1.5.2: Relative Strengths of Acids and Bases


Learning Objectives

• Assess the relative strengths of acids and bases according to their ionization constants

We can rank the strengths of acids by the extent to which they ionize in aqueous solution. The reaction of an acid with water is given by the general expression:

$\ce{HA}(aq)+\ce{H2O}(l)⇌\ce{H3O+}(aq)+\ce{A-}(aq)$

Water is the base that reacts with the acid $$\ce{HA}$$, $$\ce{A^{−}}$$ is the conjugate base of the acid $$\ce{HA}$$, and the hydronium ion is the conjugate acid of water. A strong acid yields 100% (or very nearly so) of $$\ce{H3O+}$$ and $$\ce{A^{−}}$$ when the acid ionizes in water; Figure $$\PageIndex{1}$$ lists several strong acids. A weak acid gives small amounts of $$\ce{H3O+}$$ and $$\ce{A^{−}}$$.

Figure $$\PageIndex{1}$$: Some of the common strong acids and bases are listed here.
Six Strong Acids Six Strong Bases
$$\ce{HClO4}$$ perchloric acid $$\ce{LiOH}$$ lithium hydroxide
$$\ce{HCl}$$ hydrochloric acid $$\ce{NaOH}$$ sodium hydroxide
$$\ce{HBr}$$ hydrobromic acid $$\ce{KOH}$$ potassium hydroxide
$$\ce{HI}$$ hydroiodic acid $$\ce{Ca(OH)2}$$ calcium hydroxide
$$\ce{HNO3}$$ nitric acid $$\ce{Sr(OH)2}$$ strontium hydroxide
$$\ce{H2SO4}$$ sulfuric acid $$\ce{Ba(OH)2}$$ barium hydroxide

The relative strengths of acids may be determined 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.

We can rank the strengths of bases by their tendency to form hydroxide ions in aqueous solution. The reaction of a base with water is given by:

$\ce{B}(aq)+\ce{H2O}(l)⇌\ce{HB+}(aq)+\ce{OH-}(aq)$

Water is the acid that reacts with the base, $$\ce{HB^{+}}$$ is the conjugate acid of the base $$\ce{B}$$, and the hydroxide ion is the conjugate base of water. A strong base yields 100% (or very nearly so) of OH and HB+ when it reacts with water; Figure $$\PageIndex{1}$$ lists several strong bases. A weak base yields a small proportion of hydroxide ions. Soluble ionic hydroxides such as NaOH are considered strong bases because they dissociate completely when dissolved in water.

The Ionization of Weak Acids and Weak Bases

Many acids and bases are weak; that is, they do not ionize fully in aqueous solution. A solution of a weak acid in water is a mixture of the nonionized acid, hydronium ion, and the conjugate base of the acid, with the nonionized acid present in the greatest concentration. Thus, a weak acid increases the hydronium ion concentration in an aqueous solution (but not as much as the same amount of a strong acid).

Acetic acid ($$\ce{CH3CO2H}$$) is a weak acid. When we add acetic acid to water, it ionizes to a small extent according to the equation:

$\ce{CH3CO2H}(aq)+\ce{H2O}(l)⇌\ce{H3O+}(aq)+\ce{CH3CO2-}(aq)$

giving an equilibrium mixture with most of the acid present in the nonionized (molecular) form. This equilibrium, like other equilibria, is dynamic; acetic acid molecules donate hydrogen ions to water molecules and form hydronium ions and acetate ions at the same rate that hydronium ions donate hydrogen ions to acetate ions to reform acetic acid molecules and water molecules. We can tell by measuring the pH of an aqueous solution of known concentration that only a fraction of the weak acid is ionized at any moment (Figure $$\PageIndex{4}$$). The remaining weak acid is present in the nonionized form.

At equilibrium, a solution of a weak base in water is a mixture of the nonionized base, the conjugate acid of the weak base, and hydroxide ion with the nonionized base present in the greatest concentration. Thus, a weak base increases the hydroxide ion concentration in an aqueous solution (but not as much as the same amount of a strong base).

For example, a solution of the weak base trimethylamine, (CH3)3N, in water reacts according to the equation:

$\ce{(CH3)3N}(aq)+\ce{H2O}(l)⇌\ce{(CH3)3NH+}(aq)+\ce{OH-}(aq)$

This gives an equilibrium mixture with most of the base present as the nonionized amine. This equilibrium is analogous to that described for weak acids.

We can confirm by measuring the pH of an aqueous solution of a weak base of known concentration that only a fraction of the base reacts with water (Figure 14.4.5). The remaining weak base is present as the unreacted form.

Example $$\PageIndex{6}$$: Formic Acid in Ant stings

Formic acid, HCO2H, is the irritant that causes the body’s reaction to ant stings.

Summary

The strengths of acids and bases in aqueous solutions can be determined by their acid or base ionization constants. Strong acids are completely ionized in aqueous solution because their conjugate bases are weaker bases than water. Weak acids are only partially ionized because their conjugate bases are strong enough to compete successfully with water for possession of protons. Strong bases react with water to quantitatively form hydroxide ions. Weak bases give only small amounts of hydroxide ion.

Glossary

percent ionization
ratio of the concentration of the ionized acid to the initial acid concentration, times 100

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