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 an exercise in futility, it will actually provide a thorough test of your laboratory skills (weighing, filtering, transferring, attention to detail, and observations), understanding of chemical reactions (types, and balancing), and calculating theoretical yields from a sequential reaction.

Introduction:

Many compounds which do not occur naturally or which 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, 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 have significantly reduced your final yield.

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

The three equations above, represent the molecular (or formula unit), total ionic and net ionic equations of 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 NO.  However, NO is extremely reactive and is converted to the brown gas NO₂ as soon as it comes in contact with the O₂ in the air..

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

In this series of reactions, a solution of NaOH neutralizes the acidic solution. After neutralization of H⁺_(aq) ions by the base, excess NaOH 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, Cu(OH)_2(s):

Cu²⁺_(aq)  +  2 OH^-_(aq)        Cu(OH)_2(s)

The formation of solid Cu(OH)_2(s) is a precipitation (metathesis) reaction were the driving force of this reaction changing blue Cu²⁺_(aq) ions to light blue Cu(OH)₂ precipitate.

The next step in the synthesis involves a decomposition reaction in which heat is used to decompose the the light blue Cu(OH)_2(s)  into the black CuO_(s) :

The CuO_(s) solid is then redissolved using sulfuric acid to form the hydrated copper (II) ion, Cu(H₂O)₄²⁺_(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:

Dissolve the solid Cu:

Obtain a spiral of Cu wire from the instructor. Determine the mass by difference on the top loading balance, using a 100 mL beaker as the container. Record the data. Convert the sample mass to moles of Cu. Dissolve the Cu by slowly adding 5 mL of concentrated HNO₃. This operation must be carried out UNDER A HOOD . The reaction, which should take about 5-10 minutes, converts copper metal to the water-soluble ionic compound Cu(NO₃)₂.

Precipitate of Cu(OH)₂:

Add 15 mL of 6N NaOH to the solution of Cu²⁺. 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 NaOH. This particular compound, Cu(OH)₂, which falls to the bottom (precipitates) is not very stable and may be readily converted to a more stable compound, 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 pale 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 250 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 250 mL beaker, add 10 mL of 3 M H₂SO₄  and swirl to dissolve completely. 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 beaker.  Add approximately two grams of 10-30 mesh zinc to this solution and gently swirl the solution.  You should notice an immediate reaction as the zinc displaces the copper form solution.  After the reactions have ceased (no more bubbles being formed), you may still have some solid zinc remaining.  Add approximately 5 mL of 6 M HCl to the solution to dissolve the remaining zinc.  Gently swirl until the zinc is gone. 

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 filter the solution.  Wash the copper with several 5 mL portions of distilled water to remove any water soluble contaminants.  Continue the suction for and additional 5 minutes after all of the liquid has been removed.  Turn off the suction and add approximately 10 mL of 95% ethanol.  Turn the suction back on and let it air dry for an additional 5 minutes.  While the copper is 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.  Weigh the watch glass, filter, and copper.  Now place it into an oven at 100°C for about 10 minutes.  Allow the watch glass to cool to room temperature and weigh again.  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.

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 Cu expected (the "theoretical yield"). Convert the actual yield to a percent of the theoretical value as follows:

Turn in your copper sample in a labeled (your name, date, lab section, yield in grams, and % yield), corked test tube.

(Updated 6/14/07 by C.R. Snelling)