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Letters Copper Content in Synthetic Copper Carbonate The article by Daniel Sheeran, “Copper Content in Synthetic Copper Carbonate: A Statistical Comparison of Experimental and Expected Results” (1), describes a new and interesting experiment that stimulated the following three comments. 1. In this paper’s introduction it is parenthetically stated that pure neutral copper carbonate, CuCO3, is not a known substance. Fortunately, the source of this information is cited (2); thus the “textbook error” can be corrected in future editions. Although apparently CuCO3 has not yet been found in nature, it has certainly been synthesized (3, 4 ). Moreover, both its solubility and its standard Gibbs energy of formation have been determined (5). 2. While for CuCO3 a high-pressure technique is required (3, 4 ), thus probably preventing its preparation in undergraduate teaching laboratories, only readily available equipment is necessary to synthesize pure malachite Cu2(OH) 2CO3 (6 ). Consequently, it is suggested that the experiment could be improved by preparing at least one well-defined copper carbonate phase prior to copper determination and statistical analysis. If permitted by the local situation, the synthesis of azurite, Cu3(OH)2(CO3)2 (7), as a second compound would be advantageous, because it should be a fruitful experience for students to find out analytically that copper carbonates with different stoichiometry can be prepared. Moreover, careful copper and carbonate analyses (as stated in the summary of Sheeran’s article) would provide a nice proof of the law of multiple proportions. 3. From the physicochemical point of view this experiment can be expanded by the construction and discussion of predominance diagrams for the system Cu2+–H2O–CO2 (5, 6 ). The solubility constants of the solid phases and the stability constants of some relevant copper complexes in the aqueous phase are shown in Table 1. Using these constants, the solid lines in the predominance diagram (Fig. 1) can easily be calculated. According to Figure 1 the thermodynamically stable phase at atmospheric conditions is CuO, tenorite. The transformations of malachite into azurite and azurite into copper carbonate occur at 25 °C and CO2 partial pressures around 0.76 atm and 4.57 atm, respectively. These partial pressures indicate at least qualitatively favorable conditions for the synthesis of the different copper carbonate phases. Literature Cited 1. Sheeran, D. J. Chem. Educ. 1998, 75, 453–456. 2. Encyclopedia of Inorganic Chemistry, Vol. 2; King, R. B., Ed.; Wiley: New York, 1994; p 836. 3. Ehrhardt, H.; Johannes, W.; Seidel, H. Z. Naturforsch. B 1973, 28b, 682. 4. Seidel, H.; Viswanathan, K.; Johannes, W.; Ehrhardt, H. Z. anorg. allg. Chem. 1974, 410, 138–148.

Figure 1. Predominance diagram for the system Cu2+–H2O–CO2 at 25 °C, I = 0. Table 1. Equilibrium Constants in the System Cu2+–H2O–CO2 at 25 q °C, I = 0 Mineral

Formula

K

log K + {2

2+



Cu(OH)2 (s)

[Cu ][H ]

Tenorite

CuO(s)

[Cu2 + ][H+ ]{2



CuCO3 (s)

[Cu ]p CO2 [H ] 2+

Azurite

Cu3 (OH)2 (CO3 )2 (s)

[C u ] p

Malachite

Cu2 (OH)2 (CO3 )(s)

[Cu ] p 2+

7.61 + {2

2+

2+

8.74

2/3 CO2 1/2 CO2

6.69

+ {2

6.47

+ {2

6.49

[H ] [H ]

+ {2



Cu(OH)2 (aq)

[Cu ][H ]

27.98



CuCO3 (aq)

[Cu2 + ]p CO2 [H+]{2

11.42



Cu(CO3 )2 2{(aq)

[Cu2 + ] p CO2 [H+]{4

2

26.46

5. Reiterer, F.; Johannes, W.; Gamsjäger, H. Mikrochim. Acta 1981, I, 63–72. 6. Schindler, P.; Reinert, M.; Gamsjäger, H. Helv. Chim. Acta 1968, 51, 1845–1856. 7. Handbuch der präparativen anorganischen Chemie, 2nd ed.; Brauer, G., Ed.; Enke: Stuttgart, 1994. H. Gamsjäger and W. Preis Institut für Physikalische Chemie Montanuniversität Leoben A-8700 Leoben, Austria

The author replies: I am grateful to H. Gamsjäger and W. Preis for correcting the inaccuracy concerning the existence of CuCO3 and I truly appreciate their interest in the experiment described in my paper as well as their suggestions for improving and expanding it. Daniel Sheeran Eastern Illinois University Charleston, IL 61920-3099

JChemEd.chem.wisc.edu • Vol. 76 No. 10 October 1999 • Journal of Chemical Education

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Chemical Education Today

Letters

The Amateur Mineral Chemist

More on Double Replacement

As an amateur mineralogist I enjoy perusing your Journal several times a year in a college library. As I am interested in testing minerals by the older, simpler, classical qualitative methods, a colleague suggested that I search JCE for interesting information. A few years ago a professor gave me several years’ back issues, which I enjoy very much. My interest is in reagents, simple inexpensive equipment, and tests that one may perform upon minerals using small quantities of reagents. I have in the past made a few reagents, such as barium chloride starting from the ore barite (admittedly a most insoluble compound, but there are ways to get around this), and salt of phosphorus (NH4HNaPO4?4H2O), for which I discovered a new recipe, although there are two cheaply available alternates which fuse to the same reactive sodium metaphosphate beads to which the salt of phosphorus fuses. My research is for alternate sources of small quantities, as well as literature (I had an article, “A Review of Some Texts on the Qualitative Chemical Testing of Minerals”, published in Mineral News of October 1994, volume 10, number 10, pp 6–7). I hope to compare notes with others interested in the same, and have been editing a source guide for the amateur mineral chemist. Do you know of others with whom I could correspond on this subject?

In his article, “Replace Double Replacement” (J. Chem. Educ. 1999, 76, 133), R. Bruce Martin deals with a fundamental topic that causes confusion among beginning students and hence is of perennial concern to chemical educators. Two decades ago I drew up detailed, stepwise directions for writing equations for such reactions in a consistent manner with liberal use of specific examples (Kauffman, G. B. J. Coll. Sci. Teaching 1979, 9, 83). This article, which also deals with the driving forces behind reactions, spectator ions, and other essential topics discussed by Martin, should prove useful to a new generation of students and instructors alike.

Dana Martin Morong 117 Piscataqua Bridge Rd. Madbury, NH 03820-6805

1340

George B. Kauffman Department of Chemistry California State University, Fresno Fresno, CA 93740-8034

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Journal of Chemical Education • Vol. 76 No. 10 October 1999 • JChemEd.chem.wisc.edu