🧪

Acids and Bases Overview

Sep 3, 2025

Overview

This lecture covers the three main definitions of acids and bases—Arrhenius, Brønsted-Lowry, and Lewis—with examples and explanations of conjugate acid-base pairs.

Arrhenius Definition

  • An Arrhenius acid releases H+ (hydronium, H3O+) ions in water.
  • An Arrhenius base releases OH– (hydroxide) ions in water.
  • Common Arrhenius acids: HF, HCl, H2SO4, HNO3; they all have hydrogen in front.
  • Common Arrhenius bases: NaOH, KOH, Ca(OH)2; they release OH– and increase pH.

Brønsted-Lowry Definition

  • A Brønsted-Lowry acid is a proton (H+) donor.
  • A Brønsted-Lowry base is a proton (H+) acceptor.
  • In HCl and H2O: HCl donates H+ (acid), H2O accepts H+ (base).
  • Conjugate acid: add H+ and increase charge by 1.
  • Conjugate base: remove H+ and decrease charge by 1.
  • Examples:
    • NH3 → Conjugate acid: NH4+, conjugate base: NH2–
    • HCO3– → Conjugate acid: H2CO3, conjugate base: CO3^2–

Conjugate Acid–Base Pairs Practice

  • NH3 + H2O ↔ NH4+ + OH–: NH3 is base, NH4+ is conjugate acid; H2O is acid, OH– is conjugate base.
  • CH3OH + H2O ↔ CH3O– + H3O+: CH3OH is acid, CH3O– is conjugate base; H2O is base, H3O+ is conjugate acid.

Lewis Definition

  • A Lewis acid is an electron pair acceptor.
  • A Lewis base is an electron pair donor.
  • Example: BH3 (Boron) accepts a lone pair from NH3 (Nitrogen).
  • In this reaction, NH3 is the Lewis base (nucleophile), BH3 is the Lewis acid (electrophile).
  • The resulting complex has nitrogen with a positive charge and boron with a negative charge.

Key Terms & Definitions

  • Arrhenius acid — Produces H+ (as H3O+) ions in water.
  • Arrhenius base — Produces OH– ions in water.
  • Brønsted-Lowry acid — Proton (H+) donor.
  • Brønsted-Lowry base — Proton (H+) acceptor.
  • Conjugate acid — Product when a base gains a proton.
  • Conjugate base — Product when an acid loses a proton.
  • Lewis acid — Electron pair acceptor.
  • Lewis base — Electron pair donor.
  • Nucleophile — Electron-rich Lewis base.
  • Electrophile — Electron-poor Lewis acid.

Action Items / Next Steps

  • Practice identifying acids, bases, and conjugate pairs using the definitions above.
  • Review homework on writing conjugate acids and bases for given species.

Full transcript