Chemistry » Acid-Base Equilibria » Relative Strengths of Acids and Bases

Summarizing Relative Strengths of Acids and Bases

Key Concepts and Summary

The strengths of Brønsted-Lowry acids and bases in aqueous solutions can be determined by their acid or base ionization constants. Stronger acids form weaker conjugate bases, and weaker acids form stronger conjugate bases. Thus 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. The strengths of the binary acids increase from left to right across a period of the periodic table (CH4 < NH3 < H2O < HF), and they increase down a group (HF < HCl < HBr < HI).

The strengths of oxyacids that contain the same central element increase as the oxidation number of the element increases (H2SO3 < H2SO4). The strengths of oxyacids also increase as the electronegativity of the central element increases [H2SeO4 < H2SO4].

Key Equations

  • \({K}_{\text{a}}=\phantom{\rule{0.2em}{0ex}}\frac{\left[{\text{H}}_{3}{\text{O}}^{\text{+}}\right]\left[{\text{A}}^{\text{−}}\right]}{\left[\text{HA}\right]}\)
  • \({K}_{\text{b}}=\phantom{\rule{0.2em}{0ex}}\frac{\left[{\text{HB}}^{\text{+}}\right]\left[{\text{OH}}^{\text{−}}\right]}{\left[\text{B}\right]}\)
  • Ka\(×\)Kb = 1.0 \(×\) 10−14 = Kw
  • \(\text{Percent ionization} = \frac{[\text{H}_3\text{O}^{+}]_{\text{eq}}}{[\text{HA}]_{0}}\;\times\;100\)


acid ionization constant (Ka)

equilibrium constant for the ionization of a weak acid

base ionization constant (Kb)

equilibrium constant for the ionization of a weak base

leveling effect of water

any acid stronger than \({\text{H}}_{3}{\text{O}}^{\text{+}},\) or any base stronger than OH will react with water to form \({\text{H}}_{3}{\text{O}}^{\text{+}},\) or OH, respectively; water acts as a base to make all strong acids appear equally strong, and it acts as an acid to make all strong bases appear equally strong


compound containing a nonmetal and one or more hydroxyl groups

percent ionization

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

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