Overview
This lecture explains precipitation reactions, how to predict products, determine solubility, balance equations, and write complete, ionic, and net ionic equations.
Precipitation Reactions & Predicting Products
- Precipitation reactions are double replacement reactions where two compounds exchange ions to form new products.
- To predict products, pair the first ion of one reactant with the last ion of the other, and vice versa.
- Use the crisscross method to write the correct chemical formula based on ion charges.
Writing and Balancing Chemical Equations
- Balance chemical equations so that the number of each atom is equal on both sides.
- Place appropriate coefficients in front of compounds to ensure atom balance.
- For polyatomic ions with multiple units, enclose them in parentheses.
Determining Phases and Solubility Rules
- Nitrates (NO₃⁻) and alkali metal compounds (e.g., Na⁺, K⁺) are always soluble in water.
- Halides (Cl⁻, Br⁻, I⁻) are soluble except when paired with Ag⁺, Pb²⁺, or Hg₂²⁺.
- Insoluble compounds form precipitates (solids), while soluble ones remain aqueous.
Writing Ionic and Net Ionic Equations
- In a total ionic equation, separate all aqueous substances into their ions; solids remain intact.
- Spectator ions appear unchanged on both sides of the equation and are omitted in the net ionic equation.
- The net ionic equation shows only the ions that combine to form the precipitate.
Example Reactions
- AgNO₃ + CaCl₂ → AgCl (s) + Ca(NO₃)₂ (aq)
- Net ionic: Ag⁺ (aq) + Cl⁻ (aq) → AgCl (s)
- Pb(NO₃)₂ + NaBr → PbBr₂ (s) + NaNO₃ (aq)
- Net ionic: Pb²⁺ (aq) + 2 Br⁻ (aq) → PbBr₂ (s)
Key Terms & Definitions
- Precipitation Reaction — Reaction forming an insoluble solid from two solutions.
- Double Replacement Reaction — Reaction where ions of two compounds exchange partners.
- Crisscross Method — Technique to write formulas by crossing ion charges as subscripts.
- Spectator Ions — Ions not involved in the actual chemical change.
- Net Ionic Equation — Equation showing only the species that change during the reaction.
Action Items / Next Steps
- Review solubility rules for common ions.
- Practice predicting products, balancing equations, and writing net ionic equations for precipitation reactions.