George F. Condike Fitchburg State College Fitchburg, Massachusetts 01420
Near 100% Student Yields with the "Cycle of Copper Reactions" Experiment
Chemistry teachers generally are familiar with the "Cycle of Copper Reactions" laboratory experiment in which a weighed amount of copper metal is reacted with nitric acid to form copper(I1) nitrate, then copper(I1) hydroxide precipitated by the addition of base, thence t o copper(I1)oxide by thermal decomposition, to copper(I1) sulfate by the addition of sulfuric acid, and finally to copper metal by having zinc metal displace copper from CuS04. Comparison of the final weight of copper obtained with the starting weight of copper gives a percentage recovery. Using a centigram balance and following usual procedural directions for this reaction sequence, our students have previously been obtaining yields in the 85% to more than 110% range, with only a few obtaining near 100%. Many sources of possible error combine to give such results, among which are incomplete removal of by-product salts when supernatant liquid is decanted from CuO, incomplete displacement.of copper metal by zinc, incomplete removal of excess zinc metal from the mixture of zinc metal and copper metal as excess zinc is reacted with acid, and the inadvertent conversion of some copper metal to black CuO in attempting to dry the final copper metal product by direct Bunsen burner heat. If a few of these errors are present, the student not only has a product of poor appearance, but has a personally unrewarding laboratory experience when his recovery is too far from 100%. A poor quantitative recovery may also confuse students as to the usefulness of yields. A careful selection of the amounts of reagents used, a precautionary washing and decantation for CuO, a fresh acid charge to assure removal of excess zinc, and a drying procedure for final copper product that does not use direct Bunsen burner heat seem to eliminate the sources of error cited inasmuch as most of our students now obtain yields very close to 100%. The copper product is a uniformly colored powder with a few metallic glints of light from occasional larger crystals formed. Our students appear to be enthusiastic about the product appearance and their quantitative results. Of course, none of the above precludes a student from making weighing errors, but an alert laboratory instructor can spot these and have weighing data checked. The starting weight of copper should be about 0.5 g weighed to nearest 0.01 g and should be in the form of # 16 or # 18 wire. Six and one-half centimeters of # 16 wire is about 0.5 g. The wire can be issued to the student preweighed as an unknown. Included below is a short summary of the essential procedure for the reaction sequence. 1) I n fume hood, using a 250-ml beaker, prepare CU(NOQ)P
by adding 4.0 ml concentrated nitric acid to copper wire supplied hy instructor. After reaction is complete, add distilled water until beaker is about half-full. 2) Add 30 ml3.0 M NaOH to precipitate Cu(0H)z. 3) Add two or three boiling chips. Then, with stirring,heat just to boiling to convert CU(OH)Z to insoluble black CuO.
4) Allow CuO to settle, then deeant supernatant liquid. Add about 200 ml very hot distilled water, settle, and decant once more. 5 ) Add 15 m16.0 M HVSOIto convert CuO to CuSOn. 6 ) In fume hwd, sddallat once 2.0-g 30-mesh zinc metal to precipitate copper metal, stirring until supernatant liquid IS colorless. When evolution of hydropen pas has become slow, decant. 7) Still in hood, add 5 ml distilled water, then 10 ml concentrated bvdrocblorie acid to react excess zinc. When hvdrogm evolution has heeome UP,, slow, remove to lnhoratog desk statlo", warm, but do not boil, and remove hent. 81 When no hydrogen evolution ran be detected visually, decant, and transfer to porcelain evaporating dish. 9) Wash product with about 5 ml distilled water, allow to settle, and decant. Repeat. 10) Wash with about 5 ml methanol (keep away from flames!), allow to settle, and decant. Finally, wash with about 5 ml acetone (keep away from flames!), allow to settle, and decant. 11) Place dish over steam bath (may he a beaker of hoiling water) at Least 5 min to dry product. 12) Transfer product to weighing paper, remove boiling chips, and after cooling, obtain product weight with a good centigram balance. 13) If time permits, repeat steps 11and 12 to determine if product is at constant weight. ~
For step 2, an equivalent amount of NaOH of another molaritv mav be used. ex.. 15 ml of 6.0 M NaOH. Boiling chips hive been includedi'n step 3 because of previous exnerience with a downward explosion if stirring is inadequate during the heating. In s t e p 6, students'should be aware that, while zinc metal is displacing copper from CuS04, zinc is also reacting simultaneobsly with excess H2S04, thus producing hydrogen gas with its attendant hazards. A three-hour laboratory session is more than adequate time to complete the experiment provided there is not excessive waiting in line to make weighings or to use fume hoods. For shorter laboratory sessions the experiment can be completed in allotted time if forewarning is given relative to preparing in advance the hot water for the washing1 decantation in step 4, and the steam bath in step 11. A student laboratorv reoort reauest can include all balanced equations involved- weighing data (with calculated nercent recoverv when initial weieht of wire is known to the student); questions about the type reaction (combination, decomposition, single replacement, metathesis or double replacement; or, in the case of the first reaction encountered. a more comolex redox reaction): and questions about the positions of zinc and copper in the electrode potential series relative to steps 6 and 7. In using this experiment we entitle it "Measuring Technique Efficiency with a Cycle of Copper Reactions." Students seem to view this title as an incentive and a challenge for good technique, and a gratifying interpretation which is made of a good recovery percentage.
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Volume 52. Number 9. September 1975 / 615
