Summary of "Chapter 8.3 - Dissociation of Water"

Summary of Chapter 8.3 - dissociation of water

This section focuses on understanding acids and bases quantitatively by introducing the pH scale and the dissociation behavior of water. It explains how water acts as an amphoteric substance (both acid and base) and how the equilibrium between hydronium ions (H₃O⁺) and hydroxide ions (OH⁻) defines the acidity or basicity of a solution.

Main Ideas and Concepts

amphoteric nature of water: Water can act as both an acid (proton donor) and a base (proton acceptor). When two water molecules interact, one donates a proton (H⁺) and the other accepts it, forming hydronium (H₃O⁺) and hydroxide (OH⁻) ions.
dissociation of water: The reaction can be represented as: 2 H₂O ⇌ H₃O⁺ + OH⁻ This equilibrium strongly favors the reactants (water molecules), so only a small amount dissociates.
Concentrations in Pure Water: At 25°C, the concentration of both H₃O⁺ and OH⁻ in pure water is 1.0 × 10⁻⁷ M. These concentrations are equal because every proton lost by one water molecule is gained by another.
ion product constant of water (Kᵥ or Kw): The product of the concentrations of hydronium and hydroxide ions is constant in any aqueous solution: Kw = [H3O+][OH-] = 1.0 × 10-14 at 25°C This relationship holds true for all aqueous solutions, not just pure water.
Relationship Between H₃O⁺ and OH⁻ Concentrations:
- If you know the concentration of one ion, you can calculate the other using: [OH-] = Kw / [H3O+] or [H3O+] = Kw / [OH-]
- Increasing the concentration of H₃O⁺ (acidic condition) decreases OH⁻ concentration, and vice versa.
Determining Acidity or Basicity:
- If [H3O+] > [OH-], the solution is acidic.
- If [OH-] > [H3O+], the solution is basic.
- If [H3O+] = [OH-], the solution is neutral (pure water).

Methodology / Instructions for Calculations

Calculate hydroxide ion concentration given hydronium ion concentration: [OH-] = 1.0 × 10-14 / [H3O+]
Calculate hydronium ion concentration given hydroxide ion concentration: [H3O+] = 1.0 × 10-14 / [OH-]
Determine whether a solution is acidic, basic, or neutral: - Compare [H3O+] and [OH-]. - Larger concentration determines acidity/basicity.
Example problem: Given [H3O+] = 1.0 × 10-5 M (coffee), calculate [OH-]: [OH-] = 1.0 × 10-14 / 1.0 × 10-5 = 1.0 × 10-9 M Since [H3O+] > [OH-], the solution is acidic.
Calculator tips for scientific notation:
- Use the "EEE" or "EXP" button to enter numbers like 1 × 10⁻¹⁴ as 1 EEE -14.
- Alternatively, use parentheses: (1 × 10^-14) / (1 × 10^-5).
- Check that exponents add up correctly (usually to -14 or -15) as a verification step.

Key Takeaways

Water self-ionizes slightly to produce equal concentrations of hydronium and hydroxide ions.
The ion product constant Kw is fundamental to understanding acid-base equilibria in aqueous solutions.
The pH