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  Home > JCE Print > Journal of Chemical Education > Issues > 2001  > November  >
In the Classroom
The Relative Acidities of Water and Methanol
Henry I. Abrash
Department of Chemistry, California State University, Northridge, CA 91330-8262

Cover
November 2001
Vol. 78 No. 11
p. 1496

Abstract
The experimental evidence for the relative acidities of water and methanol is reviewed. Because of solvent effects, a comparison of either autoprotolysis constants or dissociation constants measured in different media does not provide a reliable indication of these relative values. The most suitable measure of the relative acidities of water and methanol is the equilibrium constant for the proton transfer between water and methoxide ion (H2O + CH3O- OHO- + CH3OH) in various water-methanol mixtures. Experimental measurements of this thermodynamic equilibrium constant, in particular the contributions of Unmack, show considerable uncertainty owing to the difficulties in estimating activity coefficients, but they strongly indicate that methanol is about twice as acidic as water. This result shows that substitution of a methyl group for a hydrogen atom does not always destabilize a negative charge on a nearby oxygen atom. The question of whether to present acidities, particularly those of solvents, in terms of dissociation constants based on concentrations rather than activities is considered. In view of the slight consideration given to the relative acidities of water and alcohols in current organic chemistry tests and the discontinuity for students caused by use of concentration-based constants in organic chemistry only, thermodynamic constants remain the most suitable way to present acidities.
More Information
*  Citation
Abrash, Henry I. J. Chem. Educ. 2001 78 1496.
*  Keywords
Acid-Base Chemistry; Equilibrium; Organic Chemistry; Physical Chemistry; Textbooks
*  History
Created:
Last Updated:
October 8, 2001
August 31, 2005
  Home > JCE Print > Journal of Chemical Education > Issues > 2001 > November > Page 1496


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