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  Home > JCE Print > Journal of Chemical Education > Issues > 1998  > May  >
Research: Science and Education
Is the Reaction Equilibrium Composition in Non-ideal Mixtures Uniquely Determined by the Initial Composition?
Jan Sefcik
California Institute of Technology, Division of Chemistry and Chemical Engineering, 139-74 Beckman Institute, c/o Debbie Chester, Pasadena, CA 91125
Cover
May 1998
Vol. 75 No. 5
p. 587
Abstract
Reaction equilibrium can be mathematically described by the equilibrium equation and the reaction equilibrium composition can be calculated by solving this equation. It can be proved by non-elementary thermodynamic arguments that for a generic system with given initial composition, temperature and pressure there is a unique stable equilibrium state corresponding to the global minimum of the Gibbs free energy function. However, when the concept of equilibrium is introduced in undergraduate chemistry and chemical engineering courses, such arguments are generally not accessible. When there is a single reaction equilibrium among mixture components and the components form an ideal mixture, it has been demonstrated by a simple, elegant mathematical argument that there is a unique composition satisfying the equilibrium equation. It has been also suggested that this particular argument extends to non-ideal mixtures by simply incorporating activity coefficients. We show that the argument extension to non-ideal systems is not generally valid. Increasing non-ideality can result in non-monotonicity of the function crucial for the simple uniqueness argument, and only later it leads to non-uniqueness and hence phase separation. The main feature responsible for this is a composition dependence of activity coefficients in non-ideal mixtures.
More Information
*  Citation
 Sefcik, Jan. J. Chem. Educ. 1998 75 587.
*  Keywords
 Physical Chemistry, Chemical Engineering, Thermodynamics, Equilibrium, Teaching/Learning Theory/Practice
*  History
 Created:
Last Updated:		 June 23, 1999
June 24, 2005
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