Private: Chapter Fourteen
Relative Strengths of Acids and Bases (14.3)
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Learning Objectives
By the end of this section, you will be able to:
- Assess the relative strengths of acids and bases according to their ionization constants
- Rationalize trends in acid–base strength in relation to molecular structure
- Carry out equilibrium calculations for weak acid–base systems
Acid and Base Ionization Constants
The relative strength of an acid or base is the extent to which it ionizes when dissolved in water. If the ionization reaction is essentially complete, the acid or base is termed strong; if relatively little ionization occurs, the acid or base is weak. As will be evident throughout the remainder of this chapter, there are many more weak acids and bases than strong ones. The most common strong acids and bases are listed in Figure 14.6.
Figure 14.6 Some of the common strong acids and bases are listed here.
The relative strengths of acids may be quantified by measuring their equilibrium constants in aqueous solutions. In solutions of the same concentration, stronger acids ionize to a greater extent, and so yield higher concentrations of hydronium ions than do weaker acids. The equilibrium constant for an acid is called the acid-ionization constant, Ka. For the reaction of an acid HA:
HA(aq) + H2O(l) ⇌ H3O+(aq) + A−(aq),
the acid ionization constant is written
Ka = [latex]\frac{[H_{3}O^{+}][A^{-}]}{[HA]}[/latex]
where the concentrations are those at equilibrium. Although water is a reactant in the reaction, it is the solvent as well, so we do not include [H2O] in the equation. The larger the Ka of an acid, the larger the concentration of H3 O+ and A− relative to the concentration of the nonionized acid, HA, in an equilibrium mixture, and the stronger the acid. An
acid is classified as “strong” when it undergoes complete ionization, in which case the concentration of HA is zero and the acid ionization constant is immeasurably large (Ka ≈ ∞). Acids that are partially ionized are called “weak,” and their acid ionization constants may be experimentally measured. A table of ionization constants for weak acids is provided in Appendix H.
To illustrate this idea, three acid ionization equations and Ka values are shown below. The ionization constants increase from first to last of the listed equations, indicating the relative acid strength increases in the order CH3CO2H < HNO2 < HSO4 − :
CH3 CO2 H(aq) + H2O(l) ⇌ H3O+(aq) + CH3CO2 −(aq) Ka = 1.8 × 10−5
HNO2(aq) + H2O(l) ⇌ H3O+(aq) + NO2 −(aq) Ka = 4.6 × 10−4
HSO4 −(aq) + H2O(aq) ⇌ H3O+(aq) + SO4 2−(aq) Ka = 1.2 × 10−2
Another measure of the strength of an acid is its percent ionization. The percent ionization of a weak acid is defined in terms of the composition of an equilibrium mixture:
% ionization = [latex]\frac{[H_{3}O^{+}]_{eq}}{[HA]_{0}} \times 100[/latex]
where the numerator is equivalent to the concentration of the acid’s conjugate base (per stoichiometry, [A−] = [H3O+]). Unlike the Ka value,