Strong and weak acids

Acids are substances that produce hydronium ions, H₃O⁺, when dissolved in water.

"Strong" acids ionize completely in water. For example,

The above reaction indicates that when hydrogen chloride gas reacts with water, hydronium ions are produced in aqueous solution. Note that the arrow points to the products, indicating that virtually ALL the HCl becomes H₃O⁺ and Cl^- ions in aqueous solution. HCl is therefore a strong acid.

"Weak" acids ionize partially in water.

Acetic acid, CH₃3COOH, does not completely ionize in water. Most of the acetic acid in solution remains un-ionized and a small fraction ionizes to form CH₃COO^- and H₃O⁺ ions. To represent this in a chemical equation, a double arrow is used. The longer arrow pointing toward the reactants indicates that acetic acid in solution remains un-ionized. CH₃COOH is a weak acid.

Examples

Names and formulae of six strong acids.

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

Names and formulae of three weak acids.

• H₃PO₄, phosphoric acid
• CH₃COOH, acetic acid
• H₂CO₃, carbonic acid

Content suitability

BCIT courses: CHEM 0011