PERCENT Y IELD: COPPER T Ra NSFORMATIONS

Transcription

1 Experiment 4 Name: 39 PERCENT Y IELD: COPPER T Ra NSFORMATIONS In this experiment, you will carry out and observe a reaction sequence where copper metal is the starting material that undergoes a series of transformations as summarized below. Copper will be recovered as the end product. The mass of Cu at the beginning and at the end of the series of reactions are theoretically equal according to the law of conservation of mass. HNO 3 NaOH heat H 2 SO 4 Zn Cu Cu(NO 3 ) 2 Cu(OH) 2 CuO CuSO 4 Cu 88 In the first reaction, copper reacts with nitric acid forming copper(ii) nitrate. Reaction 1: Cu(s) + 4HNO 3(aq) Cu(NO 3) 2(aq) + 2NO 2(g) + 2H 2O(l) Copper(II) nitrate then reacts with sodium hydroxide to form copper(ii) hydroxide. Reaction 2: Cu(NO 3) 2(aq) + 2NaOH(aq) Cu(OH) 2(s) + 2NaNO 3(aq) Copper(II) hydroxide is converted to copper(ii) oxide by heating. Reaction 3: Cu(OH) 2(s) CuO(s) + H 2O(g) Copper(II) oxide reacts with sulfuric acid, forming copper(ii) sulfate. Reaction 4: CuO(s) + H 2SO 4(aq) CuSO 4(aq) + H 2O(l) Copper (II) sulfate reacts with zinc to produce metallic copper. Reaction 5: CuSO 4(aq) + Zn(s) Cu(s) + ZnSO 4(aq) The percent yield for this sequence of reactions can be calculated as follows: % yield = actual yield theoretical yield 100 The actual yield is the amount of copper actually recovered, which is the mass of the copper product obtained. The theoretical yield is the amount of copper expected to be recovered. Theoretically, after undergoing all of these reactions, the amount of copper produced in Reaction 5 should be equal to the amount of copper that you start with in Reaction 1. This is in accordance with the law of conservation of matter; the amount of Cu is conserved all throughout the reactions. For example, if 0.56 g of Cu was used in the experiment, 0.56 g Cu is the theoretical yield. If 0.41 g Cu was actually recovered, the percent yield is 73%. 1

2 PROCEDURE 1. Obtain ~0.4 g of copper. Record the weight. Record the initial color and appearance of the copper. 2. Place the copper in a 400-mL beaker. Proceed to the hood where the concentrated HNO 3 (16 M solution) is. Measure 5 ml of it with a graduated cylinder, transferring to from the bottle to the graduated with a dropper. Then, add it to the beaker. Brown NO 2 fumes will evolve vigorously. The reaction beaker should be kept in the hood until the reaction is completed, that is, when fumes are no longer released and all the copper wire has disappeared. CAUTION: HNO 3 is very corrosive to the skin. Handle carefully. If spilled on skin, immediately wash off with plenty of water for several minutes. Since NO 2 is toxic, the reaction must be carried out only in the hood. Clean up spills. 3. Remove from the hood once all copper has dissolved. Cautiously add 100 ml of water. Note the color of the solution. 4. Slowly and cautiously, add, while stirring, 15 ml of 6 M NaOH to the beaker. Note the color of the solid produced. CAUTION: This reaction will release a lot of heat. 5. Place a magnetic stir bar in the beaker. Then place the beaker at the center of a heat/stir plate. Turn on the stirrer to medium speed. Turn on the heat to high. When the mixture has boiled, reduce the temperature to ~200 C and gently heat for 5 minutes. Note the color of the solid produced. CAUTION: Stirring prevents bumping and spattering. The sudden spattering of the mixture is dangerous, so the stirrer should be kept on during this stirring process. 6. Remove the beaker from the hot plate and allow to cool and the precipitate to settle. Obtain a large (i.e. 400 ml) waste beaker. Decant the solution into this waste beaker, pouring off as much of the liquid as possible, taking care that none or minimal solid is lost. Dispose of the liquid in the waste beaker in the designated main waste container. 7. Add 100 ml of distilled water and place on the hot plate. Heat on high and stir the precipitate for 5 minutes. Remove from the hot plate, let cool, and decant the liquid once more. This step is used to wash off any residue on the precipitate left from the previous reactions. 2

3 8. Obtain a 250 ml beaker and an evaporating dish. Make sure the evaporating dish fits in the beaker as shown in Figure 1, but do not place it on the beaker. Fill the beaker up with about 150 ml of water and begin heating the water. (This will be needed later when drying your sample). 9. Add 20 ml of 3 M H 2SO 4 to the precipitate from step 7. Stir until all the solid is reacted and heat if necessary. Note the color of the solution produced. 10. Use a stir bar retriever to retrieve the stir bar from the beaker. With the bar stuck to the retriever, rinse both thoroughly with plenty of tap water. CAUTION: Do not drop the stir bar down the drain of the sink. 11. In the hood, add in 0.5 g small portions, while stirring, of powdered zinc metal to the solution, until the solution becomes colorless. Do not continue until your solution is colorless. 12. Still in the hood, add 10 ml of 6 M HCl to remove the excess zinc, stirring the mixture with stirring rod as needed. Gas evolution will occur. 13. When the zinc has completely disappeared and bubbling has stopped, stop stirring. Return to lab bench and allow the copper to settle and cool. 14. Decant the liquid into the waste beaker. 15. Wash the solid by pouring ~10 ml water then decanting. Repeat with another 10-mL water wash and decant. 16. Weigh a clean, dry evaporating dish. Transfer the copper to this dish, using as little water as possible to wash the copper from the beaker. Decant the excess water in the evaporating dish. 17. Put the magnetic stir bar into the 250 ml beaker and turn on the stirring to medium speed. When the water boils, reduce the heat to ~ 200 C, and continuing the boiling. Place the evaporating dish on the beaker (note the beaker will be hot). The set-up is shown in Figure 1. 3

4 evaporating dish with product heat/stir plate 250-mL beaker with 150 ml water and stir bar Figure 1. Set-up for drying the product 18. Turn off both heating and stirring when all the liquid in the dish has evaporated and the product (solid) is dry. Using tongs, remove the dish from the beaker and allow to cool to room temperature. Wipe the outside of the dish to dry. Weigh the evaporating dish with the copper and record. Observe and record the color and appearance of the copper product. CLEAN-UP Dispose of wastes in designated containers in the front hood. The contents of the waste beaker go to the container labeled filtrates. Wash all glassware used. Return materials where they belong. Make sure the heat/stir plate is turned off. Do not lose the stir bar down the sink drain. 4

5 Name: Partner s Name: Date: _ PERCENT YIELD: COPPER TRANSFORMATIONS DATA AND OBSERVATIONS Mass of copper starting material Color and appearance of copper Product in the Procedure Step 3 solution Step 4 solid Step 5 solid Step 9 solution Color Chemical Formula of the Copper Product Mass of evaporating dish Mass of evaporating dish with copper product Mass of copper product Color and appearance of product POST LAB QUESTIONS Show clearly the complete calculations with correct number of significant figures and units. 1. Based on the conservation of matter, how much copper should you have at the end of the experiment (what is your theoretical mass of copper)? 2. Calculate the percent yield. 5

6 3. If your percent yield is more than 100%, list 2 factors that would cause your mass to be higher than expected. (Hint: in which steps could something have added mass to your sample) 4. If your percent yield is less than 100%, list 2 factors that would cause your mass to be lower than expected. (Hint: in which steps could you have lost mass from your sample) 5. Compare the color and appearance of the copper starting material and the copper product. Why might they not look the same? 6