Precipitation Reactions and Solubility

Learning Goals

• Define what a precipitation reaction is.
• Identify precipitation reactions.
• Predict solubility of common inorganic compounds using solubility rules.

Precipitation Reactions

• A reaction where dissolved substances react to form solid products.
• Also known as double displacement, double replacement, or metathesis reactions.
• Involves exchange of ions.

Examples in Nature and Industry

• Lead iodide: Forms a bright yellow precipitate, historically used in artist pigments.
• Kidney stones: Formed from calcium oxalate in the body.
• Coral reefs: Formed from calcium carbonate precipitates extending coral structure.

Vocabulary

• Solubility: Maximum concentration of a substance that can dissolve under given conditions.
  • Soluble: Substances with large solubility.
  • Insoluble: Substances with low solubility, tend to precipitate.
• Precipitate: Substance forms when concentration exceeds solubility.

Example

• Thallium chloride: Solubility of 3.2g/L at 20°C. Exceeding this results in a precipitate.

Solubility Rules

• Essential for predicting whether substances dissolve or precipitate.
• Textbooks provide summaries in tables, listing soluble and insoluble ionic compounds.

Soluble Ionic Compounds

• Group 1 cations (e.g., sodium, ammonium): Always soluble, no exceptions.
• Halogens (chloride, bromide, iodide): Soluble, except with silver, mercury, lead.
• Fluoride: Soluble, except with group 2 metals, lead, iron.
• Molecular ions (acetate, bicarbonate, nitrate, chlorate): Always soluble.
• Sulfate: Soluble, except with silver, barium, strontium.

Insoluble Ionic Compounds

• Carbonate, chromate, phosphate, sulfide: Insoluble, except with group 1 cations, ammonium.
• Hydroxide: Insoluble, except with barium, group 1 cations.

Writing Precipitation Reactions

• Example 1: Sodium Chloride and Silver Nitrate

  • Dissociate reactants: NaCl → Na⁺ + Cl⁻; AgNO₃ → Ag⁺ + NO₃⁻.
  • Products: NaNO₃ (soluble), AgCl (precipitate).
  • Write molecular, complete ionic, and net ionic equations.

• Example 2: Potassium Iodide and Lead Nitrate

  • Dissociate reactants: KI → K⁺ + I⁻; Pb(NO₃)₂ → Pb²⁺ + 2 NO₃⁻.
  • Products: KNO₃ (soluble), PbI₂ (precipitate).
  • Write molecular, complete ionic, and net ionic equations.

Key Process

• Use solubility tables to determine if products are soluble or form precipitates.
• Write equations to reflect ion exchange and solubility outcomes.