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Confusing Quantitative Descriptions of Brønsted Lowry AcidBase Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators
Author(s) -
Meister Erich C.,
Willeke Martin,
Angst Werner,
Togni Antonio,
Walde Peter
Publication year - 2014
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.201300321
Subject(s) - chemistry , equilibrium constant , oxonium ion , acid–base reaction , base (topology) , dimensionless quantity , chemical equilibrium , solvent , acid dissociation constant , hydroxide , value (mathematics) , thermodynamics , ion , aqueous solution , inorganic chemistry , organic chemistry , mathematical analysis , physics , mathematics , machine learning , computer science
In chemistry textbooks, the p K a,H 2 Ovalue of water in the solvent water at 25 °C is sometimes given as 14.0, sometimes as 15.7. This is confusing. The particular chemical reaction considered is the one in which water as Brønsted Lowry acid reacts with water as Brønsted Lowry base in water as solvent to yield equal concentrations of hydrated oxonium and hydroxide ions, H 3 O + (aq) and HO − (aq), respectively. This reaction is also known as the ‘self‐ionization’ of water for which the equilibrium constant is abbreviated as K w with its known value of 10 −14.0 at 25 °C, i.e. , p K w (25 °C)=14.0. Identical values for p K a,H 2 Oand p K w at a fixed temperature appear reasonable, since K a,H 2 Oand K w refer to one and the same reaction. Therefore, reasons for the apparent disagreement between the ‘thermodynamically correct’ p K a value for water (14.0 at 25 °C) and the value reported in most organic chemistry textbooks (15.7) should be discussed when teaching acidbase chemistry. There are good arguments for introducing, from the very beginning, the concepts of activity and thermodynamic standard states when teaching quantitative aspects of chemical equilibria. This also explains in a straightforward way why all thermodynamic equilibrium constants, including K w , are dimensionless, and why p K H 3 O +(25 °C)=0.