Chapter 15.3

Understanding Lewis Acids and Bases

  • Lewis Acid and Base Definitions
    • Lewis Base: A substance that can donate a pair of electrons.
    • Lewis Acid: A substance that can accept a pair of electrons.

Characteristics of Lewis Bases

  • Electron Pair Donor:
    • Common Lewis bases include anions and molecules with unshared (lone) pairs of valence electrons.
    • Example: Ammonia (NH₃) is a Lewis base as the nitrogen atom has a lone pair that can be donated.

Characteristics of Lewis Acids

  • Electron Pair Acceptor:
    • Typical Lewis acids are cations (e.g., H⁺) and certain neutral molecules that can accept additional electrons.
    • Characteristics:
    • Lewis acids often have empty valence orbitals.
    • Example: H⁺ is considered a Lewis acid because it has an empty 1s orbital.

Relation to Brønsted-Lowry Definitions

  • Comparison of Definitions:
    • Every Brønsted-Lowry acid or base is also a Lewis acid or base.
    • A Brønsted-Lowry acid donates H⁺ while a Brønsted-Lowry base accepts H⁺.
    • Therefore, compounds that do not act as Lewis acids or bases cannot be Brønsted-Lowry acids or bases.

Identifying Lewis Acids and Bases

Strategy:
  1. Draw Lewis Structures: Analyze the reactants and products.
  2. Determine Electron Participants: Identify which is the likely electron donor (Lewis base) and which is the electron acceptor (Lewis acid).
  3. Identify Common Patterns:
    • Lewis Acid: Typically a cation or an electron-deficient molecule.
    • Lewis Base: An anion or a molecule containing atoms with lone pairs.

Practice Problem Example

  • Reaction: HCH₃CO₂ (aq) + NH₃ (aq) → CH₃CO₂⁻ (aq) + NH₄⁺ (aq)
    • To identify roles:
    • HCH₃CO₂ is likely the electron acceptor (Lewis acid).
    • NH₃ is likely the electron donor (Lewis base).

Practice Problem Analysis

  • Highlighted Question: In the reaction, identify the roles of reactants:
    • Determine if the highlighted reactant is a Brønsted-Lowry acid/base or Lewis acid/base.
    • Conclusion: NH₃ acts as a Lewis base.

Reactants Acting as Neither Lewis nor Brønsted-Lowry Acids/Bases

  • Not All Reactions Involve Lewis/Brønsted-Lowry Concepts:
    • When covalent bonds break without lone pair participation, the process differs from Lewis acid-base reactions.
    • Example: In the reaction
      2extCH<em>4(g)+extBr</em>2(g)→2extCH<em>3extBr(g)+extH</em>2(g)2 ext{CH}<em>4 (g) + ext{Br}</em>2 (g) → 2 ext{CH}<em>3 ext{Br} (g) + ext{H}</em>2 (g)
    • The reactants do not act as Lewis or Brønsted-Lowry acids or bases since they do not involve electron transfer in the same manner as formal acid-base reactions.