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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 10 Issue 3 (2005) pp 181-185

Understanding the Thermodynamics of Equilibria across Semipermeable Membranes

João Paulo M. Ferreira

Escola Superior de Biotecnologia—Universidade Católica Portuguesa, Rua Dr. António Bernardino de Almeida, 4200-072 Porto, Portugal,
Received April 1, 2005. Accepted May 2, 2005.

Published online: 24 May 2005

Abstract. Osmotic equilibrium is ubiquitous in biological systems and it also underlies a number of technological processes, such as reverse osmosis. Quite often, students know how to calculate osmotic pressures in ideal diluted solutions, but they do not have a deep understanding of the driving forces for transport and how these relate to thermodynamic variables. This article derives, in a simple and straightforward way, the conditions for equilibria through selective membranes, starting with ideal gas mixtures. Changes with physical conditions of several thermodynamic properties are evaluated. Then, the analysis is extended to ideal liquid mixtures, pointing out the similarities and differences with the previous situation. An expression for the reversible work associated with a change in concentration is derived, and it is applied to the process of desalination of seawater.

Key Words: In the Classroom; physical chemistry; thermodynamics

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Issue date: June 1, 2005

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