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JCE WebWare: Web-Based Learning Aids
William F. Coleman Wellesley College Wellesley, MA 02481
A New Java Animation in Peer-Reviewed JCE WebWare Just added to JCE WebWare, Computer Simulations of Salt Solubility uses a Java applet and Web browser to present an animated illustration of differences in the solubility of salts. This animation illustrates differences in solubility due to differences in the entropy of solvation. The animation can also be used to illustrate equilibrium concepts qualitatively. This WebWare is appropriate for use in both high school and college introductory chemistry courses. This animation and its classroom applications are discussed fully in the accompanying article, Using Computer Simulations To Teach Salt Solubility, by the same authors, on page 170 of this issue of the Journal. Read it for details about how to use this animation to help students in your classroom! You can view and download this and other animations in the peer-reviewed and open review collections of JCE WebWare at http://www.JCE.DivCHED.org/JCEDLib/WebWare/. Computer Simulations of Salt Solubility by Victor M. S. Gil, Department of Chemistry, University of Coimbra, Portugal; João C. M. Paiva, Department of Chemistry, University of Porto, Portugal
Edward W. Fedosky
W
University of Wisconsin–Madison Madison, WI 53715
The pairs chosen for these animations are NaCl/CaCO3 and MgCO3/MgSO4. In the former case, both dissolution phenomena are almost athermic, hence the major differences are in configurational disorder and solvation effects. In the latter pair, the major difference lies in thermal disorder, as the dissolution of magnesium sulfate is quite exothermic. The program simulates the “before” and the “after” states for the dissolution of equal amounts of the two salts of each pair in identical amounts of water. Changes in total entropy and in its components are represented qualitatively in bar graphs. In the case of NaCl versus CaCO3, the animation illustrates that the small solubility of CaCO3 in water is due to the decreased mobility of the water molecules associated with the small, dipositive calcium ion. This decrease results in a decrease in total entropy upon dissolution of calcium carbonate, and thus the amount dissolved is very small. On the other hand, the dissolution of NaCl is substantial because it yields a gain in total entropy since there is not a similar decrease in motility.
Keywords: First-Year Undergraduate / General; High School / Introductory Chemistry; Computational Chemistry; Equilibrium; Thermodynamics Requires: Web browser with Java enabled
Computer Simulations of Salt Solubility provides an animated, visual interpretation of the different solubilities of related salts based on simple entropy changes associated with dissolution: configurational disorder and thermal disorder. This animation can also help improve students’ conceptual understanding of chemical equilibrium before any quantitative interpretation of equilibrium constants is attempted. The animation compares pairs of compounds chosen according to the following criteria. Paired together are: • Familiar salts of very different solubility • Salts of similar packing structures • Salts whose solubility is either not significantly affected by ion reaction with water (such as acid–base and complex formation reactions), or affected in a way that opposes the observed solubility differences • Salts that essentially require discussion of only configurational disorder or only thermal disorder
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Figure 1. A screenshot from Computer Simulations of Salt Solubility comparing the dissolution of NaCl and CaCO3.
Vol. 83 No. 1 January 2006
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Journal of Chemical Education
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For the pair MgCO3/MgSO4, where the solvation effects are similar, the increased thermal disorder of dissolved MgSO4 is illustrated by greater particle agitation. The dissolution of MgSO4 yields greater thermal entropy and particle movement, and thus it is much more soluble than MgCO3. A word of reservation is due. First, the simulations are qualitative in nature, and no rigorous calculations of the contributions to entropy changes are illustrated. Also, when using these pictorial simulations, students should be alerted specially to the unrealistically small number of particles represented and the unrealistically large spaces between them, to the symbolic representation of ions and molecules, and to qualitative illustrations of only some molecular motions. In addition, exchange processes involving water molecules of the hydration sphere are not shown.
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Journal of Chemical Education
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Figure 2. A still image from Computer Simulations of Salt Solubility of the animation comparing the dissolution of MgCO3 and MgSO4
Vol. 83 No. 1 January 2006
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