Experiment 4

Chemical Changes: Reactions of Copper

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

Instead of using a piece of copper wire, as described in the printed protocol, you will use a penny (you must use a 1970 or later 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.

To do this experiment follow the modified protocol shown below:

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

• Place a single penny (you should have determined its mass) into a 250-mL Erlenmeyer flask.
• Add 20 mL concentrated nitric (HNO₃) acid.
• Let the penney completely dissolve, or if you believe all the copper has dissolved, you can remove the zinc core using a pair of forceps.
• Add about 20 mL of DI water before proceeding with the next step.

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

• Add about 50 mL of 6 M NaOH in small increments, with stirring.
• The bluish Cu(OH)₂ solid also contains the Zn(OH)₂, which is white.
• Using litmus paper, check the pH to make certain the liquid is basic (red litmus turns blue).

3. Cu(OH)₂ (s) → CuO (s) + H₂O (l)

• Heat your sample, with stirring, until all the blue color disappears, leaving a grey slurry (your mixture will contain both the black CuO and the white ZnO).
• Collect your sample using vacuum filtration and a Büchner funnel (you must use #2 filter paper, not #3 filter paper).
• Remove the filter from your funnel and place in the bottom of a 250-mL beaker (with the solids on the top side).

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

• Add about 20 mL of the 3 M H₂SO₄ and let it dissolve, with occassional swirling of the beaker to completely dissolve the solid material.
• When all the solid has been dissolved, the solution should be a light blue in color.
• Add about 40 mL of DI water, swirl, and then remove the filter paper.

Cover your sample with a layer of parafilm and place on the cart for storage until the next lab period. The blue color of the liquid is due to the Cu²⁺ ion. The Zn²⁺ ion is colorless, but is also present. (If your liquid has pieces of the filter paper in it, you must do a filtration prior to doing the Mg reduction step.)

During the second day of your lab, you will then reduce your Cu²⁺ to produce Cu metal using Mg. Don't worry about the Zn²⁺ ion which also gets reduced (producing Zn metal), because any Zn metal produced will either react with the sulfuric acid or it will help to reduce the Cu²⁺ into the Cu metal. By doing this reduction step (starting with Mg and your solution of ions) you will produce only Cu metal, and all the zinc and magnesium will be converted, or stay, as ions.

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

• If your liquid has pieces of the filter paper in it, you must do a Büchner funnel vacuum filtration prior to adding the Mg filings.
• Pour the clear filtrate into either a 400-mL or 600-mL beaker.
• Add the required amount of Mg filings (see published protocol for amount) to your beaker, a few pieces at a time.
• Continue to add more Mg until all the blue color in the liquid is gone (Cu²⁺ is blue in solution; therefore, when it is all reduced to Cu, the blue color will disappear).
• If any bubbles are apparent, then all the Mg has not yet been reacted. If in doubt, add up to 10 mL of 3 M H₂SO₄ to assist in the dissolving. Do not filter until no more gas (bubbles) is produced.

To collect your copper do the following:

• Filter the final mixture (the liquid must be colorless, but will continue the reddish or black-looking copper) to collect your Cu metal via Büchner funnel and vacuum filtration.
• Wash the solid material with copious amounts of water to remove any acid from the filter paper (residual acid will cause the filter paper to turn black when heated).
• Keep the vacuum on for at least 5 min after your final wash to remove as much water as possible.
• Dry your Cu metal (along with a preweighed piece of filter paper) in the drying oven (use a pre-weighed evaporating dish or watch glass to put your filter in).

Weigh your dry Cu sample and determine the amount of copper present (remember to subtract the mass of the pre-weighed evaporating dish and filter paper). Determine the percent yield of Cu based on the mass of the original penny using the %Yield formula given below.

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)

• 20 mL concentrated nitric acid
• 50 mL 6 M NaOH
• 40 mL H₂SO₄ (20 mL to dissolve solid CuO and 20 mL to dissolve excess Mg after last step)
• Use only #2 Büchner filter paper, do not use #3 paper as it is too thick and gets clogged during the CuO filtration

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