Notes on the Strength of Acids and Bases

  • Strength of Acids and Bases

    • Acid Strength:

    • Strong acids: Dissociate 100% in water (e.g., HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3).

    • Weak acids: Do not fully dissociate, retaining H+. Example: Acetic acid (HC2H3O2).

  • Dissociation of Acids

    • Strong Acid (HA):

    • 100% dissociation of HA in solution: HA→H++A−HA \rightarrow H^+ + A^-

      • Conductive solution due to high ion concentration.

    • Weak Acid (HA):

    • Partial dissociation: HA⇌H++A−HA \rightleftharpoons H^+ + A^-

      • Low conductivity since a small fraction dissociates.

  • Strength of Bases

    • Strong Bases:

    • Alkali metal hydroxides and some alkaline earth metal hydroxides fully dissociate (e.g., NaOH, Ca(OH)2).

    • Weak Bases:

    • Not fully dissociated; example: Ammonia (NH3).

  • Conductivity of Solutions

    • Strong Acid vs. Weak Acid:

    • 2M HCl (strong, very conductive) vs. 2M HC2H3O2 (weak, poor conduction).

  • Conjugate Acid/Base Pairs

    • Strong acid has weak conjugate base.

    • Strong base has weak conjugate acid.

  • pH and Salt Hydrolysis

    • Salts can produce neutral, acidic, or basic solutions based on the parent acid/base.

    • Examples:

    • Strong Acid + Strong Base = Neutral.

    • Strong Acid + Weak Base = Acidic.

    • Weak Acid + Strong Base = Basic.

  • Acid and Base Dissociation Constants

    • Ka and Kb define strength of weak acids and bases, respectively.

    • Larger values indicate stronger acids/bases.

  • Calculating Ka and Kb

    • For weak acid: HA+H2O⇌H3O++A−HA + H2O \rightleftharpoons H3O^+ + A^-

    • Ka = \frac{[H3O^+][A^-]}{[HA]}$

    • For weak base: NH3+H2O⇌NH4++OH−NH3 + H2O \rightleftharpoons NH4^+ + OH^-

    • Kb = \frac{[NH4^+][OH^-]}{[NH3]}$

  • Equilibrium Constants (Ka)

    • Weak acids reach equilibrium with low ionization.

    • A stronger acid has a larger Ka (easier proton donation).

  • Calculating pH and Ka from Concentration

    • Use [H^+] = 10^{-pH} to find concentration and set up ICE charts (Initial, Change, Equilibrium).

    • Example:

    • Weak acid at 0.15 M gives pH = 2.96

    • Calculate % Ionization: [H+][HAinitial]×100%\frac{[H^+]}{[HA_{initial}]} \times 100\%

  • Buffers and Weak Acid Calculation

    • Example for another weak acid: H2C2O4 with a pH of 1.28 to find Ka:

    • Set up dissociation table and solve using ICE chart.

  • pH and Kb for Weak Bases

    • Calculate Kb similarly using pOH and hydroxide concentration (OH-).

    • Example with NH3 and CN- to find Kb values.

  • Final Calculations for pH and K

    • Comprehensive examples demonstrating how to calculate pH, concentration, and K values for both acids and bases using log and dissociation principles.