Relative Strengths of Acids and Bases

Acid-Base Strength

  • Strength is the extent of acid or base ionization in water.

    • Strong acids/bases ionize nearly completely.

    • Weak acids/bases ionize minimally (most acids/bases).

Ionization Constant

  • Quantifies the relative strength of an acid (K_a).

    • Model reaction: HA(aq)+H<em>2O(l)⇌H</em>3O+(aq)+A−(aq)HA(aq) + H<em>2O(l) \rightleftharpoons H</em>3O^+(aq) + A^-(aq)

    • K<em>a=[H</em>3O+][A−][HA]K<em>a = \frac{[H</em>3O^+][A^-]}{[HA]}

    • Large K_a: high ionization, strong acid.

    • Small K_a: low ionization, weak acid.

    • K_a approaches infinity for strong acids.

    • K_a values for weak acids are experimentally determined and tabulated (see Appendix H).

Base Ionization Constant

  • Quantifies the relative strength of a base (K_b).

    • Model reaction: B(aq)+H2O(l)⇌HB+(aq)+OH−(aq)B(aq) + H_2O(l) \rightleftharpoons HB^+(aq) + OH^-(aq)

    • Kb=[HB+][OH−][B]K_b = \frac{[HB^+][OH^-]}{[B]}

    • Stronger base = larger K_b.

    • Ionization constants for some weak bases appear in Appendix I.

Percent Ionization

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  • ([H<em>3O+]or[OH−][HA]</em>initialor[B]initial)∗100(\frac{[H<em>3O^+] or [OH^-]}{[HA]</em>initial or [B]_initial}) * 100

    • Less useful than Ka/Kb because it varies with initial concentration.

    • Decreases as concentration increases (Le Chatelier's principle).

Conjugate Acid-Base Pairs

  • K<em>a∗K</em>b=KwK<em>a * K</em>b = K_w

    • K<em>a=K</em>wKbK<em>a = \frac{K</em>w}{K_b}

    • K<em>b=K</em>wKaK<em>b = \frac{K</em>w}{K_a}

    • Stronger acid/base = weaker conjugate partner.

Leveling Effect

  • Acids stronger than H<em>3O+H<em>3O^+ completely ionize in water, producing only H</em>3O+H</em>3O^+.

  • To measure relative strength of strong acids, use less basic solvents (e.g., ethanol).

    • Same logic applies to strong bases (use less acidic solvents).

Binary Acids

  • Strength increases down and to the right on the periodic table (inversely related to electronegativity).

    • Weaker bond between H and atom = stronger acid.

Ternary Acids and Bases

  • Structure: Atom bound to oxygen and then hydrogen (E-O-H).

    • Central atom (E) with low electronegativity: releases OH−OH^-, behaving as a base.

    • Central atom (E) with high electronegativity: releases H+H^+, behaving as an acid.

Oxyacids

  • Non-metal bound to oxygen; high electronegativity = acids.

  • Increasing the oxidation number of the central atom increases acidity of oxyacid.

Amphoteric compounds

  • Elements with intermediate electronegativity (metalloids) can behave as both acids and bases.