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Unformatted text preview: Austin Peay State University Department of Chemistry Chem 1121 Solubility and Thermodynamics of Potassium Nitrate Revision F7 Page 1 of 9 Suggested reading: Chang text pages 700 706; 775 781; 782 786 Cautions Potassium Nitrate (KNO 3 ) is an oxidizer and skin irritant. Open flames and hot water can cause burns. Purpose The purpose of this experiment is to measure the solubility of potassium nitrate as a function of temperature and use the data to calculate the equilibrium constant, enthalpy, entropy, and free energy of the dissolution reaction. Introduction When a salt dissolves in water it will dissociate into ions. In aqueous solution potassium nitrate (KNO 3 ) dissociates according to the following reaction. KNO 3(s) K + (aq) + NO 3- (aq) Once a sufficient amount of K + and NO 3- are in solution, the ions will recombine into solid potassium nitrate, KNO 3 . At this point a pair of ions will recombine for every pair that dissociates, and the concentrations of the ions in solution will remain constant. The reaction is said to be at equilibrium. The two-sided arrow indicates that the reaction is reversible and undissolved solid is in equilibrium with the dissolved ions. This is known as a saturated solution. The equilibrium constant expression, shown in equation (1), defines the equilibrium constant (K sp ) for KNO 3 dissolving in water. K sp = [K + ][NO 3- ] (1) The sp stands for the solubility product, and the brackets around the ions symbolize molar concentration (M or mol/L). The equilibrium constant is not only concerned with solubility but also is expressed as a product of the molar concentration of ions. The value for K sp for a compound changes with the temperature. Thermodynamics may be used to understand what energy changes occur when a salt dissolves in water. The energy difference between the solid salt and its dissolved ions is known as the enthalpy change ( H), and the relative disorder of the dissolved ions is an indication of the entropy change ( S). A positive enthalpy change will occur if heat must be added to dissolve the salt in water. The enthalpy change will be negative if the dissolution process releases heat. The entropy change for a solid salt solid dissolving in water should be positive because the dissolved ions possess more disorder than a solid salt molecule. The free energy change ( G) for a salt dissolving in water indicates whether the process will occur spontaneously. A negative free energy change implies that the salt will spontaneously dissolve in water while a positive free energy change indicates that the process is non-spontaneous. The Gibbs-Helmholtz equation, shown in equation (2), is a mathematical process is non-spontaneous....
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This note was uploaded on 05/09/2008 for the course CHEM 108 taught by Professor Fink during the Spring '08 term at Tulane.
- Spring '08