Experiment 19   Preparation of a Cu(I) Chloride

Discussion

Chemists often use a series of reactions to obtain a desired product.  The reactions typically are of different types and can be present in any order in a series of reactions.  We can group most chemical reactions into just a few classes of reactions, listed below. 

Combination reaction             

In a combination reaction, two or more substances combine to form another substance.  This reaction can be represented by                                               

A  +  B  ®  AB

An example of a combination reaction is the reaction between Cu(II) ions and carbonate to form copper(II) carbonate, as shown in equation (3). 

Decomposition reaction

In a decomposition reaction, one compound breaks down into two or more substances.  This reaction is the reverse of a combination reaction and can be represented by

AB  ®  A  +  B 

An example of a decomposition reaction is the breakdown of the CuCl₄^3- complex ion into Copper(I) chloride and chloride ions, as shown in equation (6). 

Metathesis or Double Displacement reaction

In a double displacement reaction the ions in two reactant compounds exchange parts.  This reaction can be represented by                                               

AB  +  CD  ®  AD  +  BC

Two of the most common types of double displacement reaction is the precipitation reaction and the acid-base reaction.  Note that no redox reaction takes place. 

Single Displacement reaction

In a single displacement reaction one substance (typically an element) displaces the part of another substance.  This reaction can be represented by                        

A  +  BC  ®  AC  +  B

Oxidation Reduction reaction

More generally speaking reactions can be classified as redox reaction.  In a redox reaction, one substance is oxidized (or gives up electrons) while another substance is simultaneously reduced (or gains electrons).  Redox reactions are very common.  For example, a combination reaction may or may not be a redox reaction, depending on the reactant molecules.

In today’s experiment, you will use oxidation-reduction reactions, Metathesis reactions, as well as combination reactions.  We will use a series of such reactions to prepare one of the less commonly encountered salts of copper, copper(I) chloride.  Most copper compounds contain copper(II), but copper(I) is present in a few slightly soluble or complex copper salts. 

The synthesis of CuCl begins by dissolving copper metal in nitric acid according to the equation (1).  This reaction involves oxidation of metallic copper by the NO₃^- ion from HNO₃ to give a blue, aqueous solution of Cu²⁺.  Simultaneous reduction of the nitrate ion produces a brown gas, NO₂. 

Cu(s) + 4 H⁺(aq) + 2 NO₃^-(aq)  ®  Cu²⁺(aq) + 2 NO₂(g) + 2 H₂O(l)                      (1)

The blue solution obtained is treated with an excess of sodium carbonate, which neutralizes the remaining acid with evolution of CO₂ and precipitates Cu(II) as the carbonate, as seen in equations (2) and (3). 

2 H⁺(aq) + CO₃^2-(aq)  ®  H₂CO₃(aq)  ® CO₂(g) + H₂O(l)                                     (2)

Cu²⁺(aq) + CO₃^2-(aq)  ®   CuCO₃(s)                                                                  (3)

The CuCO₃ will be purified by filtration and washing.  The purified substance is then dissolved in hydrochloric acid to form a copper-chloride complex ion.  Excess carbonate will react with hydrochloric acid and is removed from the solution as carbon dioxide, as shown in equation (4). 

CuCO₃(s) + 2 H⁺(aq) + 4 Cl^-(aq)  ® CuCl₄^2-(aq) + CO₂(g) + H₂O(l)                     (4)

In yet another reaction (5), excess copper metal is added to the highly acidic solution.  The addition of metallic copper reduces the Cu(II) to Cu(I) while elemental Cu is oxidized to Cu(I) in a reaction called a disproportionation reaction.  In the presence of excess chloride, the copper will be present as a CuCl₄^3- complex ion.  Addition of this solution to water destroys the complex, and white CuCl precipitates.

CuCl₄^2-(aq) + Cu(s) + 4 Cl^-(aq)  ®  2 CuCl₄^3-(aq)                                              (5)

CuCl₄^3-(aq)  ®  CuCl(s) + 3 Cl^-(aq)                                                                   (6)

Because CuCl is readily oxidized, care must be taken to minimize its exposure to air during its preparation and while it is being dried.

You can find a similar experiment at

http://www.wpi.edu/Academics/Depts/Chemistry/Courses/CH1010/Stream1/cucycle.html

Materials and Equipment

Cu turnings                                        15 M HNO₃                 

Büchner funnel                                  400 mL beaker

6 M HCl                                               Na₂CO₃           

acetone

Experimental Procedure   

1.  Obtain a 1-gram sample of copper metal turnings, a Büchner funnel, and a filter flask from the cart.  Weigh the copper metal on the analytical balance to the nearest 0.001 g.

2.  Cut the metal into small pieces and put the metal in a 150-mL beaker.  Under the hood, carefully add 5 mL 15 M HNO₃.  

Brown NO₂ gas will be evolved and an acidic blue solution of Cu(NO₃)₂ produced.  If it is necessary, you may warm the beaker gently with a Bunsen burner to dissolve all of the copper.  When all of the copper is dissolved, add 50 mL of water and allow it to cool.  Be sure that no more fumes of NO₂ are present before you proceed to the next step. 

3.  Weigh out about 5 grams of sodium carbonate in a small beaker on a top loading balance.  With your spatula, add small amounts of the Na₂CO₃ to the solution, adding the solid as necessary when the evolution of CO₂ subsides.  Stir the solution well after each addition.  When the acid is neutralized, a blue-green precipitate of CuCO₃ will begin to form.  At that point, add the rest of the Na₂CO₃ stirring the mixture well to ensure complete precipitation of the copper carbonate.

4.  Set up a suction filtration apparatus.  Make sure the filter paper fits the funnel exactly.  Transfer the precipitate to the Büchner funnel.  Turn on the suction to separate the solid from the solution.  Use your rubber policeman and a spray from your wash bottle to make a complete transfer of the solid.  Wash the precipitate well with distilled water with suction on.  Allow suction to continue for a minute or two to dry the solid. 

5.  Transfer the solid CuCO₃ to a 150-mL beaker.  To dissolve the solid copper carbonate, begin by adding 10 mL water.  Then, slowly, add 30 mL 6 M HCl to the solid, stirring continuously.  When the CuCO₃ has all dissolved, add 1.5 g Cu turnings to the beaker and cover it with a watch glass. 

6.  Using a hot plate, heat the mixture in the beaker to the boiling point and keep it at that temperature, just simmering, for about 20 minutes.  It may be that the dark-colored solution that forms will clear to a yellow color before that time is up, and if it does, you may stop heating and proceed to the next step. 

7.  While the mixture is heating, add 150 mL distilled water to a 250-mL beaker and put the beaker in an ice bath.  Cover the beaker with a watch glass and label it with your name.  After you have heated the acidic Cu-CuCl₂ mixture for about 20 minutes, the solution should be light colored (step 6).  If no clear liquid is obtained, continue heating for another 10 minutes.  If you have a light-colored solution, carefully decant the hot liquid into the beaker of water, taking care not to transfer any of the excess Cu metal to the beaker.  White crystals of CuCl should form.  Continue to cool the beaker in the ice bath to promote crystallization and to increase the yield of solid. 

9.  Cool 25 mL distilled water, to which you have added five drops 6 M HCl, in an ice bath.  Put 20 mL acetone into a separate small beaker.  Filter the crystals of CuCl by suction.  Swirl the beaker to aid in transferring the solid to the funnel.  Just as the last of the liquid is being pulled through; wash the CuCl with 1/3 of the acidified cold water.  Rinse the last of the CuCl into the funnel with another portion of the acidified water and use the final 1/3 to rewash the solid.  Turn off suction and add ½ of the ice cold acetone to the funnel; wait about 10 seconds and turn on the suction.  Repeat this operation with the other half of the acetone.  Draw air through the sample for a few minutes to dry it.  If you have properly washed the solid, it will be pure white; if the moist compound is allowed to come into contact with air, it will tend to turn pale green, due to oxidation of Cu(I) to Cu(II).  Weigh the CuCl to the nearest 0.001 g. 

Calculate the percentage yield for this reaction. 

Waste Disposal

CuCO₃ should be disposed of in the solid waste beaker.  The contents after the second filtration include an appreciable amount of copper, and they should also be put in the waste beaker.