Brønsted called the second constant the “ rational” acidity constant (which we distinguish as K* a,H2O), in which he included a molar concentration of water. 5 One value was the “ conventional”, thermodynamically-sound equilibrium constant, in which the activity of the water is used, and assigned its correct value of unity (1) because it is the solvent in a dilute solution. The origin of the value of 15.7 for the pK a of water can be tracked back at least as far as 1928, when Johannes Brønsted postulated that there could be two different values for the p K a of water. 3,4 This value can also be obtained by the same process used to calculate the pK a of all other water-soluble compounds that can act as acids in aqueous solution: from the analysis of thermodynamic or electrochemical data for these aqueous solutions. The value of 14.0 for the pK w of water at 25✬ has been experimentally measured. Two Values for the Same Equilibrium Constant? Much of the information in this module comes from two review papers with more comprehensive coverage of the topic. This module describes the derivation of these two values and describes why the value of 15.7 should not be used. In many organic chemistry textbooks and some biochemistry texts, however, the pKa of water at 25✬ is listed as 15.7. In most general chemistry textbooks, the pK a of water at 25 ✬ is listed as 14.0. For Further Consideration: Dissociation Constant of Methanol.Note: Regarding the Erroneous claim that Ka = Keq.
0 kommentar(er)
