Acids and bases can be classified by the number of H+ or OH- ions they can produce (basicity/acidity) and by the degree of ionization (strength).
Basic Knowledge
Basicity/acidity: The number of H+ ions produced when one molecule of an acid (or one formula unit) ionizes. For a base, it is the number of OH- ions.
・Monoprotic acid: HCl, HNO3, CH3COOH
・Diprotic acid: H2SO4, H2CO3
・Triprotic acid: H3PO4
・Monobasic base: NaOH, KOH, NH3
・Dibasic base: Ca(OH)2, Ba(OH)2
Strength is judged by the degree of ionization α, the proportion of dissolved molecules that ionize. When α ≒ 1 (almost complete ionization), the substance is a strong acid or strong base; when α is small (only partial ionization), it is a weak acid or weak base.
Basicity/acidity (Number of H+ or OH- ions that can be produced per molecule (formula unit))
Degree of ionization α (Proportion of molecules that ionize. 0 ≦ α ≦ 1)
Strong acid (HCl, H2SO4, HNO3. α ≒ 1)
Weak acid (CH3COOH, H2CO3, H2S. α is small)
Strong base (NaOH, KOH, Ca(OH)2, Ba(OH)2)
Weak base (NH3, Mg(OH)2, Cu(OH)2)
Further Exploration (Background and Significance)
Basicity/acidity and strength are different concepts. For example, H2SO4 is a diprotic strong acid, CH3COOH is a monoprotic weak acid, and H3PO4 is a triprotic acid of moderate strength.
Representative strong acids and bases (essential memorization):
・Strong acids: HCl, HNO3, H2SO4
・Strong bases: NaOH, KOH, Ca(OH)2, Ba(OH)2 (many hydroxides of alkali metals and alkaline-earth metals)
Other substances can generally be considered weak acids or weak bases. Weak acids and weak bases are written with a reversible reaction arrow (“⇄”).
- Basicity/acidity: number of H+ (OH-) ions that can be produced
- Strength: determined by the degree of ionization α
- Strong acids: HCl, HNO3, H2SO4
- Strong bases: NaOH, KOH, Ca(OH)2, Ba(OH)2
- Representative weak acids: CH3COOH, H2CO3
- Representative weak base: NH3
- Strong acids/bases use “→”; weak acids/bases use “⇄”
Points of Caution (Common Confusions)
① Basicity/acidity and strength are different concepts. Diprotic does not mean strong acid. ② NH3 is a weak base but produces OH- in aqueous solution. ③ H2SO4 is a strong acid, while H2SO3 is a weak acid. They are easy to confuse. ④ Concentrated or dilute (concentration) and strong or weak (degree of ionization) are different. A dilute strong acid is not the same as a concentrated weak acid.
Practice
- State the basicity/acidity of H2SO4 and CH3COOH.
- Name three strong acids and three strong bases.
- Briefly explain what the degree of ionization α means.