Electrolytic Dissociation, Ion Exchange

Electrolytes

Electrolytes: substances whose aqueous solution or melt conducts electric current (acids, bases, salts).

• Strong: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄ and soluble salts.
• Alkalis: NaOH, KOH, Ba(OH)₂, Ca(OH)₂ (strong).
• Weak: CH₃COOH, HF, H₂S, NH₃·H₂O. Also H₂O, H₂CO₃, H₂SO₃.

Degree of dissociation: α = N(dis) / N(total).

When a weak electrolyte is diluted, its degree of dissociation increases (Ostwald's dilution law).

Ion exchange

A reaction goes to completion if a precipitate, a gas or a weak electrolyte (water) forms.

• Precipitates and gases are written in molecular form (in the ionic equation). A weak electrolyte too.
• Precipitates: AgCl (white), AgI (yellow), BaSO₄ (white), Cu(OH)₂ (blue), Fe(OH)₃ (brown).

pH

pH = −lg[H⁺]. Ion product of water: [H⁺][OH⁻] = 10⁻¹⁴.

pH	Medium
pH < 7 (0-6)	acidic
pH = 7	neutral
pH > 7 (8-14)	alkaline

• pH = 7: neutral medium.
• In 0.01 M HCl, pH = 2 ([H⁺] = 10⁻²).
• In 0.001 M NaOH, pOH = 3, pH = 11.

Indicators

• Litmus: red in acid, blue in alkali.
• Methyl orange: red in acid, yellow in alkali.
• Phenolphthalein: raspberry (crimson) in alkali.