CYCLE OF COPPER REACTIONS

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1 CYCLE OF COPPER REACTIONS PURPOSE: The goal of the experiment is to observe a series of reactions involving copper that form a cycle and calculate the percent recovery of the initial copper mass. INTRODUCTION: A schematic of the cycle of copper reactions is shown below. Solid copper is initially dissolved in concentrated nitric acid (1), producing Cu(NO3)2 and gaseous NO2. To this solution, sodium hydroxide is added (2), resulting in the precipitation of Cu(OH)2. Upon heating (3), the Cu(OH)2 decomposes to CuO. Sulfuric acid is added to the CuO (4), producing CuSO4. Finally, solid zinc is added (5) and solid copper is regenerated. By massing the copper before and after this cycle of reactions, the percent recovery can be calculated. Percent Recovery = mass Cu (after) mass Cu (before) 100 SAFETY PRECAUTIONS Concentrated (16 M) nitric acid is a potent oxidizing agent, which may produce chemical burns, skin discoloration, and lung irritation. As such, the nitric acid must remain in the fume hood at all times. In addition, the dissolution of copper wire in the nitric acid produces toxic NO2 gas; this reaction must also take place in the fume hood. Finally, methanol is flammable and its vapors are toxic. Use it in the fume hood and keep it away from open flames. PROCEDURE: For best results, use the same balance for all mass measurements. Reaction (1) 1. Obtain and record the mass of a precut piece of copper wire using the analytical balance. Coil the wire into a flat spiral and place it into a 250-mL beaker.

2 2. In the fume hood, add ~5 ml concentrated nitric acid to the beaker. Allow the reaction to run to completion and record your observations. There should be no solid copper remaining. At the end of this step, you may take the beaker back to the lab bench. Reaction (2) 3. Add ~100 ml DI water to the beaker; then add ~30 ml 3.0 M NaOH to the beaker, continuously stirring. Record your observations. Reaction (3) 4. Add two boiling beads to the beaker and place it on a hot plate. Gently heat the mixture, with stirring, for ~30 minutes or until the reaction is complete. Do not let the solution boil. Record your observations. 5. Remove the beaker from the hot plate, let it cool, and allow the solid to settle to the bottom. 6. Decant most of the liquid (use a dropper) and dispose of it in the appropriate waste container. Do not risk losing your solid product by trying to remove all of the liquid. 7. Add ~200 ml hot DI water to the beaker, stir, and allow the precipitate to settle again. 8. Decant the liquid in the same manner as before. Reaction (4) 9. Add ~15 ml 6.0 M H2SO4 to the beaker, stirring constantly. Record your observations. Reaction (5) 10. Return to the hood and add ~2.0 g of zinc metal to the beaker, stirring constantly. 11. Continue stirring until the solution becomes completely colorless (check against a white paper). Record your observations. 12. When gas evolution becomes slow, look for any grains of unreacted zinc in the beaker. If there is any zinc present, add ~10 ml of 6.0 M HCl and warm gently. If there is no zinc, proceed directly to step Let the beaker cool completely. This is a good stopping point if you don t have time to complete the entire experiment in one day. Cover the beaker with a watch glass and carefully place it in your drawer. 14. Record the mass of your evaporating dish on the analytical balance. 15. Decant the solution and transfer the precipitate to the evaporating dish. 16. Add ~5 ml DI water to the precipitate and stir. Let the precipitate settle and decant the liquid. Repeat this step with a second portion of DI water. 17. Repeat step 16 with ~5 ml methanol. 18. Completely dry the product by placing the evaporating dish on a hot plate. Once it is cool, remove the boiling beads and record the mass of the evaporating dish/product. 19. Calculate the mass of the recovered copper (product). Is this mass larger than the starting mass? If the answer is yes, your product is still wet. Put it back on the hot plate. If not, proceed to step Calculate the percent recovery of the copper.

3 PRELABORATORY ASSIGNMENT NAME: DATE: 1. Identify each cycle of copper reaction (1 5) as a redox reaction, precipitation reaction, acid-base reaction, or decomposition reaction. 2. Give balanced chemical equations for each of the reactions described in the cycle of copper (1 5). 3. A g piece of copper is added to ml of 16 M HNO3, producing g of copper(ii) nitrate. What is the percent yield of the reaction? 4. What volume (ml) of 3.0 M NaOH is required to react with g copper(ii) nitrate? What mass of copper(ii) hydroxide will form, assuming 100% yield?

4 RESULTS AND CALCULATIONS Reaction (1) Initial Mass Copper (g): Reaction (2) Reaction (3) Reaction (4) Reaction (5) Final Mass of Copper (g): Percent Recovery:

5 POST-LAB QUESTIONS 1. Other than water, what is removed during the decanting step in Reaction (3)? 2. The gas produced upon adding the zinc metal to the mixture in Reaction (5) is hydrogen. Give the balanced chemical equation for this reaction. What type of reaction is this? 3. What (other than water) is removed during the washing and decanting steps of Reaction (5)? 4. Assuming no calculation errors, why would a percent recovery > 100% or a percent recovery < 100% be obtained? Give at least one reason for each. 5. What volume (ml) of 2.00 M sulfuric acid is required to react completely with g copper(ii) oxide? 6. If 2.00 g zinc is reacted with g copper(ii) sulfate, how many grams of zinc remain unreacted? What volume (ml) of 6.0 M HCl is required to react will the excess zinc?