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The Synthesis and Analysis of Copper(I) Iodide A First-Year Laboratory Project Lara A. Margolis, Richard W. Schaeffer, and Claude H. Yoder* Department of Chemistry, Franklin and Marshall College, Lancaster, PA 17604-3003; *
[email protected] Copper forms two oxidation states of relatively similar stability. In aqueous solution Cu(I) and Cu(II) can be related by the disproportionation reaction 2Cu+ → Cu + Cu2+ which has an equilibrium constant of 106. The significance of this constant can be appreciated by the observation that a solution that contains 0.001 M Cu+ will also contain 1 M Cu2+. The relative stability of the two oxidation states can of course be influenced greatly by the presence of an anion that precipitates one or both of the ions. In the earth’s crust, chalcocite, Cu2S, is more abundant than covellite, CuS, and cuprite, Cu2O, is more abundant than tenorite, CuO. When chloride or bromide is the anion, the cupric salts are more stable (but the cuprous salts can be prepared from the cupric salt by reaction with copper or other reducing agent). When iodide is the anion the +1 oxidation state is favored, presumably as a result of the insolubility of CuI and the greater reducing ability of the iodide ion. The Project We report here a laboratory project designed to allow students to explore the preparation and analysis of the binary compound formed by copper and iodine. The preparation can be done by one of two methods: addition of iodide to an aqueous solution of cupric chloride as reported by Wilhelm (1) and Kauffman and Pinnell (2), Cu2+ + 2I ᎑ → CuI + 1⁄2 I2 or the direct reaction of the elements in a procedure directly analogous to that reported for SnI4 (3). Cu + 1⁄2 I2 → CuI Because the reaction of the elements in toluene requires several hours, each student can set up the direct reaction and, while that reaction proceeds unattended, can prepare the same compound by mixing aqueous solutions of cupric chloride and potassium iodide. After filtration and washing, the products of both reactions can be analyzed gravimetrically for both copper and iodine. The percent copper is obtained by dissolving and oxidizing the sample in nitric acid. Addition of sodium hydroxide until the solution becomes basic results in the precipitation of Cu(OH)2, which is converted to CuO by gentle heating. The weight of CuO is converted to percent Cu. Triplicate determinations of the same product gave an average percent copper of 35.1 with a standard deviation of 0.6%. The percent iodine is obtained by heating another sample of the product with nitric acid. The nitric acid converts the iodide to iodine, which is liberated from a hot solution and collected on a “cold finger” above the solution. This procedure does not require special apparatus and is sufficiently quanti-
tative to permit a definitive identification of the product. Triplicate determinations of the same product yielded an average of 63.0% iodine with a standard deviation of 3.0%. The theoretical percentages of copper and iodine in CuI are 33.4 and 66.6, whereas the percentages for CuI2 are 20.0 and 80.0. The experimentally determined values are entirely sufficient, therefore, to establish the empirical formula of the compound and to determine that CuI, not CuI2, has been formed. Hazards Although toluene is not extremely toxic (LC50, inhalation, rat = 49 g/cm3/4 h; LD50, oral, rat = 636 mg/kg), all reagents should be handled in the hood. Because of the corrosive nature of nitric acid, special care must be taken to avoid contact with skin. We recommend the use of gloves and, of course, appropriate safety goggles. Extensions and Discussion Topics The project can be expanded by the preparation of other iodides such as SnI4 and SnI2, or by analysis of Cu2+ by other procedures such as colorimetry and analysis of I᎑ by standard volumetric methods (3, 4 ). The instructor should discuss the existence of both oxidation states, but not reveal the greater stability of Cu(I) in the presence of iodide. In addition to the relative advantages of each reaction and a comparison of products and yields, the students should be encouraged to think about one or more of the following: 1. The amount of energy required to form Cu+ and Cu2+ in the gaseous state, relative to the energy requirements for the formation of the +1 and +2 ions of neighboring zinc. 2. The amount of energy liberated when the gaseous Cu+ and Cu2+ ions are hydrated (the hydration energy) and use this along with the ionization energies (question 1) and the heat of formation of the gaseous atom to obtain the heat of formation of both Cu+(aq) and Cu2+(aq). 3. The effect of lattice energy in the heat of formation of the solids (in addition to ionization energy and heat of formation of gaseous atom). 4. The difference in the reaction of Cl2 and I2 with copper, given the difference in the electrode potentials for the two halogens. 5. Rationalization of the product using the hard/soft acid–base principle. 6. Rationalization of the relative solubilities of CuCl vs CuCl2 and CuI vs CuI2 using the hard/soft acid–base principle. 7. The general conversion of hydroxides to oxides upon heating and the reasons for carrying out this step in the gravimetric analysis of copper.
JChemEd.chem.wisc.edu • Vol. 78 No. 2 February 2001 • Journal of Chemical Education
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In the Laboratory 8. The relative volatility of I2 that allows the iodine to be determined by sublimation. 9. The general features of gravimetric analyses illustrated by the copper and iodine analyses.
Of course, the type of question that the student can be expected to engage will depend on the content of the course. The project can also be performed to advantage in a collaborative or discovery laboratory. For example, the effect of a thiosulfate wash of the crude product in the aqueous preparation, the length of reaction time and the temperature of reaction in the direct reaction, and the duration of sublimation time and heating in the iodine analysis are all variables that can be explored by different groups or members of a group. In
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order to save time, the CuI can be produced only from the aqueous reaction, and some groups can determine the copper content while others determine the iodine content. WSupplemental
Material Instructions for students and additional information for the instructor are available in this issue of JCE Online. Literature Cited 1. Wilhelm, D. L. J. Chem. Educ. 1973, 50, 436. 2. Kauffman, G. B.; Pinnell, R. P. Inorg. Synth. 1960, 6, 3. 3. Schaeffer, R. W.; Chan, B.; Molinaro, M.; Morissey, S.; Shenk, S.; Yoder, C. S.; Yoder, C. H. J. Chem. Educ. 1997, 74, 575–577. 4. Wheatlan, D. A. J. Chem. Educ. 1973, 50, 854.
Journal of Chemical Education • Vol. 78 No. 2 February 2001 • JChemEd.chem.wisc.edu
