Resumen de pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water?
Todd P. Silverstein, Stephen T. Heller
Since at least the 1960s, organic chemistry textbooks have featured pKa tables for organic acids that include values for H2O and H3O+ (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 (pKa = 15.54) is a stronger acid than water; from this, organic chemists have concluded that pKa 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, pKa(H2O) = 14.00.
