Confusing Quantitative Descriptions of BrønstedLowry AcidBase Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators. Issue 1 (January 2014)

• Record Type: Journal Article
• Authors: Meister, Erich C.; Willeke, Martin; Angst, Werner; Togni, Antonio; Walde, Peter
• Is Part Of: Helvetica chimica acta. Volume 97:Issue 1(2014:Jan.)
• Page Start: 1
• Page End: 31
• Publication Date: 2014-01
• Subjects: Chemistry -- Periodicals; Chemistry, Analytical -- periodicals; Chimie -- Périodiques; 540
• DOI: 10.1002/hlca.201300321
• Languages: English
• ISSNs: 0018-019X

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>In chemistry textbooks, the p<italic>K</italic><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgg40b37zmn" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math altimg="urn:x-wiley:0018019X:media:HLCA201300321:tex2gif-inf-1" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext> </mml:mtext><mml:mrow><mml:mtext>a, H</mml:mtext><mml:msub><mml:mtext> </mml:mtext><mml:mrow><mml:mtext>2</mml:mtext></mml:mrow></mml:msub><mml:mtext>O</mml:mtext></mml:mrow></mml:msub></mml:math></alternatives> value 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 <italic>Brønsted</italic><italic>Lowry</italic> acid reacts with water as <italic>Brønsted</italic><italic>Lowry</italic> base in water as solvent to yield equal concentrations of hydrated oxonium and hydroxide ions, H<sub>3</sub>O<sup>+</sup>(aq) and HO<sup>−</sup>(aq), respectively. This reaction is also known as the 'self‐ionization' of water for which the equilibrium constant is abbreviated as <italic>K</italic><sub>w</sub> with its known value of 10<sup>−14.0</sup> at 25 °C, <italic>i.e.</italic>, p<italic>K</italic><sub>w</sub>(25 °C)=14.0. Identical values for p<italic>K</italic><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgg40b37zdb" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math altimg="urn:x-wiley:0018019X:media:HLCA201300321:tex2gif-inf-6" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext> </mml:mtext><mml:mrow><mml:mtext>a, H</mml:mtext><mml:msub><mml:mtext> </mml:mtext><mml:mrow><mml:mtext>2</mml:mtext></mml:mrow></mml:msub><mml:mtext>O</mml:mtext></mml:mrow></mml:msub></mml:math></alternatives> and p<italic>K</italic><sub>w</sub> at a fixed temperature appear reasonable, since <italic>K</italic><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgg40b37zgf" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math altimg="urn:x-wiley:0018019X:media:HLCA201300321:tex2gif-inf-9" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext> </mml:mtext><mml:mrow><mml:mtext>a, H</mml:mtext><mml:msub><mml:mtext> </mml:mtext><mml:mrow><mml:mtext>2</mml:mtext></mml:mrow></mml:msub><mml:mtext>O</mml:mtext></mml:mrow></mml:msub></mml:math></alternatives> and <italic>K</italic><sub>w</sub> refer to one and the same reaction. Therefore, reasons for the apparent disagreement between the 'thermodynamically correct' p<italic>K</italic><sub>a</sub> value for water (14.0 at 25 °C) and the value reported in most organic chemistry textbooks (15.7) should be discussed when teaching acidbase 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 <italic>K</italic><sub>w</sub>, are dimensionless, and why p<italic>K</italic><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgg40b37zk3" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math altimg="urn:x-wiley:0018019X:media:HLCA201300321:tex2gif-inf-14" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext> </mml:mtext><mml:mrow><mml:mtext>H</mml:mtext><mml:msub><mml:mtext> </mml:mtext><mml:mrow><mml:mtext>3</mml:mtext></mml:mrow></mml:msub><mml:mtext>O</mml:mtext><mml:msup><mml:mtext> </mml:mtext><mml:mrow><mml:mtext>+</mml:mtext></mml:mrow></mml:msup></mml:mrow></mml:msub></mml:math></alternatives>(25 °C)=0.</p> </abstract>