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  Home > JCE Print > Journal of Chemical Education > Issues > 2003  > December  >
Research: Science and Education
The A to G of Chemical Equilibrium: Aspects Depicted by Helmholtz Energy Using Its Relationship to Gibbs Energy
Arthur E. Burgess
School of Contemporary Sciences, University of Abertay Dundee, Dundee DD1 1HG Scotland
Cover
December 2003
Vol. 80 No. 12
p. 1476
Abstract
The approach to chemical equilibrium at constant V, T is considered for various heterogeneous reactions and processes. Analysis is made showing how A and G conveniently interrelate enabling typical 2-D and 3-D Helmholtz energy curves to be drawn across a range of T to illustrate water vaporization and the thermal decomposition of calcium carbonate and ammonium chloride solids. A dip in free energy forms when gas component(s) change in pressure and amount as each reaction proceeds. Equilibrium is consequently attained by the opposing influence of these changes bringing the free energy to a minimum or the entropy to a corresponding maximum. Significantly this effect does not depend on a contribution from a free energy (or entropy) of mixing. The Helmholtz energy curves for the decomposition of ammonium chloride into ammonia and hydrogen chloride are obtained by using (equilibrium) vapor pressures for this two-gas formation and the effect of adding more of one of these gases is allowed for and depicted in comparison plots.
Supplement
A supplementary discussion available in this issue of JCE Online considers CO2 dissolution in water to see whether a similar approach allows Helmholtz energy charts to depict free energy minima for gas solutes sparingly dissolving in water at constant V, T.
*  Contents JCE2003p1476W.doc (Microsoft Word)
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*  Citation
 Burgess, Arthur E. J. Chem. Educ. 2003 80 1476.
*  Keywords
 Equilibrium; Physical Chemistry; Thermodynamics
*  History
 Created:
Last Updated:		 October 31, 2003
February 28, 2005
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