Relative Strengths of Acids and Bases Quiz

PDF page 783: 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.

PDF page 783: 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.

PDF page 783: 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

PDF page 784: 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) + H O(l) ⇌ H O (aq) + A (aq), 2 3 the acid ionization constant is written + − [H O ][A ] 3 Ka = [HA] 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 H O and 3 −

PDF page 784: 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) + H O(l) ⇌ H O (aq) + A (aq), 2 3 the acid ionization constant is written + − [H O ][A ] 3 Ka = [HA] 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 H O and 3 −

PDF page 784: 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, and a partial listing is given in Table 14.3. 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 < HSO : 4 −5 − + CH CO H(aq) + H O(l) ⇌ H O (aq) + CH CO (aq)

PDF page 784: 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, and a partial listing is given in Table 14.3. 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 < HSO : 4 −5 − + CH CO H(aq) + H O(l) ⇌ H O (aq) + CH CO (aq)

PDF page 784: 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, and a partial listing is given in Table 14.3. 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 < HSO : 4 −5 − + CH CO H(aq) + H O(l) ⇌ H O (aq) + CH CO (aq)

PDF page 784: where the numerator is equivalent to the concentration of the acid's conjugate base (per stoichiometry, [A ] = [H3O ]). Unlike the Ka value, the percent ionization of a weak acid varies with the initial concentration of acid, typically decreasing as concentration increases. Equilibrium calculations of the sort described later in this chapter can be used to confirm this behavior.

PDF page 784: where the numerator is equivalent to the concentration of the acid's conjugate base (per stoichiometry, [A ] = [H3O ]). Unlike the Ka value, the percent ionization of a weak acid varies with the initial concentration of acid, typically decreasing as concentration increases. Equilibrium calculations of the sort described later in this chapter can be used to confirm this behavior.

PDF page 785: Just as for acids, the relative strength of a base is reflected in the magnitude of its base-ionization constant (Kb) in aqueous solutions. In solutions of the same concentration, stronger bases ionize to a greater extent, and so yield higher hydroxide ion concentrations than do weaker bases. A stronger base has a larger ionization constant than does a weaker base. For the reaction of a base, B: + − B(aq) + H O(l) ⇌ HB (aq) + OH (aq), 2 the ionization constant is written as + − [HB ][OH ] K = b [B] Inspection of the data for three weak bases presented below shows the base strength increases in the order − −

PDF page 786: b b The inverse proportional relation between Ka and Kb means the stronger the acid or base, the weaker its conjugate partner. Figure 14.7 illustrates this relation for several conjugate acid-base pairs.

PDF page 788: strong acids in water but weak acids in ethanol (strength increasing HCl < HBr < HI). The right column of Figure 14.8 lists a number of substances in order of increasing base strength from top to bottom. Following the same logic as for the left column, species listed above water are weaker bases and so they don’t undergo base ionization when dissolved in water. Species listed between water and its conjugate base, hydroxide ion, are weak bases that partially ionize. Species listed below hydroxide ion are strong bases that completely ionize in water to yield hydroxide ions (i.e., they are leveled to hydroxide). A comparison of the acid and base columns in this table supports the reciprocal relation between the strengths of conjugate acid-base pairs. For example, the conjugate bases of the strong acids (top of table) are all of negligible strength. A strong acid exhibits an immeasurably large Ka, and so its conjugate base will exhibit a Kb that is essentially zero: strong acid :

PDF page 788: conjugate base : K = Kw /Ka = Kw /∞ ≈ 0

PDF page 787: solution of any strong acid is H3O (aq), meaning that hydronium ion is the strongest acid that may exist in water; any stronger acid will react completely with water to generate hydronium ions. This limit on the acid strength of solutes in a solution is called a leveling effect. To measure the differences in acid strength for “strong” acids, the acids must be dissolved in a solvent that is less basic than water. In such solvents, the acids will be “weak,” and so any differences in the extent of their ionization can be determined. For example, the binary hydrogen halides HCl, HBr, and HI are