Overview
This lecture covers the fundamentals of acid-base chemistry, focusing on proton transfer reactions, acid-base theories, conjugate pairs, and the auto-ionization of water.
Importance of Acid-Base Chemistry
- Acid-base chemistry is essential for understanding biochemistry, aquatic systems, and air chemistry.
- Acid-base reactions involve the transfer of a hydrogen ion (proton) between species.
Acid-Base Theories
- The Arrhenius theory defines acids as substances releasing hydrogen ions and bases as releasing hydroxide ions in water.
- The Bronsted-Lowry theory defines acids as proton donors and bases as proton acceptors, broadening the acid-base definition.
Examples of Proton Transfer Reactions
- Hydrochloric acid (HCl) donates a proton to water, producing hydronium ions (H₃O⁺) and increasing system acidity.
- Acetic acid (ethanoic acid) donates a proton to water; water acts as the base in both cases.
Conjugate Acid-Base Pairs
- A conjugate acid-base pair differs by one proton.
- When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.
- Example: HCl/Cl⁻ and H₂O/H₃O⁺ for hydrochloric acid; CH₃COOH/CH₃COO⁻ and H₂O/H₃O⁺ for acetic acid.
- Ammonia (NH₃) acts as a base, accepting a proton from water to form NH₄⁺ and OH⁻.
Water's Role in Acid-Base Chemistry
- Water can function as both a Bronsted-Lowry acid and base.
- Water undergoes auto-ionization, serving as both proton donor and acceptor, forming H₃O⁺ and OH⁻.
The Ionization Constant for Water (Kw)
- The auto-ionization of water is represented as: 2H₂O ⇌ H₃O⁺ + OH⁻.
- The equilibrium constant for this reaction, Kw, equals [H₃O⁺][OH⁻].
- At 25°C, Kw = 1 × 10⁻¹⁴, so [H₃O⁺] = [OH⁻] = 1 × 10⁻⁷ mol/L.
- Kw increases with temperature, affecting ion concentrations and acidity.
Key Terms & Definitions
- Proton transfer reaction — Movement of a hydrogen ion (proton) from one species to another.
- Bronsted-Lowry acid — Proton donor.
- Bronsted-Lowry base — Proton acceptor.
- Conjugate acid-base pair — Pair of species differing by one proton.
- Auto-ionization of water — Water molecules reacting to form hydronium and hydroxide ions.
- Kw (Ionization constant for water) — Product of hydronium and hydroxide ion concentrations in water.
Action Items / Next Steps
- Prepare for upcoming content on pH calculations using Kw.
- Review equilibrium and rate concepts for better understanding changes in Kw with temperature.