A Cycle of Copper Reactions

Transcription

1 EXPERIMENT A Cycle of Copper Reactions PURPOSE To demonstrate a series of copper reactions: starting with copper metal, oxidizing the metal to put it into solution and then, form a copper hydroxide, an oxide, a sulfate, and lastly reducing the copper ion to produce copper metal again. Validate conservation of mass and of moles; we should be able to recover as much copper as we start with since careful attention to the procedure should have the amount of copper we started with during each step of the cycle of reactions. Practicing standard chemical techniques: quantitative transfers by decanting a supernatant liquid without losing the solid and successful completion of series of reactions. Recording observation and interpreting them in terms of classification of chemical process. MATERIALS AND EQUIPMENT Copper wire, zinc metal (granulated), 16 M HNO3, 3 M NaOH, 6 M H2SO4, 6 M HCl, methanol, centigram balance, hot plate, glass stirring rod, ceramic tile, two 250 ml beaker. BACKGROUND In this experiment you will start with a measured mass of copper metal and carry out a sequence of reactions that end with the recovery of the metallic copper. A dazzling change of color and change in physical appearance that helps in the identification, and classification of each reaction accompany each step of this cycle. We put elemental copper through five different chemical reactions in order to convert it into different compounds. By the end of the fifth reaction, the copper will be back to its elemental state. Step I Step II Step III Step IV Step V metal blue solution blue gelatinous solid black solid blue solution metal copper hydrated copper hydroxide oxide hydrated copper copper Cu(s) Cu 2+ (aqueous) Cu(OH)2(s) CuO(s) Cu 2+ (aqueous) Cu(s) Step I: Dissolving Copper & forming hydrated copper ion The concentrated nitric acid provides the nitrate ions which oxidize the copper metal to copper (II) ion while itself undergo a reaction to form nitrogen monoxide (NO) which rapidly reacts with oxygen in the air to form nitrogen dioxide (NO2), a brown gas. When the reaction mixture is diluted with water, the copper(ii) ion binds to six water molecules in this process. The hydrated (surrounded by water) copper ion forms an octahedral complex. The physical change you should observe is the copper-colored metal vanishing as the solution turns pale blue {from [Cu(H 2 O) 6] 2+, the hexaaquacopper(ii) ion} and a brown gas (NO 2 ) is evolved. Cu (s) + 4 H 3 O + (aq) + 2 NO 3 - (aq) [Cu (H 2 O) 6] 2+ (aq) + 2 NO 2 (g) Page 1

2 UNDER HOOD Heat UNDER HOOD Step II: Forming Hydroxides In this step two reactions are carried out by addition of sodium hydroxide. First, the hydroxide ions neutralize the excess acid from the first part and then they bind to the copper (II) ions. This binding is stronger than binding of water molecules. As a result, hydroxide ions can displace the water molecules from the copper (II) ion, yielding copper hydroxide, Cu (OH) 2, a blue gelatinous precipitate. [Cu (H 2 O) 6] 2+ (aq) + 2 OH - Cu (OH) 2 (s) + 6 H 2 O (l) In Part II, two reactions are carried out by adding NaOH (aq). In the first reaction, the hydroxide ions (OH ) from the NaOH (aq) neutralize the excess hydronium ions (H3O+) left over from the previous part: Step III: Forming Oxides Heating copper hydroxide produces copper oxide, CuO, a black solid. Cu (OH) 2 (s) CuO (s) + H2O (l) Step IV: Regeneration of hydrated copper ion Sulfuric acid dissolves copper oxide, regenerating the copper (II) ion, which once again binds to water. CuO (s) + 2 H 3 O + (aq) + 3 H 2 O (l) [Cu (H 2 O) 6] 2+ (aq) Step V: Forming Copper metal Finally, zinc metal reduces the hydrated copper (II) ion back to metallic copper while itself is oxidized to zinc (II) ions. At the same time, some of the zinc metal, which is present in excess, reduces hydronium ions to H 2. [Cu (H 2 O) 6] 2+ (aq) + Zn (s) Cu (s) + Zn 2+ (aq) + 6 H 2 O (aq) Zn (s) + 2 H 3 O + (aq) Zn 2+ (aq) + H 2 (g) + 2 H 2 O (l) Cu (s) 4 ml 16 M HNO 3 Cu 2+ (aqueous) 30 ml 3.0 M NaOH (aq) Cu(OH)2(s) Zn (S) Cu 2+ (aqueous) 15 ml 6 M H 2 SO 4 (aq) Cu(OH)2(s) Page 2

3 PROCEDURE SAFETY Nitric acid, sodium hydroxide, and sulfuric acid are corrosive. Several of the chemicals are toxic. Nitric acid stains skin. If you spill these reagents on yourself, rinse the affected area with water. The gas produced when copper reacts with nitric acid is highly toxic. Perform this reaction under a hood. Wear gloves and goggles Step 1: Dissolving Copper & forming hydrated copper ion 1- Measure the mass of a 250 ml beaker using centigrade balance and record it in your lab book. 2- Mass about 0.50 g a piece of copper to the nearest tenth of a milligram (± g) on the analytical balance and record the mass in your lab book. 3- Place the copper in a 250 ml beaker and under the FUME HOOD, add about 4 ml of 16 M nitric acid. Keep the beaker tilted to ensure copper is immersed entirely in the acid. Occasionally swirl the solution around in the beaker to facilitate dissolving copper and to remove NO 2 gas. Reaction is complete when there is no trace of metal and no evolution of toxic brown gas. 4- Record your observation in your in your notebook. 5- After the reaction is completed and there is no trace of metal copper, fill the beaker half full with deionized water and then take the beaker back to your lab bench. Step II: Forming Hydroxides 1- While stirring the solution with a glass rod, add very slowly 30 ml of 3.0 M NaOH to the beaker containing the copper solution. 2- Keep stirring the solution until the pale blue color turns to gelatinous solid blue color. 3- Record your observation and identify the products. Wait until the precipitate settles before you make your observations. Be sure to note the color of the precipitate as well as the color of the solution. Step III: Forming Oxides 1- Fill the second 250 ml beaker with 200 ml deionizes water and place the beaker on the hot plate. This hot water will be used to rinse the solid product 2- Place the beaker containing copper solution on the hot plate and heat both beakers until they have just barely reached boiling. 3- Continue stirring the copper solution with a glass rod to prevent "bumping" (a phenomenon caused by the formation of a large bubble in a locally overheated area of the liquid in the beaker). 4- When the transformation is complete (the color change of the precipitate to black is completed and the solution is colorless), remove the beaker from the hot plate and let it cool down on Page 3

4 ceramic tile. Do not place the hot beaker directly on the lab bench). Continue stirring for a minute or two, and then STOP stirring to allow the solid to settle. 5- Carefully decant the supernatant liquid into a clean 500 ml beaker. Do not to lose any precipitate. 6- Wash the solid two or three times, each time adding about 50 ml of hot deionized water and swirling the mixture, then allowing the solid to settle and decanting off the supernatant into a clean beaker. Step IV: Regeneration of hydrated copper ion 1- While stirring, add 15 ml of 6 M H 2 SO 4 solution to the beaker containing the precipitate. 2- Record your observations in your lab book and then carefully carry the beaker to the fume hood. Step V: Forming Copper Metal 1- Mass about 2.0 g of granular zinc using centigrade balance. 2- Under the FUME HOOD, add all the zinc metal at once and keep stirring until the supernatant liquid is colorless. If the Zn is completely dissolved but the solution is still blue, add more zinc metal. 3- If you can see any silvery grains of unreacted zinc, add 10 ml of 6 M HCl and warm, but do not boil the solution. 4- When no hydrogen evolution can be detected by eye, allow the Cu metal to settle at the bottom of the beaker. Without losing any of the solid, carefully decant the supernatant liquid into the 400 ml beaker that used as a waste container. 5- Wash the copper metal three times using 20-mL portions of deionized water by stirring and then allowing the solid to re-settle deionized water using decanting producer. 6- Wash the copper metal again three times with 5-ml portions of methanol. Washing with methanol will reduce the time needed for the drying step. 7- Under the FUME HOOD, dry the copper by gentle heating using hot plate. Avoid excessive heat! 8- Once the copper metal appears to be dry, let the beaker cool down to room temperature and measure and record the mass of beaker and copper on the analytical balance. 9- Reheat the beaker again (gently), allow to cool to room temperature and re-measure and record the mass in your notebook. If the mass of beaker and copper agree to within 5 mg between the first and second massing then you can consider to have reached constant mass and you may proceed and determine the mass of the recovered copper. Otherwise continue heating until you reach constant mass. 10- Record all your observation in the lab notebook. Page 4

5 LAB REPORT Copper Cycle Reactions Name Section Date Mass of copper wire Mass of clean, dry 250 ml Mass of recovered copper metal and 250 ml beaker Mass of final copper sample Calculate the percent recovery of copper. Show your calculations! Describe your observations for each step including colors, gases formed, etc. Classify the observed reactions as many ways as possible: Step I: Step II: Step III Step IV Step V Write a brief discussion of your results including a statement of the final percent recovery of copper and a discussion of reasons why the recovery differs from 100%.. Page 5

6 Pre-lab Questions: 1. Write a balanced chemical equation including phase labels for the reaction between aqueous copper (II) nitrate and aqueous sodium hydroxide 2. Nitrogen monoxide (NO) and nitrogen dioxide (NO2) are toxic, corrosive gases that significantly lower blood pressure when inhaled. How are these gases produced in today s experiment? What should you do to protect yourself against their toxicity? 3. Iron reacts with oxygen from the atmosphere to produce iron (III) oxide, also known as rust (Fe2O3). What chemical species is oxidized in this reaction? What is the reducing agent? 4. One student couldn t find any nitric acid so he tried to dissolve his copper sample in hydrochloric acid instead. Unfortunately his copper wouldn t dissolve in HCl solution. Why will copper dissolve in nitric acid but not in hydrochloric acid? Post-lab Questions: 1. Copper (II) hydroxide is converted into copper (II) oxide by heating the test tube containing Cu(OH)2 in a hot water bath. Is it necessary to use distilled water in this water bath? Why or why not? 2. Copper metal doesn t rust in the presence of oxygen at room temperature. However, it will react with O2 at elevated temperatures. Write a balanced chemical equation describing the formation of copper (II) oxide when copper metal is heated in air. 3. When zinc is dissolved in sulfuric acid a gas is produced. What is the chemical identity of this gas? How is it produced? 4. A student started with a g sample of copper which he took through the series of reactions described in this experiment. At the end of the experiment he obtained g of a black product. What was his percent yield? What is the most likely source of the error in his experiment? (Hint: consider question 2 above Page 5