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Enduring Understanding 3.B.2: Acid-Base Reactions

  • Acid-Base reactions involve transfer of protons, usually in solution.
  • In an acid-base neutralization, protons (H+) are transferred from an acid to a base (OH-), producing a salt and water.
  • Example:
    • HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
  • In this case, hydrochloric acid transfers its proton to sodium hydroxide, to give sodium chloride and water.

  • Strong acids dissociate completely in water to give hydrogen ions and an anion. Examples: HCl, HBr, H2SO4
  • Weak acids only dissociate slightly in water, giving a smaller amount of hydrogen ions and anions. Examples: CH3COOH (acetic acid), HF

  • Many compounds are amphoteric. This means they can act as both acids (H+ donors) and bases (H+ acceptors).
  • Example 1: Water is amphoteric: It can act as a base, gaining a proton to form hydronium (H3O+); or it act as an acid, losing a proton to form hydroxide (OH-)
  • Water as base: HBr + H2O → H3O+ + Br-
  • Water as acid: NH3 + H2O → OH- + NH4+
  • Example 2: Hydrogen sulfate ion (HSO4-) can also act as an acid or base:
  • HSO4- + NH3 → NH4+ + SO42-
  • HSO4- + HBr → H2SO4 + Br-

  • When an acid loses a proton, the anion (or other species) formed is called the conjugate base of the acid.
  • When a base gains a proton, the cation (or other species) formed is called the conjugate acid of the base.
  • Example:
    • CH3COOH + NH3 → CH3COO- + NH4+
  • Acetate (CH3COO-) is the conjugate base of acetic acid (CH3COOH)
  • Ammonium (NH4+) is the conjugate acid of ammonia (NH3)
  • The stronger the acid or base, the weaker the corresponding conjugate base or acid.
  • Examples:
  • HCl (very strong acid), Cl- (very weak base).
  • NaOH (very strong base), H2O (Very weak acid)
  • CH3COOH (weak acid), CH3COO- (moderately weak base)

  • Sample Problem: Of the following two reactions, which is the stronger conjugate base?
    • HBr + H2O → H3O+ + Br-
    CH3COOH + OH- → CH3COO- + H2O
  • First, identify the conjugate bases: Br- is the conjugate base of HBr, and CH3COO- is the conjugate base of CH3COOH.
  • The stronger the acid, the weaker the conjugate base. HBr is a very strong acid, so it will have a very weak conjugate base. CH3COOH is a much weaker acid, so it will have a stronger conjugate base.
  • Therefore, the stronger conjugate base is acetate, CH3COO-


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