Acids and bases are classified as strong or weak based on how completely they ionize in water. This distinction matters because it controls pH, conductivity, reaction speed, and how much acid or base is available to react. A strong acid or base does not mean concentrated, and a weak acid or base does not mean harmless.
Strength describes particle behavior at the molecular level, not the amount poured into a beaker.
Strong acids and strong bases dissociate almost completely into ions, so their pH can often be calculated directly from concentration. Weak acids and weak bases only partially ionize and form an equilibrium mixture of molecules and ions. Acid strength is measured with Ka, base strength with Kb, and larger values mean more ionization.
Common examples help students recognize patterns, such as HCl as a strong acid, CH3COOH as a weak acid, NaOH as a strong base, and NH3 as a weak base.
Key Facts
- Strong acid: HA(aq) -> H+(aq) + A-(aq), nearly 100% ionization in water.
- Weak acid: HA(aq) ⇌ H+(aq) + A-(aq), partial ionization and equilibrium.
- Strong base examples include Group 1 hydroxides such as NaOH and KOH, and soluble Group 2 hydroxides such as Ca(OH)2 and Ba(OH)2.
- pH = -log[H3O+] and pOH = -log[OH-].
- At 25 °C, pH + pOH = 14.00 and Kw = [H3O+][OH-] = 1.0 x 10^-14.
- For a weak acid, Ka = [H3O+][A-]/[HA]; for a weak base, Kb = [BH+][OH-]/[B].
Vocabulary
- Strong acid
- A strong acid is an acid that ionizes almost completely in water to produce hydronium ions.
- Weak acid
- A weak acid is an acid that only partially ionizes in water and reaches an equilibrium between molecules and ions.
- Strong base
- A strong base is a base that dissociates almost completely in water to produce hydroxide ions.
- Weak base
- A weak base is a base that reacts only partially with water to produce hydroxide ions and its conjugate acid.
- Ionization
- Ionization is the process in which neutral molecules form ions when dissolved or reacting in water.
Common Mistakes to Avoid
- Confusing strong with concentrated: strong means nearly complete ionization, while concentrated means many moles of solute per liter of solution.
- Assuming all acids have the same pH at the same concentration: a 0.10 M strong acid gives much more H3O+ than a 0.10 M weak acid because the weak acid only partially ionizes.
- Forgetting coefficients in strong bases: Ca(OH)2 produces two OH- ions per formula unit, so 0.10 M Ca(OH)2 gives 0.20 M OH- if it fully dissolves.
- Using Ka or Kb for strong acids and bases in simple pH problems: strong acids and bases are treated as fully dissociated, so equilibrium calculations are usually unnecessary.
Practice Questions
- 1 Find the pH of 0.010 M HCl, assuming it fully ionizes in water.
- 2 Find the pOH and pH of 0.025 M NaOH at 25 °C, assuming it fully dissociates.
- 3 A 0.10 M solution of HCl has a lower pH than a 0.10 M solution of CH3COOH. Explain this difference using degree of ionization and particle behavior in water.