Experiment 4

Chemical Changes: Reactions of Copper

Outline of Experiment:

This experiment involves starting with copper, dissolving copper in concentrated nitric acid, and through a sequence of reactions producing metallic copper.  This addendum describes the mechanisms of doing the experiment, including the filtrations, mixing of reagents, and the ultimate production of copper.

The reactions below are the reactions you will perform to produce new copper.  The first and last reactions are oxidation and reduction reactions.  The other reactions utilize Cu2+ as the copper ion.

Determining when chemical reactions occur in an important part of chemistry.  Some of the indicators of chemical reactions are:

In this experiment, you will observe each of these indicators.  After you have completed the experiment, you should be able to identify which indicator(s) is present in experimental step.

Note: Please go to the very end of this addendum to see information about using a single penny instead of the copper wire outlined in the published protocol packet. If your instructor tells you to use a penny, do not use the instructions for each of the reactions. You must follow the instructions listed in the section which describes the use of the penny. These instructions, specifically for the use of a penny, can be printed from the following link:

Reactions in Experiment IV:

1. Cu (s) + 4 HNO3 (aq) → Cu(NO3)2 (aq) + 2 NO2 (g) + 2 H2O (l)

2. Cu(NO3)2 + 2 NaOH (aq) → Cu(OH)2 (s) + 2 NaNO3 (aq)

3. Cu(OH)2 (s) → CuO (s) + H2O (l)

4. CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)

5. CuSO4 (aq) + Mg (s) → Cu (s) + MgSO4 (aq)

Procedures and Protocols

Before you begin your experiment, you will need to check out from the stockroom a 250-mL vacuum filter flask and a vacuum trap assembly.  Your instructor will show you how to use the vacuum trap and filter flask, along with the Büchner funnel.  Do not turn in your vacuum trap and filter flask until after you have collect all of your Cu2+-containing liquid following the Reaction #4 protocol described below.

Reaction #1. Cu (s) + 4 HNO3 (aq) → Cu(NO3)2 (aq) + 2 NO2 (g) + 2 H2O (l)

For Reaction #1, use the amount of concentrated HNO3 described (if you spill any acid clean it up immediately, using NaHCO3 to neutralize it is necessary).  This part of the experiment must be conducted in the fume hood because toxic NO2 gas is produced.  You can swirl the reaction periodically to help facilitate reaction.  When all the copper metal has dissolve, and the flask is mostly free of the brownish NO2 gas, you can take this flask back to your bench for the remainder of the reactions.

The reaction, using the half-reaction method for balancing oxidation-reduction equations, is shown below.

Cu (s) + 2 H+ + 2 HNO3 (aq) → Cu2+ + 2 NO2 (g) + 2 H2O (l)

How do you get this equation? Can you get this equation using the half-reaction method to balance oxidation-reduction equations?

Reaction #2. Cu(NO3)2 + 2 NaOH (aq) → Cu(OH)2 (s) + 2 NaNO3 (aq)

For Reaction #2, add NaOH to produce solid Cu(OH)2.  Place a stirring bar in your flask, and put it on a stirring hot plate (do not turn on the heat!). You should observe some solid blue material forming when you add the NaOH, but which subsequently dissolves.  The Cu(OH)2 you produce reacts with the excess HNO3 and dissolves.  You will need to continue to NaOH until the solid Cu(OH)2 stays, and does not dissolve when the contents of the flask are mixed by swirling.

(Note: You will need at least 26 mL of 6 M NaOH for each 10 mL of HNO3 used in Reaction #1.)

If you think you have added enough NaOH, dip a glass stirring rod into your mixture and touch this stirring rod to a piece of red litmus paper.  If the liquid is basic, it will turn the red litmus blue.  Be careful since the solid Cu(OH)2 is blue, and might confuse you.  Therefore, after applying a small amount of liquid to the litmus, examine the litmus paper from the opposite side from where you applied the liquid.  If the litmus stays blue, then your solution is basic and you can stop adding NaOH.  If the paper stays red (pink), you still need to add NaOH. You cannot add too much NaOH, so don't worry about adding too much.

Reaction #3. Cu(OH)2 (s) → CuO (s) + H2O (l)

For Reaction #3, you will heat your Cu(OH)2-containing mixture to dehydrate (remove water) it.  Carefully add a small stirring bar to the flask and place the flask on top of a stirring heating plate.  Turning the stirrer on and then turn on the heating unit.  The mixture must be heated to boiling.  You will observe when the reaction is complete as the blue Cu(OH)2 is replaced by the CuO, which is black.  When all the blue chemical has disappeared, you can stop the heating.  Remove the flask from heating plate and collect the CuO in a Büchner funnel.  Place a piece of filter paper in the funnel, and place in a vacuum flask.  Turn on the vacuum and using a small amount of water wet the paper until it adhers tightly to the funnel.  Pour the contents of the flask into the funnel.  Using a wash bottle rinse any solid residue from the flask into the funnel.  After the liquid has been sucked through the funnel rinse the solid material on the filter with a little water (it does not need to be hot water).

Reaction #4. CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)

For Reaction #4, you will dissolve the CuO, present on the filter paper, with 3 M H2SO4.  This is accomplished with the filter still in the Büchner funnel.

  1. Remove the rubber stopper from the Büchner funnel and place the funnel into an Erlenmeyer flask.  (The black CuO is on the filter paper in the funnel.  Keep your filter flask until you have completed step #6 below.)
  2. Add the appropriate amount of 3 M H2SO4.  Gently rotate the funnel to allow the black solid to come in contact with the H2SO4.  A blue liquid will appear as the CuO dissolves.  Some of this liquid may drip into the flask.  Continue to swirl the funnel until all of the CuO has dissolved (no black solid remaining).  If all the H2SO4 flows through the funnel, and there is still some black CuO present, you can transfer some of the collected blue liquid (the filtrate) back onto the filter paper, which will allow you to continue to dissolve the CuO.  (You can transfer the Büchner funnel into your clean filter flask, and pour the filtrate back into the funnel, and let the rest of the CuO dissolve.)
  3. After all the CuO has dissolved, put the rubber stopper back onto the funnel, and place  itinto the top of your vacuum flask.  (If some of the filtrate is in your flask, don't worry, as you will be collecting more, and transferring into your Erlenmeyer flask for storage.) 
  4. Turn on the vacuum, and collect any liquid which had been adhering to the filter paper and funnel.  You can wash with a few mL of DI water, to recover all the Cu2+-containing liquid.
  5. Transfer the filtrate back into the flask you used previously, which probably already contains some of the blue liquid, and cover with Parafilm until the next lab period.  
  6. Using a piece of labelling tape, put your name, and the identity of its contents (e.g., DLR110 Exp 4) on the flask prior to placing it on the cart for storage until the next lab period.  Do not store in your locker.

This blue liquid, which contains the Cu2+ ion, will be used during the next lab to produce Cu metal.  Store on the cart.

Wash your filter flask and vacuum assembly thoroughly with soap and water.  You should turn in your filter flask and vacuum assembly after you have collected all of your Cu2+-containing liquid from the Büchner funnel .  

Reaction #5. CuSO4 (aq) + Mg (s) → Cu (s) + MgSO4 (aq)

For Reaction #5, you will use the blue liquid from Reaction #4 (stored from the last lab period) to react with Mg to produce new Cu.  The Mg is the reducing agent (provides electrons) and the Cu2+ ion gets reduced to Cu (Mg2+ ion is produced).  

As the Cu2+ ion reacts, metallic Cu forms.  Continue to swirl the mixture until the blue color disappears.  If large pieces of Cu form, you should crush these with a stirring rod to remove the Cu from any imbedded Mg pieces.  The reaction is complete when all the Cu2+ ion has reacted, which corresponds to a complete disappearance of the blue color.  When the blue color is gone and no Mg pieces remain (no bubbles; H2 gas being produced), your reaction is complete.

Collect the Cu metal (reddish-brown solid) in a Büchner funnel containing a pre-weighed piece of filter paper (you will need to check out a vacuum trap and vacuum flask as in the previous lab).  Use a wash bottle to remove all Cu from the flask, and rinse the filter with water.  Rinse the filter with a few mL of acetone (a volatile organic solvent, helping to remove water from the filter). Remove the acetone-treated filter with Cu pieces, from the Büchner funnel and place in an evaporating dish or watch glass.  Place your material in the drying oven to thoroughly dry (at least 20-30 min).  

Remove the dry Cu (and filter paper) from the oven and let it cool prior to weighing.  Determine the mass of Cu recovered and calculate a percent yield.  Using the formula shown below, your percent yield is calculated using your actual yield (the mass of dry Cu recovered) divided by the theoretical yield (mass of Cu wire you started with) and multiply by 100.  

Optional protocol using a penny as the source of copper (at the discretion of your instructor).

Instead of using a piece of copper wire, you will use a penny. Current pennies are made from a piece of zinc coated with a thin layer of copper. The modern penny is not 100% pure copper as it was prior to the 1960's. You will dissolve the entire penny in the concentrated nitric acid.

The protocol for this lab, starting with a penny is available here.

 

To obtain a value for %Yield, you need to know the Theoretical Value you should obtain, based on balanced equations, and if 100% of the reactant(s) is converted to product.  After obtaining your Experimental Value, divide it by the Theoretical Value, then multiply by 100 to get %Yield.

Questions:

  1. Why does the copper not dissolve in the H2SO4 mixture in the last step?
  2. Practice doing the oxidation-reduction reaction for Cu combining with concentrated HNO3, which is the first reaction. (You will see this reaction again on quizzes, the lab exam and the regular exam.)
  3. If you used dilute HNO3, instead of the concentrated acid used in this experiment, you would produce NO (g) instead of the NO2 (g). Show the correct oxidation-reduction equation for the production of NO.

Reagents needed for use of a penny (per group)

Balancing Redox (Oxidation-Reduction) Reactions

After you identify the elements (or compounds) that will be oxidized or reduced, set up two half-reactions, one for oxidation (loss of electrons) and for reduction (gain of electrons).  You do not need to determine which one is oxidation or reduction, but you need to identify the correct compounds, or elements, that change oxidation state.

  1. Balance the number of atoms1
  2. Balance the number of oxygen atoms by adding water2
  3. Balance the number of hydrogen atoms by adding H+ (protons)3
  4. Balance charge by adding electrons (e-) to the side with the most positive (least negative) charge4
  5. Balance the number of electrons gained and electrons lost5
  6. For reactions in base (OH-), as your last step, add OH- ions to each side to convert H+'s into water

1Balance the non-oxygen and non-hydrogen atoms, which are usually those undergoing oxidation or reduction
2Add water to make the number of O-atoms the same on both sides
3Add H+ to make number of H-atoms the same on both sides
4Determine the charge on each side; add enough electrons to make charge same on both sides (electrons added to least negative side, or most positive side).
5If necessary multiply each reaction by an interger to give same number of electrons gained and lost

Example Redox Balancing:
Reaction to Balance:
Cu + HNO3
→
Cu2+ + NO2
Copper Half Reaction:
Cu
→
Cu2+ + 2e-
Nitrogen Half Reaction:
1e- + H+ + HNO3
→
NO2 + H2O
Final Nitrogen Half Reaction (x2):
2e- + 2H+ + 2HNO3
→
2NO2 + 2H2O
Final Equation:
Cu + 2H+ + 2HNO3
→
Cu2+  2NO2 + 2H2O

 

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Copyright © Donald L. Robertson (Modified: 09/20/2006)