Overview
This lecture covers acid-base equilibrium, reviewing core concepts including Bronsted-Lowry theory, strong and weak acids/bases, pH calculations, salt hydrolysis, and relevant equilibrium constants (Ka, Kb, Kw), as well as strategies for calculations.
Acid-Base Equilibrium Introduction
- Acid-base equilibrium involves reversible reactions where acids donate and bases accept protons (Bronsted-Lowry theory).
- Use ICE (Initial, Change, Equilibrium) tables to systematically solve equilibrium problems.
- Acid: proton donor; Base: proton acceptor.
- Upon ionization, acids form conjugate bases; bases form conjugate acids.
- Conjugate acid-base pairs differ by one H⁺.
Strong and Weak Acids/Bases
- Strong acids/bases ionize completely in water (e.g., HCl, NaOH).
- Weak acids/bases ionize partially (e.g., ethanoic acid, ammonia).
- The extent of ionization is characterized by the equilibrium constants Ka (acid) and Kb (base).
- The strength of an acid/base is unrelated to its concentration.
Calculating pH, pOH, and Related Concepts
- pH = –log₁₀[H⁺], pOH = –log₁₀[OH⁻]; [H⁺] = 10^–pH.
- Kw = [H⁺][OH⁻]; at 25°C, Kw = 1.0 × 10⁻¹⁴; pKw = 14.
- For dilute solutions (<10⁻⁷ M), consider water autoionization when calculating [H⁺].
- For strong acids/bases, [H⁺] or [OH⁻] equals the analytical concentration.
- For weak acids/bases, set up an ICE table; assume degree of ionization is small for simplifying calculations.
- α (degree of dissociation) = amount ionized / initial amount.
Acid/Base Strength & Equilibria
- Higher Ka (lower pKa) → stronger acid; higher Kb (lower pKb) → stronger base.
- For conjugate pairs: Ka × Kb = Kw; pKa + pKb = 14 (at 25°C).
- Diluting weak acids increases their degree of dissociation; Ka remains constant.
Salt Hydrolysis
- Salts from strong acid + strong base = neutral solution (no hydrolysis).
- Salts from weak acid + strong base = basic solution (anion hydrolyzes to form OH⁻).
- Salts from strong acid + weak base = acidic solution (cation hydrolyzes to form H₃O⁺).
- For salts from weak acid + weak base, compare Ka (cation) and Kb (anion) to predict pH.
- Cations with high charge densities (e.g., Al³⁺, Fe³⁺) acidify solution by hydrolyzing water.
Key Terms & Definitions
- Bronsted-Lowry Acid — a substance that donates a proton (H⁺).
- Bronsted-Lowry Base — a substance that accepts a proton (H⁺).
- Conjugate Acid/Base Pair — species differing by a single proton.
- Ka — acid dissociation constant, measures acid strength.
- Kb — base dissociation constant, measures base strength.
- Kw — ionic product of water ([H⁺][OH⁻]), temperature dependent.
- pH — negative log of hydrogen ion concentration.
- ICE Table — tool for tracking concentrations in equilibrium calculations.
- Degree of Dissociation (α) — fraction of substance ionized.
- Salt Hydrolysis — reaction of salt ions with water to affect pH.
Action Items / Next Steps
- Practice setting up and solving ICE tables for weak acid/base equilibria.
- Review and solve exercises on pH and salt hydrolysis calculations.
- Memorize common strong and weak acids/bases and their typical behavior.
- Prepare for section four: buffer solutions.