11.8: The Scale of Acidity- pKa Values

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Many acid-base reactions take place in water, one of the most universal solvents. Water also has the dual capability of acting as a proton donor or as a proton acceptor. It makes sense, then, to develop a scale of acidity based on the behavior of the substance of interest towards water. Since most acid-base reactions are equilibrium processes, the equilibrium constant of the reaction between an acid (or base) and water forms the basis for the pK_a scale. Most general and organic chemistry textbooks contain adequate discussions of this parameter. What is of interest to us here is the relationship between pK_a and acidity. The equation

\(pK_{a} = -log K_{a} = log(\frac{1}{K_{a}})\)

shows that such relationship is inverse. The stronger the acid (i.e. the higher its acidity constant K_a), the lower its pK_a value, and viceversa. Tables of pK_a values usually show the acids and their conjugate bases arranged by order of decreasing (or increasing) acidity. Chemistry students should become proficient at reading and using data presented in such tables.