COPPER CYCLE EXPERIMENT 3

Transcription

1 COPPER CYCLE EXPERIMENT 3 INTRODUCTION One simple way to state the aim of chemistry is: The study of matter and its transformations. In this experiment, a copper sample will appear in five different forms by going through four chemical reactions. Cu(NO 3 ) 2 NaOH Cu(OH) 2 Heat CuO H 2 SO 4 CuSO4 Zn Cu reaction 1 reaction 2 reaction 3 reaction 4 Generically, the vast majority of chemical reactions can be classified as one of the following types: A + B C Combination A B + C Decomposition AB + CD AD + CB Double Displacement AB + C AC + B Single Displacement You will perform the four reactions and record observations. Based upon your observations, and knowing the reaction type (given in the procedure), you will have to write a balanced equation for each reaction. To help you get started, reaction 1 is discussed below. THEORY Reaction 1 is a double displacement reaction: Cu(NO 3 ) 2 (aq) + 2 NaOH (aq) Cu(OH) 2 (s) + 2 NaNO 3 (aq) in which both reactants (copper nitrate and sodium hydroxide) are initially in aqueous solutions. When Cu(NO 3 ) 2 dissolves into water, it separates in three ions: 1 Cu 2+ and 2 NO 3. In aqueous solution, Cu(NO 3 ) 2 could then be written as Cu 2+ (aq) and 2 NO 3 (aq). NaOH is also soluble into water and could then be written as Na + (aq) and OH (aq) when in solution. Cu(OH) 2 is not soluble and then precipitates out of the solution as a solid. NaNO 3 is soluble and then present as Na + (aq) and NO 3 (aq) in the solution. Equation of reaction 1 could then be written as: Cu 2+ (aq) + 2 NO 3 (aq). + 2 Na + (aq) + 2 OH (aq) Cu(OH) 2(s) + 2 Na + (aq) + 2 NO 3 (aq) And the net ionic equation would be: Cu 2+ (aq) + 2 OH (aq) Cu(OH) 2(s) 1

2 When going through the procedure, carefully observe the precipitates and the solutions. Copper will be present as a dissolved compound, as a solid compound, or as a metallic solid. Since the copper ion Cu 2+ produces a caracteristic blue color when in solution, you will be able to tell at any time whether copper is present into the solution or in the solid precipitate. APPARATUS AND CHEMICALS Apparatus ml Erlenmeyer flask ml beaker ml beaker 1 filter flask 1 Buchner funnel 1 watch glass 1 medium test tube 1 glass rod 1 stirring hot plate 1 magnetic stir bar 1 analytical balance 2 clamps 2 filter papers (5 and 10 cm) Chemicals copper nitrate sodium hydroxide 6M sulfuric acid 7M hydrochloric acid 6M methanol zinc strips 2

3 PROCEDURE Reaction 1 Addition of hydroxide ions to a solution containing copper ions results in the precipitation of copper hydroxide. This is a double displacement reaction Cu(NO 3 ) 2 NaOH Cu(OH)2 Accurately dispense approximately 10 ml of Cu(NO 3 ) 2 solution from the common buret at the end of the aisle directly into a 125 ml Erlenmeyer flask. Record the exact volume of the solution to 2 decimal places in Table 2. To this solution, add 5 ml of 6 M sodium hydroxide from the dispenser at the end of your aisle. Wash down the inside walls of the flask with distilled water, then add additional water to bring the total volume to 75 ml. Reaction 2 Subsequent heating of the copper hydroxide results in its decompostion to copper oxide and water. Cu(OH) 2 Heat CuO Borrow a magnetic stir bar from your demonstrator. (Bring your student ID card). Place the magnetic stir bar in the reaction flask. Clamp the flask at the neck with an extension clamp and clamp this to the hot plate stand under a mini-fumehood. (A demonstration on how to set up a mini-fumehood will be made in the pre-lab talk.) Heat the solution while stirring gently. If the solution begins to boil, remove the reaction flask from the heat. Keep your face away from the open neck. To prevent the solution from bumping, be sure to stir the mixture during the entire heating procedure. Place the minifumehood near the neck of the reaction flask. Heat the solution until the blue color (due to the copper ion in solution) is completely gone. The filtrate from the copper oxide precipitate should be colorless. Any residual blue color indicates that there are still some unreacted copper ions present in the solution. After precipitating all of the copper as copper oxide, clamp the extension clamp with the flask to the bench stand nearest you. Wash down the inside walls of the flask with distilled water, then add additional water to bring the total volume to 125 ml and allow the precipitate to settle to the bottom of the flask for 3-5 minutes. Show the precipitate to your demonstrator and record your observations in Table 1. Separate the copper oxide from the filtrate (liquid) by decanting. The object when decanting is to pour off as much liquid as possible while leaving the solid in the flask. A small amount of the solid usually comes off with the filtrate. In order to collect this solid, set up a Buchner funnel on a side-arm flask as described under Vacuum filtration (p.5). Slowly pour the filtrate through the funnel. If the side-arm flask becomes full, disconnect the water aspirator and dispose of the filtrate. When finished, turn off the water aspirator. 3

4 Reaction 3 Addition of an acid to a metal oxide is an example of a double displacement reaction. CuO H 2 SO 4 CuSO4 Clamp the extension clamp with the reaction flask to the bench stand. Remove the Buchner funnel from the filter flask and place it on the reaction flask. Dispense about 5 ml of 7 M H 2 SO 4 from a dispenser at the end of your aisle into a test tube. Slowly pour the acid through the Buchner funnel to dissolve any CuO collected on the filter paper during the decanting procedure. Remove the Buchner funnel and swirl the flask until all the CuO in the reaction flask has dissolved. Record your observations in Table 1. Reaction 4 Addition of zinc to a copper sulfate solution precipitates solid copper in a single displacement reaction. Zn CuSO 4 Cu Clamp the reaction flask on the stand of the hot plate so that the flask sits on the plate's surface and heat for about 5 minutes under the mini-fumehood. (If the reaction begins to boil, remove the flask from the heat, but keep it under the fumehood. You may need to raise the fumehood.) While still stirring, turn off the heat and add a piece of Zn metal to the flask. If the Zn reacts completely before the solution becomes colorless, add one more piece of zinc. When the solution becomes colorless, fish out (with tweezers) the piece of unreacted zinc, and dispense about 10 ml of 6M hydrochloric acid (from the common buret) directly into the reaction flask. This will get rid of any excess zinc. When bubbles of gas no longer appear, decant the solution through a Buchner funnel. Wash the copper with five successive 10 ml portions of water (approximate volume) and decant off the liquid between washing. Finally, transfer all of the precipitated copper onto the funnel using water from the wash bottle to aid in the transfer. Filter off all the water. Wash the precipitate itself with 20 ml of methanol and continue drawing air through for 3-5 minutes to aid in the drying of the copper. Label a watch glass and weigh it, record the weight (to 4 decimal places) in Table 2. Transfer the copper to the watch glass and place it in the oven until the copper is dry (approximately ten minutes). Cool for 5-10 minutes. Reweigh the watch glass and the copper, record the weight in Table 2. Examine your sample. Does it look pure? Record your observations in Table 3. Show your sample to your demonstrator for evaluation. The quality of the product depends on the color and dryness as well as the absence of impurities. Copper oxidizes easily, if left exposed to the air or heated in the air for too long, copper powder will gradually turn to black CuO. Dispose of the copper in the appropriate collection beaker near the sink. 4

5 Vacuum filtration Fit a Buchner funnel into a 125 ml filter flask. Place a small filter paper (diameter = 4.25 cm) inside the Buchner funnel. Clamp the flask. Connect the filter flask to the water tap aspirator nearest you. Wet the filter paper with distilled water from a wash bottle. Turn on the water tap aspirator halfway. The suction will pull the paper into the funnel and seal off the holes. (If suction is lacking in the setup, consult your demonstrator.) Pour the filtrate through the filter paper first, then transfer the solid unless decanting (which means that the solid is to stay in the original flask or beaker). When the filter flask becomes full, close the water tap aspirator, disconnect the Buchner funnel, and empty the filter flask either into another flask if the filtrate is to be used or down the drain if the filtrate is not needed. Reconnect the funnel and complete the filtration procedure. To vacuum trap 5

6 CALCULATIONS 1. From the weight of recovered copper and the volume of copper nitrate used, calculate the molarity of the copper nitrate solution assuming that no copper was lost during the reactions. Molarity = moles copper / liter solution Moles of copper = grams of copper / molecular weight of copper 2. From the announced molarity of the copper nitrate solution (not your calculated molarity) and the volume of solution that was used, calculate the theoretical yield (in grams) of copper. Moles copper liter solution x Volume of solution x grams copper mole copper = theoretical grams copper 3. From the weight of recovered copper and the theoretical yield of copper, calculate the percent yield of copper. percent yield = grams coppper recovered theoretical grams copper x 100% Percent yield is the weight of copper which you obtained, divided by the total weight of copper you started with in the form of copper nitrate, multiplied by 100%. A good experiment will give you a high % recovery of a pure product. 6

7 Data sheet, hand in before leaving the laboratory Copper Cycle Experiment 3 - Data Name First Last McGill ID number Demonstrator Section Day Time Date Table 1 Description of the reactions Reaction Observations 1 Cu(NO 3 ) 2 NaOH Cu(OH) 2 2 Cu(OH) 2 Heat CuO TA s initials 3 CuO H 2 SO 4 CuSO 4 4 CuSO 4 Zn Cu 7

8 Data sheet, hand in before leaving the laboratory Table 2 - Quantitative measurements Parameter Volume of Cu(NO 3 ) 2 solution Reading or Value Weight of watch glass Weight of watch glass and copper Announced molarity of Cu(NO 3 ) 2 solution Table 3 - Final product purity Student Color Dryness Impurities TA* *for TA use only TA s initials 8

9 Lab report, hand in within 24 hours Copper Cycle Experiment 3 - Lab report Name First Last McGill ID number Demonstrator Section Day Time Date Complete Table 4 by writing a balanced equation for each reaction. Use your observations to determine which species are solids and which remain in solution. Table 4 Chemical equations of the reactions Reaction Type Equation 1 double displacement Cu(NO 3 ) 2 (aq) + 2 NaOH (aq) Cu(OH) 2 (s) + 2 NaNO 3 (aq)

10 Lab report, hand in within 24 hours Table 5 - Calculations Parameter Net weight of recovered copper Value Calculated molarity of Cu(NO 3 ) 2 solution Equivalent weight of Cu in the solution (using the announced molarity) Percent yield QUESTIONS 1. Student Calamity reported a final yield of 50%. List four things she might have done wrong to account for this yield. 2. Student Overachiever reported a final yield of 115%. List two things he might have done wrong to account for this yield. 10

11 Lab report, hand in within 24 hours 3. Of the four copper compounds: Cu(NO 3 ) 2, Cu(OH) 2, CuO, CuSO 4, observed in this experiment, which two are the least soluble? 4. Suppose that, for reaction 1, you have not added enough sodium hydroxide to precipitate all the copper as copper hydroxide. a) How could you tell that not all the copper had precipitated? b) What effect would this have on your final yield? 11

12 Questions and problems 1. Write the name and chemical formula for two copper compounds used or formed in this experiment. 2. For the reaction CuSO 4 Zn Cu, how do you know when enough Zn has been added? 3. In an experiment similar to the one you will perform, the following data was obtained: Volume of Cu(NO 3 ) 2 solution used: Weight of the watch glass: Final weight of the watch glass + copper: ml g g a) How many grams of copper ions were present in the Cu(NO 3 ) 2 solution? b) How many grams of zinc would be required in the last step of the experiment, to recover all of the copper assuming that none was lost in any previous step? c) What is the calculated molarity of the copper nitrate solution? d) If the announced molarity of the copper nitrate solution was 0.89 M, what was the percent recovery of copper? 4. What acid is used in the last step of reaction 4 to get rid of the excess Zn? 5. In reaction 1,what should logically be the limiting reactant? Why? Answers 1) copper nitrate Cu(NO 3 ) 2 copper hydroxyde Cu(OH) 2 2) the solution turns from blue to colorless 3) a) g Cu 2+ b) g Zn c) M d) 93% 4) HCl 5) Cu(NO 3 ) 2 is the limiting reactant. Our goal is to transform ALL the copper nitrate, in order to recover all of the copper as Cu (s) at the end. (Therefore, an excess amount of NaOH should be used.) 12