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Factors That Influence Relative Acid Strength in Water: A Simple Model

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Michael J. Moran

Department of Chemistry, West Chester University of Pennsylvania, West Chester, PA 19383

J. Chem. Educ., 2006, 83 (5), p 800

DOI: 10.1021/ed083p800

Publication Date (Web): May 1, 2006

Section:

History, Education, and Documentation

Abstract

The pK_a's of diverse aqueous acids H–A correlate well with the sum of two gas-phase properties: the H–A bond-dissociation enthalpy (BDE) and the electron affinity (EA) of the A^• radical. Acidity correlates poorly with each of these separately. It is suggested that rather than bond strength alone or bond polarity, the sum of the enthalpies of these two steps is a fairly good indicator of relative acidity, especially when comparison is made among acids of a given type. Electronegative groups attached elsewhere on the molecule have a greater effect on the acidity not in the weakening or polarization of the H–A bond, but rather in enhancing the electron affinity of the radical, or put another way, stabilizing the conjugate base.

Keywords (Audience):

First-Year Undergraduate / General

Keywords (Domain):

Inorganic Chemistry

Keywords (Feature):

Research: Science and Education

Keywords (Pedagogy):

Misconceptions / Discrepant Events

Keywords (Subject):

Acids / Bases

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This article has been cited by 2 ACS Journal articles (2 most recent appear below).

Estimation of Standard Reduction Potentials of Halogen Atoms and Alkyl Halides
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The Journal of Physical Chemistry B2011 115 (4), 678-684
- Estimation of Standard Reduction Potentials of Halogen Atoms and Alkyl Halides
  Abdirisak A. Isse, Ching Yeh Lin, Michelle L. Coote, and Armando Gennaro
  The Journal of Physical Chemistry B2011 115 (4), 678-684
  Standard reduction potentials, SRPs, of the halogen atoms have been calculated in water on the basis of an appropriate thermochemical cycle. Using the best up-to-date thermodynamic data available in the literature, we have calculated EX•/X−o̵ values of ...
  Abstract | Full Text HTML | Hi-Res PDF | PDF w/ Links
Acidities of Water and Methanol in Aqueous Solution and DMSO
Daqing Gao
Journal of Chemical Education2009 86 (7), 864
- Acidities of Water and Methanol in Aqueous Solution and DMSO
  Daqing Gao
  Journal of Chemical Education2009 86 (7), 864
  The relative acidities of water and methanol have been a nagging issue. In gas phase, methanol is more acidic than water by 36.0 kJ/mol; however, in aqueous solution, the acidities of methanol and water are almost identical. The acidity of an acid in ...
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