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  Home > JCE Print > Journal of Chemical Education > Issues > 1999  > February  >
In the Classroom
How Thermodynamic Data and Equilibrium Constants Changed When the Standard-State Pressure Became 1 Bar
Richard S. Treptow
Department of Chemistry and Physics, Chicago State University, Chicago, IL 60628
Cover
February 1999
Vol. 76 No. 2
p. 212
Abstract
In 1982 the IUPAC recommended that the pressure used to define the standard state of a substance be changed from 1 atm to 1 bar. The principal effect of the change is a slight increase in the entropy and a slight decrease in the free energy of any gas. Since gases are defined to behave ideally in their standard state, the changes can easily be expressed and calculated. The choice of standard-state pressure also affects the entropy change, free energy change, and thermodynamic equilibrium constant for any reaction involving a change in the number of moles of a gas. Practical equilibrium constants are equivalent for both pressures. The 1-atm standard-state pressure continues to be used in general chemistry courses. Its use may continue, but instructors and textbook authors must apply conversion formulas to any 1-bar data taken from the literature. The 1-bar pressure should be introduced in more advanced courses.
More Information
*  Citation
 Treptow, Richard S. J. Chem. Educ. 1999 76 212.
*  Keywords
 Thermodynamics; Physical Chemistry; Equilibrium; Gases; Chemical Information
*  History
 Created:
Last Updated:		 June 15, 1999
June 22, 2005
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