Resumen de Activity-Based Approach For Teaching Aqueous Solubility, Energy, and Entropy
Laura Eisen, Nadia Marano, Samantha Glazier
We describe an activity-based approach for teaching aqueous solubility to introductory chemistry students that provides a more balanced presentation of the roles of energy and entropy in dissolution than is found in most general chemistry textbooks. In the first few activities, students observe that polar substances dissolve in water, whereas nonpolar substances do not, and that in some cases, dissolution is endothermic. Because energy minimization alone cannot explain these observations, we introduce entropy descriptively as another factor promoting dissolution. The increase in entropy due to mixing can counteract unfavorable energy changes, whereas unfavorable entropy changes due to reorganization of the water explain why nonpolar substances do not dissolve. In another activity, students mix combinations of positive and negative ions and formulate a simple rule that predicts the solubility of most ionic compounds: salts in which the cations and anions are multiply charged are likely to be insoluble; other salts are likely to be soluble. Students later discover that the sign and magnitude of the entropy changes explain why the rule works so often. Salts that are exceptions to the rule generally have unusually large enthalpies of dissolution.
