Overview
This lecture explains precipitation reactions in ionic compounds, solubility rules, identifying spectator ions, and how to predict whether a precipitate will form in given chemical reactions.
Precipitation Reactions
- Precipitation reactions occur when two solutions combine and form an insoluble solid, called a precipitate.
- Example: Mixing silver nitrate solution (AgNO₃) with sodium chloride solution (NaCl) forms a white precipitate of silver chloride (AgCl).
Writing Reaction Formulas
- All ions in solution are initially written in the full (molecular) reaction.
- Spectator ions (ions unchanged during the reaction) can be excluded in the net ionic equation.
- Net ionic equation for AgNO₃ + NaCl: Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
Additional Precipitation Example
- Mixing barium nitrate (Ba(NO₃)₂) with sodium sulfate (Na₂SO₄) forms a white precipitate of barium sulfate (BaSO₄).
- Net ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Solubility Rules (Rules of Thumb)
- All sodium, potassium, and ammonium salts are soluble.
- All nitrates (NO₃⁻) are soluble.
- Most chlorides are soluble except AgCl and PbCl₂.
- Most sulfates are soluble except BaSO₄, PbSO₄, and somewhat insoluble CaSO₄.
- All carbonates, phosphates, and hydroxides are insoluble, except with sodium, potassium, or ammonium.
Predicting Precipitate Formation (Sample Questions)
- Mixing barium nitrate with ammonium chloride: No precipitate forms since all resulting salts are soluble.
- Mixing lead nitrate with potassium iodide: Yellow precipitate of lead iodide (PbI₂) forms; net ionic equation: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s).
Limewater and Carbon Dioxide Reaction
- Bubbling CO₂ into limewater (Ca(OH)₂ solution) forms cloudy CaCO₃ (solid).
- Reaction: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
- Excess CO₂ converts CaCO₃ into soluble calcium hydrogen carbonate: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq).
Key Terms & Definitions
- Precipitate — An insoluble solid formed from a reaction in solution.
- Spectator ion — An ion present in solution that does not participate in the actual chemical reaction.
- Net ionic equation — A chemical equation showing only the ions and compounds involved in the reaction.
- Solubility — The ability for a substance to dissolve in a solvent (usually water).
Action Items / Next Steps
- Review and memorize solubility rules.
- Practice writing full and net ionic equations for precipitation reactions.
- Prepare for exam questions involving the prediction of precipitate formation.