw in the  Empirical Formula lab, the copper will coat the zinc.  So you will have to physically remove it by shaking the zinc plate periodically.  After the copper has been completely removed from solution (solution is colorless), there still may be a small amount of copper clinging to the zinc.  Use your scoopula to gentle scrape any remaining copper from the zinc.  After you are done with the zinc, clean it with water, dry it with paper towels and return to your instructor.

You will isolate the copper solid by suction filtration as you did for the CuO above.  However, it is important to weigh your filter paper before you begin the filtration process.  Set your filter paper with water and quantitatively transfer the copper to the filter.  Wash the copper with several 5 mL portions of distilled water to remove any water soluble contaminants.  Continue the suction for an additional 5 minutes after all of the liquid has been removed.  While the copper is air drying, weigh your watch glass (be sure to place some sort of identifying mark on it first, your grease pencil works well for this).  Transfer all of the copper and the filter paper to the watch glass.  Try to get the filter paper to transfer in a single piece.  Now place your watch glass in the oven (~100C) for about 10 minutes.  Allow the watch glass to cool to room temperature and weigh.  Continue this heating, cooling, weighing cycle until you obtain two consecutive weights that agree to within 0.01 g.  This process is known as weighing to constant dryness, and assures us that all of the water has been removed from the copper.  You can now determine the amount of copper obtained by difference.

Calculate the Yield:

The theoretical yield of product is the amount expected on the basis of starting amounts with no consideration for loss along the way. Assuming the same number of moles of product as of starting material, calculate the mass of copper expected (the "theoretical yield"). Convert the actual yield to a percent of the theoretical value as follows:

% yield  =  100%  x  (Actual yield / Theoretical yield)

(Updated 9/13/13 by C.R. Snelling)