Reactions of Copper

Reactions of Copper INTRODUCTION Reactions of Copper INTRODUCTION In this experiment you will do a series of reactions with the element copper. These reactions will involve the use of some new techniques and some interesting color changes. In addition, they will illustrate some of the classes of chemical reactions that occur in nature, for example, oxidation-reduction reaction. It is important that you make careful observations as your carry out these procedures. It is also very important that you do the questions as they appear in the procedures. This will keep you from falling into the trap of the cookbook approach that was discussed earlier and help you more fully understand the principles involved in the experiment. In the first reaction you will dissolve elemental copper and make a solution of copper nitrate. This solution will then be treated with a base that causes a precipitation. The resulting precipitate will then be heated to produce an oxide of copper. The oxide will then be treated with sulfuric acid to produce a blue colored solution of copper(II) sulfate. Finally, the copper ion will be reduced by zinc to produce copper metal that you will collect and dry. In some steps of this experiment you are given amounts of chemicals to use so that you have an idea of the scale at which you should work. In other steps you are left to make your own decisions. Keep in mind the things you learned about chemical reactions in the last experiment. They will be of use to you here. 21 Reactions of Copper PROCEDURE PROCEDURE You will work in pairs on this experiment. Each student will submit a separate postlaboratory report. Stock Chemicals Used Chemical Maximum Amount Used 6M Hydrochloric Acid Small Amount 6M Nitric Acid Small Amount 3M Sulfuric Acid Small Amount 6M Sodium Hydroxide 10-20 mL Copper Wire 0.5 g Granular Zinc, 20 mesh ~1g Conc. Nitric Acid (15M) ~4 mL Safety: Carry out all steps that produce fumes in the hoods as described. Be especially careful with acids and bases. Wear your goggles. 1. Making a Solution of Copper(II) Nitrate: Accurately weigh out 0.500 - 0.530 g sample of copper wire and record the measurement in your notebook. Using your pencil or pen, coil the wire into a flat spiral and place it into a clean 250 mL Erlenmeyer flask. Measure 4 mL of concentrated nitric acid found in the FUMEHOOD into a graduated cylinder. Keep the flask and graduated cylinder in the hood and pour the nitric acid into the flask. Make observations regarding any changes that occur during the reaction. Be careful not to breathe any of the fumes! Make notes of your observations in your notebook. Swirl the flask to be sure all the copper dissolves. If all the copper fails to dissolve within 10 minutes, then add an additional mL of nitric acid. Once all the copper has dissolved then add about 100 mL of deionized water, which is not usual but needed for this particular step, and take the flask to your laboratory bench. Be sure that no more fumes are being created before you remove the flask from the fume hood. Question A: This reaction is an oxidation-reduction reaction that is somewhat complicated. 4 HNO3(aq) + Cu(s)  Cu(NO3)2(aq) + 2 H2O(l) + 2 NO2(g) Identify the elements oxidized and reduced in this reaction and indicate how many moles of electrons are transferred between the elements being oxidized and reduced. 2. Synthesis of solid copper(II) hydroxide: The second reaction that you should perform is the synthesis of solid copper(II) hydroxide from the copper(II) nitrate you made in the last step. This may be accomplished by adding 6 M sodium hydroxide to the copper solution. Add a 22 Reactions of Copper PROCEDURE minimum of sodium hydroxide, but also be sure that you have added enough to complete the reaction. This volume will be around 10 - 20 mL. Question B: How do you determine that you have added enough sodium hydroxide? Question C: Write the balanced chemical equation for the reaction in this step. 3. Formation of the Oxide: To change the hydroxide to the oxide, slowly and carefully heat the solution containing the precipitate over a hot plate. Be sure to stir the solution continuously during the heating with a glass stir rod. This will avoid the possibility of "bumping" which can occur when a large steam bubble forms within a solution due to a local region of overheating. Bumping will cause a loss of product in addition to possible injury! Watch the solution carefully and record observations. When the reaction is complete, remove the hot plate and continue to stir for a couple of minutes to avoid bumping of the cooling solution. Then stop stirring and allow the product to settle. Once the product has settled, decant the solution and place it in the waste bottle that can be found in the fume hood. Be careful not to lose any solid. Next wash the solid with hot deionized water. After the solid has settled, decant off the wash and place it in the waste bottle. Save the solid for the next step. Question D: Write the balanced chemical equation for the reaction in this step. Assume the products are copper(II) oxide and water. 4. Formation of Copper(II) Sulfate: Copper(II) oxide will react with sulfuric acid to produce copper(II) sulfate. Perform this reaction using 3 M sulfuric acid. As with all reactions, use enough sulfuric acid to get the job done, but do not generate excess waste. In preparation for the formation of the solid copper, quantitatively transfer your solution from your 250 mL Erlenmeyer flask to a 250 mL beaker. The term quantitatively transfer indicates that after you have poured the solution out of your 250mL Erlenmeyer flask you should wash any solid left in the flask with a small amount of deionized water (less than 5 mL), adding the rinse to the solution in the 250 mL beaker with your solution. You should repeat this wash and rinse 2 more times. The decantant can be poured down the sink with copious amount of water. Now move back to the fume hood. Question E: Write the balanced chemical equation for this reaction. Assume the products are copper(II) sulfate and water. 5. Formation of Copper Metal: Working in the fume hood, quickly add 1.0 g of zinc metal and stir. The reaction will be complete when the solution is colorless. Record your observations. When the reaction is complete, decant the solution into a clean beaker. Examine the solid, if you note any solid zinc then add 10 mL of 6 M HCl and stir the solution until the 23 Reactions of Copper DATA ANALYSIS zinc is no longer present. When gas is no longer being produced, decant this solution. The decantant should be poured into the cation metal waste container. Question F: Write the balanced chemical equation for the reaction in this step. Assume the reactants are copper(II) sulfate and zinc, and that the products are copper metal and zinc sulfate. Question G: Indicate the reducing agent and the oxidizing agent in this reaction. Question H: Was the reaction between zinc and hydrochloric acid endothermic or exothermic? What was the gas produced in the reaction? 6. Recovery of Copper Metal: You will now transfer the copper from the beaker into the casserole dish. First weigh your dry casserole dish. Using your wash bottle, transfer all the solid to the casserole dish. Stir the solid in the water to wash away water soluble impurities. Decant the water and wash two more times with 5 mL aliquots of deionized water. Carefully and slowly heat the casserole on a hot plate. Watch this carefully to keep the solid from bumping. Continue the heating and stir the precipitate vigorously. Once the solid is completely dry, allow the casserole to cool, and weigh the casserole. Question I: Calculate the percent yield you obtained. This can be found by dividing the actual yield you obtained by the theoretical yield. The theoretical yield is the ideal yield you would have if there were no loss or contamination during the experiment. The actual yield is the yield you actually obtained in the experiment. Question J: Give reasons as to why you might expect your percent yield to be low. Give reasons as to why you might expect the percent yield to be high. Explain why your percent yield is high (>100%) or low (<100%)? Clean-Up. Clean all glassware that was used before leaving the laboratory. Clean the casserole by soaking it in 6 M nitric acid solution in the hood. Do not use concentrated nitric acid for this step. DATA ANALYSIS In step 1 of the experimental procedure, copper metal is added to concentrated nitric acid. The reaction between copper metal and concentrated nitric acid is an oxidation-reduction reaction that is somewhat complicated. 4 HNO3(aq) + Cu(s)  Cu(NO3)2(aq) + 2 H2O(l) + 2 NO2(g) Identify the element or elements that undergo OXIDATION. Identify the element or elements that undergo REDUCTION. Considering which elements are being oxidized and reduced, determine how many moles of electrons are transferred between the elements being oxidized and reduced. 24 Reactions of Copper DATA ANALYSIS In step 2 of the experimental procedure, you added 6 M sodium hydroxide to the copper(II) nitrate solution to form copper(II) hydroxide. You added a minimum amount of sodium hydroxide, but enough to complete the reaction. How do you determine that enough sodium hydroxide has been added? In step 2 of the experimental procedure, you added 6 M sodium hydroxide to the copper(II) nitrate solution to form solid copper(II) hydroxide. What is the BALANCED chemical equation for the reaction in this step? In step 3 of the experimental procedure, the copper(II) hydroxide was heated and decomposed to copper(II) oxide and water. What is the BALANCED chemical equation for this reaction? In step 4 of the experimental procedure, 3 M sulfuric acid is added to the copper(II) oxide to produce copper(II) sulfate and water. What is the BALANCED chemical equation for this reaction? In step 5 of the experimental procedure, 1.0 g of zinc is added to the copper(II) sulfate solution to produce copper metal and zinc sulfate. What is the BALANCED chemical equation for this reaction? In step 5 of the experimental procedure, 1.0 g of zinc is added to the copper(II) sulfate solution to produce copper metal and zinc sulfate. What are the oxidizing and reducing agents? In a 2nd reaction in step 5 of the experimental procedure, 6 M HCl is added to copper metal to remove any excess zinc metal. Is the reaction between the HCl and Zn exothermic or endothermic and what gas is released in this reaction? Hint: An exothermic reaction releases heat and an endothermic reaction absorbs heat. In step 1 of the experimental procedure, you were instructed to weigh between 0.500g and 0.530g of copper wire. Enter the precise mass of your copper wire. Report your mass to a thousandth of a gram, i.e. 0.512 g. In the final step of the experimental procedure, you dried your recovered copper. Enter your precise mass of the recovered copper. Report your mass to a thousandth of a gram, e.g. 0.512 g. Calculate the percent yield you obtained. Report your percentage to 3 significant digits, e.g. 89.3%. What factors would cause the percent yield to be LOW? What factors would cause the percent yield to be HIGH? Summarize the main results of this week's experiment. Explain why your percent yield is greater than 100% or lower than 100%. 25