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Strong Acids & Bases

Chemistry: IB Diploma Higher Level, topic 6.3.1

Strong vs Weak Bases

Bases are metallic oxides or hydroxides (or ammonia) that neutralize acids to form salt and water. An alkali is a soluble base - one that dissolves in water.

Strong base

Strong base

  • Group I hydroxides, such as aqueous sodium hydroxide and potassium hydroxide are examples of strong bases.
  • Strong bases fully ionize in aqueous solutions.
  • NaOH(aq) → Na+(aq) + OH-.
  • KOH(aq) → K+(aq) + OH-.
  • As strong bases fully ionize, the initial concentration of a strong base equals the concentration of OH- ions.
  • For example, 1M KOH gives 1M OH- ions.
Group II hydroxides

Group II hydroxides

  • Group II hydroxides are also strong bases. An example is Ca(OH)2.
  • Due to stoichiometry, 1M of a group II hydroxide gives 2M OH- ions.
  • For example, 1M Ca(OH)2 gives 2M OH- ions.
Weak base

Weak base

  • Weak bases don't fully ionize in aqueous solutions.
  • Ammonia solution is an example of a weak base.
  • It exists mainly as NH3 molecules. Only a few of these react with water molecules to form ions.
  • NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq).

Strong vs Weak Acids

Acids are proton donors that neutralize bases to form salt and water. An acid can be strong or weak - this will affect its dissociation.

Strong acid

Strong acid

  • Aqueous HCl, HBr, HI, HClO4, HNO3 and H2SO4 are examples of strong acids.
  • Strong acids fully ionize in aqueous solutions.
  • HCl(aq) + H2O(l) → H3O+(aq) + Cl-
  • HNO3(aq) + H2O(l) → H3O+(aq) + NO3-
  • As strong acids fully ionize, the initial concentration of a monoprotic strong acid equals the concentration of H3O+ ions.
  • For example, 1M HCl gives 1M H3O+ ions.
Diprotic acids

Diprotic acids

  • Diprotic strong acids ionize to give two hydronium ions in solution. An example is H2SO4.
  • Due to stoichiometry, 1M of a diprotic acid gives 2M H3O+ ions.
  • For example, 1M H2SO4 gives 2M H3O+ ions.
Weak acids

Weak acids

  • Weak acids don't fully ionize in aqueous solutions.
  • Acetic acid is an example of a weak acid.
  • Acetic acid exists mainly as CH3COOH molecules. Only a few of these react with water molecules to form ions.
  • CH3COOH(aq) + H2O(l) ⇌ H3O+(aq) + CH3COO-(aq)

Test yourself on Strong Acids & Bases

  • Which of these are examples of strong bases?

  • Which of the following is a weak base?

  • Which of these are examples of strong acids?

  • Which of the following is a weak acid?

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