Overview
This lecture covers the autoionization of water, the concepts and formulas for pH and pOH, and their mathematical relationship, as well as the classification of aqueous solutions as acidic, neutral, or basic.
Autoionization of Water
- Autoionization is the self-ionization of water without adding acids or bases.
- It occurs in very small (partial) amounts, called partial dissociation.
- The reaction: 2 H₂O ⇌ H₃O⁺ + OH⁻ (or H₂O ⇌ H⁺ + OH⁻).
- The dissociation constant (K) is defined as: K = [H₃O⁺][OH⁻] / [H₂O]².
- The concentration of water [H₂O] is considered constant and is combined with K to give the ionic product Kw.
- Kw = [H₃O⁺][OH⁻] and is called the ionic product of water.
- At 25°C, Kw = 1 × 10⁻¹⁴ mol²/L².
pH and pOH
- pH indicates how acidic or basic a solution is.
- pH formula: pH = -log₁₀[H⁺].
- pH is defined as the negative logarithm (base 10) of hydrogen ion (H⁺) concentration.
- pOH formula: pOH = -log₁₀[OH⁻].
- pOH is the negative logarithm (base 10) of hydroxyl ion (OH⁻) concentration.
Relationship Between pH and pOH
- Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ at 25°C.
- Taking log: log₁₀([H⁺][OH⁻]) = log₁₀(1 × 10⁻¹⁴).
- This leads to: pH + pOH = 14.
Types of Aqueous Solutions
- Acidic solution: pH < 7, [H⁺] > [OH⁻].
- Neutral solution: pH = 7, [H⁺] = [OH⁻].
- Basic (alkaline) solution: pH > 7, [H⁺] < [OH⁻].
Key Terms & Definitions
- Autoionization of Water — The process by which water molecules dissociate into ions without external input.
- Partial Dissociation — Incomplete splitting of molecules into ions.
- Dissociation Constant (K) — Ratio of product concentrations to reactant concentrations at equilibrium.
- Ionic Product of Water (Kw) — The product [H⁺][OH⁻] in pure water at equilibrium.
- pH — Negative logarithm (base 10) of hydrogen ion concentration, indicates acidity.
- pOH — Negative logarithm (base 10) of hydroxide ion concentration, indicates basicity.
Action Items / Next Steps
- Write the derived formulas and definitions in your notebook.
- Practice numerical problems using the pH, pOH, and Kw formulas.
- Review the previous lectures on ion equilibrium if not already done.
- Prepare for the next class on aqueous solutions.