Confusing Quantitative Descriptions of BrønstedLowry AcidBase Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators

In chemistry textbooks, the p K 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 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 and p K w at a fixed temperature appear reasonable, since K and 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 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 K w , are dimensionless, and why p K (25 °C)=0.