Acids and bases -- L 7 Acid Base Equilibrium Constant Expressions

Equilibrium Constants for Acidic and Basic Systems

  • Acidic systems use Ka, defined by the expression Kc.

  • Basic systems use Kb, also defined by Kc.

  • Integration of concepts of pH and pOH, as well as the Kw (ion product of water).

Definition of Ka

  • Ka Expression: Derived from the equilibrium chemical reaction.

    • For hydrogen chloride (HCl):

      • Strong acid; predominant reaction goes nearly to completion, resulting in large Ka value.

    • Examples:

      • HCl -> H+ + Cl-

      • Ka = [H+][Cl-]/[HCl] (water is omitted as a liquid).

  • For acetic acid (weak acid):

    • Ka Expression:

      • Acetic acid ionizes minimally (1.3%); Ka values are smaller.

      • Example reaction: CH3COOH ⇌ H+ + CH3COO-

    • Ka = [H+][CH3COO-]/[CH3COOH] (water is omitted).

Types of Questions in Acid-Base Chemistry

  1. Determining Ka from Concentration of Hydronium:

    • Example: Carbonic acid with [H+] calculated from pH = 3.48.

    • Reaction: H2CO3 + H2O ⇌ H3O+ + HCO3-

    • Use ICE table for changes in concentrations and plug into Ka expression.

    • Find Ka: 4.4 × 10⁻⁷.

  2. Determining Concentration of Hydronium from Ka:

    • Given [HCOOH] = 0.200 M, Ka = 1.8 × 10⁻⁴.

    • Set up ICE table to find concentrations and plug into Ka expression; solved to find [H+] = x = 6.0 × 10⁻³ M and pH = 2.22.

    • Confirm results with ICE table mechanics confirming stoichiometry.

Understanding Kb and Strong Bases

  • Kb: Base ionization constant, similar to Ka for acids.

  • Strong bases (e.g., NaOH) dissociate fully in water.

    • Example: NaOH → Na+ + OH-

    • Kb expression follows the same rules as Ka.

Relationship Between Ka and Kb

  • Mathematical Relationship:

    • For any conjugate acid-base pair:

      • Ka × Kb = Kw (1.0 × 10⁻¹⁴ at 25°C).

    • Example: Ammonia (NH3) and ammonium ion (NH4+).

Example of Using Ka and Kb Relationship

  • Determine Kb for weak base (sodium benzoate): Ka of benzoic acid = 6.3 × 10⁻⁵.

    • Calculate Kb using Kw = Ka × Kb.

    • Kb = 1.6 × 10⁻¹⁰.

Final Example: Finding Kb from Hydroxide Concentration

  • Given pH of NH3 solution:

    • pH = 11.32

    • Calculate [H+] = 10⁻¹¹.32 using hydronium concentration relations.

    • Determine [OH-] from Kw and find Kb using ICE table.

    • Final calculated Kb values are consistent: 1.7 × 10⁻⁵, 1.8 × 10⁻⁵ (both acceptable).

Summary and Recommendations

  • Acknowledge different types of questions and approaches in acid-base equilibrium.

  • Emphasis on practice questions for comprehensive understanding.

  • Preview of the next lecture on pH and buffer systems.