Acids and bases -- L 4 The Autoionization of Water

Autoionization of Water

  • Definition: Spontaneous ionization of water molecules that leads to the formation of hydronium (H₃O⁺) and hydroxide (OH⁻) ions.

  • Electrical Conductivity:

    • Pure water is a poor conductor of electricity due to a low concentration of ions.

    • Conductivity arises from the spontaneous ionization of water, forming charged particles (ions) that can carry electric current.

  • Equilibrium System:

    • Reaction: 2 H₂O ⇌ H₃O⁺ + OH⁻.

    • Equilibrium constant expression: Kc = [H₃O⁺][OH⁻]. This constant is designated as Kw.

Ionization Constant of Water (Kw)

  • Value at Standard Ambient Temperature and Pressure (SATP):

    • Kw = 1.00 x 10⁻¹⁴ at 25°C.

    • Indicates the product of the concentrations of hydronium and hydroxide ions in pure water.

  • Concentrations in Pure Water:

    • At neutral pH, [H₃O⁺] = [OH⁻] = 1.00 x 10⁻⁷ moles per liter.

Calculations Using Kw

  • Deriving Ion Concentrations:

    • For given concentrations, can calculate both [H₃O⁺] and [OH⁻] using Kw.

    • Example: If [H₃O⁺] = 1.3 x 10⁻² moles per liter, find [OH⁻] = Kw / [H₃O⁺] = 7.7 x 10⁻¹³ moles per liter.

Strong Bases and Acids

  • Sodium Hydroxide (NaOH):

    • Strong base, ionizes completely in solution.

    • Calculation example: Dissolving 2.50 g in 2.00 L yields an initial concentration of 0.03125 moles per liter.

    • Final [OH⁻] = Initial [NaOH] = 0.03125 moles per liter.

  • Complete Ionization of Strong Acids:

    • Strong acids like HCl also ionize completely, providing equivalent concentrations of H₃O⁺ ions.

pH Scale

  • Definition: Measure of the acidity or alkalinity of a solution based on the concentration of hydronium ions.

  • Formula: pH = -log[H₃O⁺].

  • Example Calculation:

    • [H₃O⁺] in pure water = 1.00 x 10⁻⁷ moles per liter leads to pH = 7.000.

  • Significant Figures: Reported pH values depend on significant digits as per the concentration's accuracy.

Acid Strength

  • Difference Between Strong and Weak Acids:

    • Strong acids: Fully dissociate (e.g., HCl); more than 99.9% ionization.

    • Weak acids: Partially dissociate (e.g., acetic acid); only 1.3% ionization.

  • Examples of Strong Acids:

    • Hydrochloric acid (HCl), nitric acid (HNO₃), sulfuric acid (H₂SO₄), etc.

  • Weak Acids: Include many other acids listed in data booklets, requiring Kc expressions for equilibrium calculations.

Percent Ionization Calculations

  • Calculating Percent Ionization:

    • For acetic acid with an initial concentration of 0.10 M and 21.3% dissociation:

      • [H₃O⁺] = 0.10 M * 0.213 = 0.00130 moles per liter.

      • pH = -log(0.00130) = 2.89.

    • Example for methanoic acid given pH and hydronium concentration measuring percent reaction.

Homework and Application

  • Expect questions that require calculating concentrations of hydronium and hydroxide ions, as well as determining pH and identifying solutions as acids, bases, or neutral.

  • Practice problems related to strong and weak acids to reinforce understanding of concepts.