An Investigation of Reducing Agents for the Copper Cycle Lab Phil Sinner and Lester Lorenz Overview Four metals were used to reduce copper from the copper(II) ion. The metals were added to acid solutions of copper (II) chloride dihydrate and copper (II) sulfate pentahydrate. The reduced copper was then recovered and percent yield was calculated. Discussion of differences of the reducing metals is included. Introduction The copper cycle lab investigation typically begins with metallic copper which is reacted with nitric acid to form the copper (II) ion in solution. Sodium hydroxide is added to produce solid copper (II) hydroxide which is heated to form copper (II) oxide. The copper (II) oxide is then reacted with an acid. If sulfuric acid is used, copper (II) sulfate is formed in aqueous solution (1). If hydrochloric acid is used, copper (II) chloride is formed in aqueous solution (2). An active metal is added to the solution to reduce the copper which can be recovered by filtration. A summary of this series of reactions is given below. Cu(s) + 4HNO3(aq) à Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l) Cu(NO3)2(aq) + 2NaOH(aq) à Cu(OH)2(s) + 2NaNO3(aq) Cu(OH)2(s) à CuO(s) + H2O(l) heat CuO(s) + H2SO4(aq) à CuSO4(aq) + 2H2O(l) OR CuO(s) + 2HCl(aq) à CuCl2(aq) + H2O(l) CuSO4(aq) + Zn(s) à Cu(s) + ZnSO4(aq) OR 3CuCl2(aq) + 2Al(s) à 3Cu(s) + 2AlCl3(aq) Problem Zinc, which is used in the CSB/SJU general chemistry laboratory, has disposal concerns that make the use of another reducing agent an attractive alternative. Three other active metals were considered for this step of the copper cycle lab. Aluminum, iron, and magnesium were chosen because of their reduction potentials. Procedure Two series of reactions were used in order to compare the two methods that were found in references (1) and (2). The first used 2.00 g of copper (II) chloride in 25 mL of 0.2M hydrochloric acid The second used a solution of 4.00 g of copper (II) sulfate pentahydrate in 25 mL of 0.1M sulfuric acid.. Each of these solutions contained approximately 1.00 g of copper, the mass of copper suggested as the starting point for the general chemistry lab. To each of these solutions an equimolar mass of the reducing metal was added. A reaction was considered finished when the blue color was absent from the solution, indicating that the copper (II) ion was no longer in solution and had been reduced to metallic copper. Some reactions were sped up by gentle heating if they appeared to occur slowly. When the reactions were finished, each was vacuum filtered on a pre-weighed filter paper, washed with water to remove any soluble salts, and then washed with several small portions of acetone to remove the water. The copper residue was then placed on a pre-weighed watch glass with the pre-weighed filter paper and placed in a 100 degree celsius oven until dry. The final mass of the residue, watch glass and filter paper was recorded when it had cooled. Calculations of percent yield were done. Results The combinations of starting material and reducing metal that worked best for us were copper (II) sulfate and zinc, as used by the CSB/SJU general chemistry lab, and the copper (II) chloride and aluminum foil (see data table). The advantages of using the copper (II) sulfate - zinc combination are that copper (II) sulfate is relatively safe to use and that good results can be obtained. The disadvantage is that disposing of the zinc may be problematic. Phil, the lab coordinator, suggested that disposal limits are down in the ppm range. The advantages of using the copper (II) chloride - aluminum foil combination is that good results can be obtained and that aluminum foil is cheap and available. The disadvantage is that copper (II) chloride is not as safe to use. Teachers may want to make the copper (II) chloride solution ahead of time for the students, avoiding some of the potential handling problems. CuCl2 CuSO4 Sample Theoretical Actual % Yield Theoretical Actual % Yield Al .784g 1.023g 130.5% Fe .779g 1.183g 151.8% 1.016g 1.303g 128.5% Mg .786g .725g * 1.009g .904g 89.6% Zn .787g 1.265g 160.7% 1.018g .986g 96.9% Al foil .787g .665g 84.5% *The sample of Mg and CuCl2 became contaminated with a yellow powder which could not be separated, so it was discarded in spite of the apparent result. Though this exercise is intended as an alternative final step in the copper cycle, it can stand-alone just as presented. The reduction of copper from copper (II) chloride by aluminum or from copper (II) sulfate by zinc requires about 30 minutes. Students must be cautioned regarding CuCl2 . Student Procedures for using the reduction of copper as a stand-alone lab exercise The following are two procedures for using the last phase of the copper cycle lab as a stand-alone lab exercise for high school students. The procedures are a very slight modification of the procedure found in the CSB/SJU general chemistry lab manual. These procedures assume that the teacher has prepared solutions for the students (see teacher notes). Copper (II) sulfate - granular zinc Equipment: 150 mL beaker, stirring rod, funnel, filter paper Materials: copper (II) sulfate/ sulfuric acid solution, granular zinc, acetone, deionized or distilled water 1. Using a 150 mL beaker, obtain 25 mL of the copper (II) sulfate solution from the buret. Measure this volume as carefully as you can as this volume determines the theoretical yield. Record the volume. 2. Add about 1.2 g of granular zinc to the beaker. You may use a stirring rod, but you should be careful not remove any material, solid or liquid, from the beaker. Gently heat the solution, but do not boil. The reaction is finished when the blue color disappears. If the blue color does not disappear, more zinc may be added. Add small amounts! 3. When you are sure that the reaction is complete, weigh a piece of filter paper and use it to filter the material in the beaker. Wash the residue with water to remove any soluble materials from the solid residue. Wash the residue with a few 5 mL portions of acetone to remove the water. Dry the residue as instructed by your teacher. ANALYSIS 1. Calculate the mass of the residue. This is your actual yield. 2. Calculate the mass of copper contained in the volume of solution you obtained. Check with your teacher for the concentration of the solution. This is your theoretical yield. 3. Calculate the percent yield. Copper (II) chloride - aluminum foil Equipment: 150 mL beaker, stirring rod, funnel, filter paper Materials: copper (II) chloride/ hydrochloric acid solution, aluminum foil, acetone, deionized or distilled water 1. Using a 150 mL beaker, obtain 25 mL of the copper (II) chloride solution from the buret. Measure this volume as carefully as you can as this volume determines the theoretical yield. Record the volume. 2. Obtain 5 or 6 1"x1" squares of aluminum foil. Add them to the beaker one at a time. You may use a stirring rod, but you should be careful not remove any material, solid or liquid, from the beaker. The reaction is finished when the blue color disappears. If the blue color does not disappear, more aluminum may be added. You may add smaller amounts of foil if the solution is very close to colorless. 3. When you are sure that the reaction is complete, weigh a piece of filter paper and use it to filter the material in the beaker. Wash the residue with water to remove any soluble materials from the solid residue. Wash the residue with a few 5 mL portions of acetone to remove the water. Dry the residue as instructed by your teacher. ANALYSIS 1. Calculate the mass of the residue. This is your actual yield. 2. Calculate the mass of copper contained in the volume of solution you obtained. Check with your teacher for the concentration of the solution. This is your theoretical yield. 3. Calculate the percent yield. A data table may be provided for the students or they may make their own. The table should include the following spaces: Volume of solution, mass of filter paper, mass of residue and filter paper. Teacher Notes Students must be able to calculate moles from the concentration of the solution, calculate mass from moles, and use good filtering technique. The related concepts of oxidation and reduction, limiting reactant, and single replacement reactions would also be helpful. Different methods of solution preparation and filtration will change the time requirements of this lab. It is recommended that the solutions be instructor prepared, especially the copper (II) chloride solution because it is a tissue irritant. Filtration without suction will take over night. Unless a fume hood is available, air dry over night in place of the acetone wash. The solution of CuCl2.2H2O is prepared by dissolving 2.5g. of CuCl2.2H2O in 25 ml of .2M of HCl for each student. The CuSO4.5H2O solution is prepared by dissolving 4g. of CuSO4.5H2O in 25 ml of .1M H2SO4 each student (heat the solution to speed the solution process). This will result in concentrations of about 1 gram of copper per 25ml of solution. Calculate the concentration of your solutions and give it to the students. References (1) CSB/SJU General Chemistry Laboratory Manual. (2) Todd, David, and Hobey, William D., J Chem Ed, 62, 177 (1985), An Improvement in the Classical Copper Cycle Experiment.