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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 12 Issue 1 (2007) pp 18-21

Determining the Enthalpy, Free Energy, and Entropy for the Solubility of Salicylic Acid with the van’t Hoff Equation: A Spectrophotometric Determination of Keq

Jose C. Barreto*, Terry Dubetz, David W. Brown, Patricia D. Barreto, Cristina M. Coates, and Allison Cobb

Department of Chemistry and Mathematics and the Whitaker Center for Science, Mathematics and Technology Education, Florida Gulf Coast University, Fort Meyers, FL 33965,
Received August 19, 2005. Accepted November 3, 2006.

Published online: 1 February 2007

Abstract. The solubility of a substance can be defined as the equilibrium constant (Ks) between the crystalline and dissolved forms of a solute in a saturated solution. In this experiment, we have experimentally determined the equilibrium constant Ks, at different temperatures for the solvation of a saturated solution of salicylic acid in water. A Van’t Hoff plot of ln Ks versus 1/T reveals the enthalpy (DH°), and entropy, (DS°) of the reaction. The free energy (DG°) of the process can then be obtained from DG° = DH°TDS°. We suggest that the students gather the data by working collaboratively as a class, sharing the data by writing Ks and T on the board during the experiment. Our experimental design lends itself to rapid data collection and fosters a cooperative team approach in the laboratory, followed by immediate data sharing. A pattern quickly develops during the data-gathering phase and any aberrant data points can be discussed and dealt with by replication. There is minimal heat transfer during the solvation of salicylic acid; indeed, the joules of heat transferred are so small that the experiment would be very difficult to perform calorimetrically (an important lesson for the students to grasp). The experiment is rapid, simple to understand and safe. It makes a superior capstone laboratory at the end of General Chemistry II because it integrates several concepts in equilibrium and thermodynamics.

Key Words: Laboratories and Demonstrations; general chemistry; equilibrium; van’t Hoff equation; spectrophotometry; colorimetry; Keq; solvation; solubility; saturation; enthalpy; entropy; free energy; calorimetry; recrystallization; precipitation; Kelvin; centigrade; thermometer; temperature

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Issue date: February 1, 2007

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