Chapter 2 Notes – Acids and Bases (Chem 2201)

Arrhenius Acids

  • Arrhenius acids are substances that dissociate in water to give H₃O⁺ ions.

  • Stronger acids dissociate to a greater degree than weaker acids.

Arrhenius Bases

  • Arrhenius bases are substances that dissociate in water to give hydroxide ions (OH⁻).

  • Stronger bases (e.g., NaOH) dissociate more than weaker bases (e.g., Mg(OH)₂).

Lewis Acids and Lewis Bases

  • Lewis bases: species with an available pair of electrons that can be donated to form a new bond; they are electron-pair donors.

  • Lewis bases: electron-pair donors.

  • Lewis acids: species that can accept these electrons to form new bonds; they are electron-pair acceptors.

Lewis Acid-Base Reactions

  • Reactions involving donation of an electron pair from a Lewis base to a Lewis acid to form a new covalent bond.

Brønsted-Lowry Acids and Bases

  • Brønsted-Lowry acid: any species that donates a proton (H⁺).

  • Brønsted-Lowry base: any species that can accept a proton.

Conjugate Acids and Bases

  • Conjugate acid: the species formed when a base accepts a proton; it can donate that proton back.

  • Conjugate base: the species formed when an acid donates its proton; it can accept that proton back.

Acid Strength

  • An acid’s strength is expressed by its extent of ionization in water.

  • General ionization: HA+H<em>2O⇌H</em>3O++A−\mathrm{HA} + \mathrm{H<em>2O} \rightleftharpoons \mathrm{H</em>3O^+} + \mathrm{A^-}

  • Acid dissociation constant: K<em>a=[H</em>3O+][A−][HA]K<em>a = \frac{[\mathrm{H</em>3O^+}][\mathrm{A^-}]}{[\mathrm{HA}]}

  • The strength of an acid is summarized by its pKa: pK<em>a=−log⁡K</em>apK<em>a = -\log K</em>a

Relative Strength of Some Common Acids and Their Conjugate Bases

  • Stronger acids have larger Ka and smaller (more negative) pKa; weaker acids have smaller Ka and larger pKa. Conjugate bases are listed accordingly.

  • Key entries (acid → conjugate base; Ka; pKa):

    • Hydrochloric acid: HCl + H₂O → Cl⁻ + H₃O⁺; Ka ≈ 1×1071\times 10^{7}; pKₐ ≈ −7-7; conjugate base Cl⁻ (very weak base).

    • Hydronium ion: H₃O⁺ + H₂O ⇌ H₂O + H₃O⁺ (conceptually); Ka ≈ 5.0×1015.0\times 10^{1}; pKₐ ≈ −1.7-1.7; conjugate base H₂O.

    • Hydrofluoric acid: HF + H₂O ⇌ H₃O⁺ + F⁻; Ka ≈ 6.8×10−46.8\times 10^{-4}; pKₐ ≈ 3.17; conjugate base F⁻ (weak base).

    • Formic acid: HCOOH + H₂O ⇌ H₃O⁺ + HCOO⁻; Ka ≈ 1.7×10−41.7\times 10^{-4}; pKₐ ≈ 3.76; conjugate base formate HCOO⁻.

    • Acetic acid: CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻; Ka ≈ 1.8×10−51.8\times 10^{-5}; pKₐ ≈ 4.74; conjugate base acetate CH₃COO⁻.

    • Hydrocyanic acid: HCN + H₂O ⇌ H₃O⁺ + CN⁻; Ka ≈ 6.0×10−106.0\times 10^{-10}; pKₐ ≈ 9.22; conjugate base CN⁻.

    • Ammonium ion: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺; Ka ≈ 5.8×10−105.8\times 10^{-10}; pKₐ ≈ 9.24; conjugate base NH₃.

    • Water: H₂O ⇌ H⁺ + OH⁻; Ka (as acid) ≈ 1.8×10−161.8\times 10^{-16}; pKₐ ≈ 15.7; conjugate base OH⁻.

    • Ethanol: CH₃CH₂OH + H₂O ⇌ CH₃CH₂O⁻ + H₃O⁺; Ka ≈ 1.3×10−161.3\times 10^{-16}; pKₐ ≈ 15.9; conjugate base ethoxide CH₃CH₂O⁻.

    • Amide ion: NH₂⁻; Ka ≈ around 10⁻³⁶; pKₐ ≈ 36; conjugate acid NH₃.

    • Methane: CH₄ + H₂O ⇌ CH₃⁻ + H₃O⁺; Ka ≈ ≪1×10⁻⁵⁰; pKₐ > 50; extremely weak acid; conjugate base CH₃⁻.

    • Methyl anion: CH₃⁻; pKₐ > 50; Ka < 10⁻⁵⁰; extremely strong base; not commonly isolated.

  • Notes on pattern:

    • A stronger acid has a weaker conjugate base; a weaker acid has a stronger conjugate base.

    • Acids and their conjugate bases are linked by equilibrium positions; stronger acids push equilibrium toward reactants, while weaker acids favor products depending on the base present.

Base Strength

  • A stronger acid has a weaker conjugate base; a weaker acid has a stronger conjugate base.

  • Acid-base reactions favor the weaker acid and the weaker base.

  • The strength of an acid is inversely related to the strength of its conjugate base.

Equilibrium Positions of Acid–Base Reactions

  • The acid–base equilibrium favors formation of the weaker acid and the weaker base.

  • The weaker acid has the larger pK_a.

  • The weaker base has the larger pK_b.

  • The weaker acid and the weaker base are always on the same side of the equation (both reactants or both products).

Example: CH₃COOH + NaOH

  • Reactants: CH₃COOH (acid) and NaOH (base).

  • Proton transfer occurs from the acid to the base: CH₃COOH + NaOH → CH₃COONa + H₂O.

  • Conjugate base: CH₃COO⁻ (in CH₃COONa).

  • Conjugate acid formed on the base side: H₂O from the proton transfer.

  • Overall, the reaction demonstrates that weaker acids/bases drive equilibrium toward the side with the weaker species (in this case, toward CH₃COO⁻ and H₂O if conditions favor weaker acid/base).