Overview
This lecture covers the fundamental definitions and properties of acids and bases, emphasizing key concepts, molecular definitions, and how to measure acidity and basicity.
Definitions of Acids and Bases
- Brønsted-Lowry acid: donates a proton (H⁺).
- Brønsted-Lowry base: accepts a proton.
- Acid-base reactions form conjugate acid-base pairs.
- The conjugate acid is the base after gaining a proton; the conjugate base is the acid after losing a proton.
- Lewis acid: accepts an electron pair.
- Lewis base: donates an electron pair.
- Lewis acids do not need hydrogen; Brønsted-Lowry acids do.
Amphoteric Substances and Water Equilibrium
- Amphoteric species can act as both acids and bases (e.g., water).
- In water, proton transfer creates hydronium and hydroxide ions, establishing an equilibrium.
- The equilibrium constant for water (Kw) at room temperature is 1 x 10⁻¹⁴.
- Kw increases as temperature rises.
- Acidic solution: [H⁺] > 1 x 10⁻⁷; basic: [H⁺] < 1 x 10⁻⁷; neutral: [H⁺] = 1 x 10⁻⁷.
Acid and Base Strength
- Strong acids easily lose protons and have weak, stable conjugate bases.
- Weak acids do not readily lose protons and have strong conjugate bases.
- Larger atoms stabilize negative charge better, making the conjugate base more stable and the acid stronger.
- More electronegative atoms stabilize charge better, increasing acidity.
- Resonance stabilization in conjugate bases (e.g., carboxylic acids) increases acid strength.
- Monoprotic acids lose one proton; polyprotic acids can lose several, but become less acidic with each loss.
- In equilibrium, the side with the weaker acid-base pair is favored.
Measuring Acidity and Basicity
- Acid strength is indicated by a lower pKa value.
- Strong acids deprotonate completely; weak acids only partially deprotonate.
- pH is the negative logarithm of hydronium ion concentration; a pH of 7 is neutral.
- pOH is the negative logarithm of hydroxide ion concentration.
- pH + pOH = 14 at room temperature.
Key Terms & Definitions
- Brønsted-Lowry acid — substance that donates a proton (H⁺).
- Brønsted-Lowry base — substance that accepts a proton.
- Lewis acid — substance that accepts an electron pair.
- Lewis base — substance that donates an electron pair.
- Amphoteric — can act as both acid and base.
- Conjugate acid/base — the product formed after acid/base reaction.
- Kw — equilibrium constant for water ionization.
- pH — negative log of hydronium ion concentration; measures acidity.
- pOH — negative log of hydroxide ion concentration; measures basicity.
- Monoprotic/polyprotic acids — acids that can lose one or multiple protons.
- pKa — measure of acid strength; lower value means stronger acid.
Action Items / Next Steps
- Practice identifying acids, bases, and conjugate pairs using both Brønsted-Lowry and Lewis models.
- Use formulas to convert between pH, pOH, [H⁺], and [OH⁻].
- Review resonance stabilization and atom size/electronegativity effects on acid strength.