Strength of Acids and Bases

Strong electrolytes are completely dissociated into ions in water. The acid or base molecule does not exist in aqueous solution, only ions. Weak electrolytes are incompletely dissociated. Here are definitions and examples of strong and weak acids and strong and weak bases.

Strong acids completely dissociate in water, forming H⁺ and an anion. There are six strong acids. The others are considered to be weak acids. You should commit the strong acids to memory:

• HCl: hydrochloric acid
• HNO₃: nitric acid
• H₂SO₄: sulfuric acid
• HBr: hydrobromic acid
• HI: hydroiodic acid
• HClO₄: perchloric acid

If the acid is 100 percent dissociated in solutions of 1.0 M or less, it is called strong. Sulfuric acid is considered strong only in its first dissociation step; 100 percent dissociation isn't true as solutions become more concentrated. 

H₂SO₄ → H⁺ + HSO₄^-

A weak acid only partially dissociates in water to give H⁺ and the anion. Examples of weak acids include hydrofluoric acid, HF, and acetic acid, CH₃COOH. Weak acids include:

• Molecules that contain an ionizable proton. A molecule with a formula starting with H usually is an acid.
• Organic acids containing one or more carboxyl group, -COOH. The H is ionizable.
• Anions with an ionizable proton (e.g., HSO₄^- → H⁺ + SO₄^2-).
• Cations
• Transition metal cations
• Heavy metal cations with high charge
• NH₄⁺ dissociates into NH₃ + H⁺

Strong bases dissociate 100 percent into the cation and OH^- (hydroxide ion). The hydroxides of the Group I and Group II metals usually are considered to be strong bases.

• LiOH: lithium hydroxide
• NaOH: sodium hydroxide
• KOH: potassium hydroxide
• RbOH: rubidium hydroxide
• CsOH: cesium hydroxide
• *Ca(OH)₂: calcium hydroxide
• *Sr(OH)₂: strontium hydroxide
• *Ba(OH)₂: barium hydroxide

* These bases completely dissociate in solutions of 0.01 M or less. The other bases make solutions of 1.0 M and are 100 percent dissociated at that concentration. There are other strong bases than those listed, but they are not often encountered.

Examples of weak bases include ammonia, NH₃, and diethylamine, (CH₃CH₂)₂NH. Like weak acids, weak bases do not completely dissociate in aqueous solution.

• Most weak bases are anions of weak acids.
• Weak bases do not furnish OH^- ions by dissociation. Instead, they react with water to generate OH^- ions.