pK[subscript a] Values in the Undergraduate Curriculum: What Is the Real pK[subscript a] of Water?

Autor/inn/en	Silverstein, Todd P.; Heller, Stephen T.
Titel	pK[subscript a] Values in the Undergraduate Curriculum: What Is the Real pK[subscript a] of Water?
Quelle	In: Journal of Chemical Education, 94 (2017) 6, S.690-695 (6 Seiten)Infoseite zur Zeitschrift PDF als Volltext Verfügbarkeit
Zusatzinformation	ORCID (Silverstein, Todd P.) ORCID (Heller, Stephen T.)
Sprache	englisch
Dokumenttyp	gedruckt; online; Zeitschriftenaufsatz
ISSN	0021-9584
DOI	10.1021/acs.jchemed.6b00623
Schlagwörter	Undergraduate Study; Organic Chemistry; Science Laboratories; Laboratory Experiments; Textbooks; Water; Misconceptions; Equations (Mathematics); Science Education History + Suchen Sie Ihr Suchwort? Grundstudium; Organische Chemie; Laboratory work; Laborarbeit; Textbook; Text book; Schulbuch; Lehrbuch; Wasser; Missverständnis; Equations; Mathematics; Gleichungslehre
Abstract	Since at least the 1960s, organic chemistry textbooks have featured pK[subscript a] tables for organic acids that include values for H[subscript 2]O and H[subscript 3]O[superscript +] (15.74 and -1.74, respectively) that are thermodynamically and chemically indefensible. Here we trace this error back to Brønsted's early contributions in the 1920s to the Brønsted-Lowry Theory of acids and bases. Organic chemists generally defend the use of these values by citing measurements of the equilibrium constant for the water + methoxide acid-base reaction that suggested that methanol (pK[subscript a] = 15.54) is a stronger acid than water; from this, organic chemists have concluded that pK[subscript a] of water must be 15.74 rather than 14.00. Here we discuss the problems that invalidate this conclusion, the most important being that it is based on the use of the pure liquid standard state (mole fraction = 1) that is quite different from the standard state for acidities determined in dilute solution (molality = 1). Using the latter standard state, the equilibrium constant for the water/methoxide reaction ranges from 4 to 70, showing water to be a "stronger" acid than methanol, and justifying the use of the thermodynamically correct value, pK[subscript a](H[subscript 2]O) = 14.00. (As Provided).
Anmerkungen	Division of Chemical Education, Inc and ACS Publications Division of the American Chemical Society. 1155 Sixteenth Street NW, Washington, DC 20036. Tel: 800-227-5558; Tel: 202-872-4600; e-mail: eic@jce.acs.org; Web site: http://pubs.acs.org/jchemeduc
Erfasst von	ERIC (Education Resources Information Center), Washington, DC
Update	2020/1/01