In the Classroom
Experimental Demonstration of Isomorphism J. Kamení cˇek* and M. Melichárek Department of Inorganic and Physical Chemistry, Palack y´ University, 771 47 Olomouc, Czech Republic; *
[email protected] In chemistry education books about crystals, there are many terms such as isomorphism, polymorphism, and allotropy (1, 2). These terms are illustrated in the literature by some examples of compounds that exhibit these phenomena: alums (isomorphism), calcite/aragonite (polymorphism of CaCO3) and graphite/diamond (allotrope of C); but many students have difficulty understanding this. Our experience shows that isomorphism and polymorphism in particular can often be confused. The best way to avoid this problem is by presenting an interesting demonstration. Generally, each welldesigned experiment enhances student’s understanding of chemistry. The effect of isomorphism may be clearly and simply demonstrated in two ways: By the preparation of mixed crystals with components in various ratios; and
reaction shown in eq 2, 7 g of (NH4)2SO4 in water (20 mL, 20 °C) and 38 g of Cr2(SO4)3ⴢ18H2O in water (100 mL, 20 °C). Filtration of solutions before growing the crystals and covering the beaker with a sheet of paper to avoid entrance of microparticles from the air is recommended (danger of the formation of satellite crystals). The formation of crystals starts when the solution is saturated owing to a decrease in temperature or evaporation of water. Finally, it is necessary to avoid air-decomposition of the prepared samples. For this purpose, silicone oil can be used. The crystals of alums were placed into test tubes with silicone oil and sealed, so that the samples might be saved for a very long time. All reagents used were from LACHEMA Brno, Czech Republic, p.a. purity. CAS Registry Numbers: [7778-80-5], [7784-31-8], [7783-20-2], [15244-38-9]. Results and Discussion
By the formation of a “double” crystal, e.g. deposition of the second phase on the surface of the initial crystal.
Alums are compounds having the general formula M MIII(SO4)2ⴢ12H2O (MI = NaI, KI, RbI, NH4I; MIII = AlIII, CrIII, FeIII, CoIII, MnIII and some others). They are appropriate materials for use because they exhibit well-formed octahedral single crystals (3). Moreover, the exchange of cations in these salts is associated with a change of color, and this feature may be exploited for experimental demonstrations. I
Large single crystals of dimensions up to 0.01 meter or more were obtained (Fig. 1). “Mixed” single crystals were prepared in a similar way, the only difference being that the initial solution was obtained by mixing the reactants in eqs 1 and 2 in various volume ratios (from 1:50 to 1:1). Thus, a set of single crystals of various shades from light to dark violet was obtained (Fig. 2).
Experimental Procedure Alums of general formula MIMIII(SO4)2ⴢ12H2O were prepared by the reaction of aqueous solutions of the appropriate sulfate salts (MI = KI, NH4I; MIII = AlIII, CrIII). For instance: 24H2O(aq) + K2SO4(aq) + Al2(SO4)3(aq) → (1) 2KAl(SO4)2ⴢ12H2O(s) (white, “colorless“) 24H2O(aq) + (NH4)2SO4(aq) + Cr2(SO4)3(aq) → 2NH4Cr(SO4)2ⴢ12H2O(s) (violet)
(2)
In principle, other alums can be used, too, but those mentioned above give the best results (growing time, shape and color of crystals). • CAUTION: chromium salts are poisonous! Generally, these reactions were performed as follows. One small crystal of the chosen alum was hung on a thin thread (0.1–0.2 mm) in the appropriate solution in a beaker for a long time (up to a week, depending on temperature, which should be constant). This arrangement is necessary to avoid the formation of multiple small crystals and deformations due to contact with the walls of the beaker. The following solutions of components were used: for the reaction shown in eq 1, 12 g of K2SO4 in hot water (50 mL, 90 °C) and 42 g of Al2(SO4)3ⴢ18H2O in warm water (100 mL, 50 °C); for the
Figure 1. Single crystal of NH4Cr(SO4)2ⴢ12H2O.
Figure 2. Mixed crystals of KAl(SO4)2ⴢ12H2O + NH4Cr(SO4)2ⴢ12H2O.
JChemEd.chem.wisc.edu • Vol. 77 No. 5 May 2000 • Journal of Chemical Education
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In the Classroom
In a second demonstration, one small violet crystal of NH4Cr(SO4)2ⴢ12H2O was placed into a solution of reactants (eq 1). The result was that a white octahedral crystal of KAl(SO4)2ⴢ12H2O grew directly on the surface of the initial violet one, which is clearly shown in Figure 3. The results of both experiments may be explained only by an assumption of the replacement of cations in structures of alums without significant changes in structure (isomorphism). This important idea can lead to discussions with students concerning the conditions of exchanges. For example, why LiAl(SO4)2ⴢ12H2O does not exist—LiI is too small to be replaced without significant distortion of the structure. The students can propose other possible MIII atoms appropriate for alums (for instance the alums of Ti, V, Cr, Mn, Fe, Co, Ga, In, Rh, and Ir are known). Conclusion This article has presented some experiments for the illustration of isomorphism using single crystals of alums. These experiments are very simple. Because they need no special arrangements or devices they may be carried out without difficulty by teachers in any type of school where chemistry is taught—and, of course, by students in laboratories, too. In our experience, all the experiments are very effective and amazing. While it may take up to one week to
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Figure 3. Double crystal of KAl(SO4)2ⴢ12H2O growing on the surface of NH4Cr(SO4)2ⴢ12H2O.
grow crystals, the surprise of students is great and they never forget the explanation of the phenomenon of isomorphism. Literature Cited 1. Fundamentals of Crystallography; Giacovazzo, C.; Monaco, H. L.; Viterbo, D.; Scordari, F., Gilli, G., Zanotti, G., Catti, M., Eds.; IUCr Book Series; Oxford University Press: New York, 1990. 2. Rao, C. N. R., Gopalakrishnan, J. New Directions in Solid State Chemistry; Cambridge University Press: Cambridge, 1986. 3. Cotton, F. A., Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1966
Journal of Chemical Education • Vol. 77 No. 5 May 2000 • JChemEd.chem.wisc.edu
