General chemistryLesson 3 of 7 in the course Acid-Base Equilibria

Acid–Base Strength, Ionization Equilibria, and Periodic Trends

5 flashcards · 5 quiz questions · 5 match pairs · 0 fill-in sentences · English

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Summary

For a weak acid, HAHA, the ionization reaction is

HA(aq)+H2O(l)⇌H3O+(aq)+A−(aq)HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)

Brackets indicate equilibrium molar concentrations. Liquid water is omitted from the expression because it is the solvent. A larger KaK_a means a greater proportion of acid has ionized and, for solutions of equal initial concentration, generally means more hydronium ions are produced. Thus, larger KaK_a indicates a stronger acid.

All flashcards

Acid-ionization constant, KaK_a
The equilibrium constant for acid ionization; a larger value indicates a stronger acid.
Base-ionization constant, KbK_b
The equilibrium constant for base ionization; a larger value indicates a stronger base.
Conjugate acid–base constants
For a conjugate pair, KaKb=KwK_aK_b=K_w at the same temperature.
Leveling effect
In water, strong acids and bases react to form hydronium and hydroxide ions.
Small-xx approximation
Approximates C−xC-x as CC when ionization is small relative to the initial concentration.

Quiz questions

  1. At 25 °C, how are the ionization constants of a conjugate acid–base pair related?Answer: KaKb=Kw=1.0×10−14K_aK_b=K_w=1.0\times10^{-14}
  2. According to water’s leveling effect, what is the strongest acid species that persists in aqueous solution?Answer: Hydronium ion, H3O+H_3O^+
  3. Why does binary acid strength generally increase down a periodic-table group?Answer: The H–A bond weakens, making proton release easier.
  4. For oxyacids with the same central element, why is the acid with the higher oxidation number generally stronger?Answer: The central atom draws electron density away, weakening the O–H bond.
  5. When should a weak-acid calculation generally retain the full C−xC-x term instead of using the small-xx approximation?Answer: When the estimated ionization exceeds about 5% of the initial concentration.

Based on Chemistry 2e by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.

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