6.3 - Strength of Acids and Alkalis

Strong and Weak Acids

• Acid strength depends on the degree of ionisation in water, not merely on concentration.
• A strong acid ionises completely in water and produces a high concentration of ${\mathrm{H}}^{+}$.
  • $\mathrm{HCl}(\mathrm{aq})\to {\mathrm{H}}^{+}(\mathrm{aq})+\mathrm{C}{\mathrm{l}}^{-}(\mathrm{aq})$
• A weak acid ionises partially in water and produces a low concentration of ${\mathrm{H}}^{+}$.
  • $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}(\mathrm{aq})\rightleftharpoons {\mathrm{H}}^{+}(\mathrm{aq})+\mathrm{C}{\mathrm{H}}_3\mathrm{CO}{\mathrm{O}}^{-}(\mathrm{aq})$
  • In the illustrated ethanoic-acid example, the degree of dissociation is 1.54%, so only a small proportion of molecules ionise.
  • The reversible arrow shows that ions can recombine; most ethanoic-acid molecules remain unionised.

Strong and Weak Alkalis

• An alkali’s strength depends on its degree of ionisation or dissociation in water.
• A strong alkali ionises completely and produces a high concentration of $\mathrm{O}{\mathrm{H}}^{-}$.
  • $\mathrm{NaOH}(\mathrm{aq})\to \mathrm{N}{\mathrm{a}}^{+}(\mathrm{aq})+\mathrm{O}{\mathrm{H}}^{-}(\mathrm{aq})$
• A weak alkali ionises partially and produces a low concentration of $\mathrm{O}{\mathrm{H}}^{-}$.
  • $\mathrm{N}{\mathrm{H}}_3(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\rightleftharpoons \mathrm{N}{\mathrm{H}}_4^{+}(\mathrm{aq})+\mathrm{O}{\mathrm{H}}^{-}(\mathrm{aq})$
• In the illustrated ammonia example, the degree of dissociation is 1.3%; a small proportion of ammonia molecules accept ${\mathrm{H}}^{+}$ from water, leaving many ammonia molecules unionised.
• At the same molarity, a strong acid normally has lower pH than a weak acid, while a strong alkali has higher pH than a weak alkali.
• Strength describes degree of ionisation; concentration describes solute amount per unit solution volume.