Overview
This lecture covers precipitation reactions, focusing on predicting products, balancing equations, and writing complete, total ionic, and net ionic equations.
Predicting Precipitation Reactions
- Precipitation reactions are double replacement reactions where the outer ions and the inner ions swap partners.
- Use ion charges to determine product formulas; the crisscross method helps assign subscripts.
- Silver nitrate reacts with calcium chloride to form silver chloride and calcium nitrate.
- Lead nitrate reacts with sodium bromide to form lead(II) bromide and sodium nitrate.
Balancing Chemical Equations
- Ensure the number of each atom is equal on both sides of the equation.
- Use coefficients to balance atoms, especially for polyatomic ions appearing in multiple compounds.
- Example: 2 AgNO₃ + CaCl₂ → 2 AgCl + Ca(NO₃)₂ (balanced form).
Determining Phases and Solubility
- Nitrates (NO₃⁻) and alkali metals (group 1) are always soluble in water (aqueous phase).
- Most chlorides and bromides are soluble except those of silver, lead, and mercury.
- Insoluble compounds (such as AgCl and PbBr₂) are written as solids (s); soluble compounds as aqueous (aq).
Writing Ionic and Net Ionic Equations
- Split aqueous compounds into individual ions in the total ionic equation; leave solids intact.
- Spectator ions—ions unchanged on both sides—are omitted from the net ionic equation.
- Net ionic equation only includes ions and compounds directly involved in forming the precipitate.
- Example: Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (net ionic for silver nitrate + calcium chloride).
Example Problems
- Silver nitrate + calcium chloride: Net ionic equation is Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
- Lead nitrate + sodium bromide: Net ionic equation is Pb²⁺(aq) + 2 Br⁻(aq) → PbBr₂(s).
Key Terms & Definitions
- Precipitation Reaction — a reaction where two solutions form an insoluble solid, called a precipitate.
- Double Replacement Reaction — a reaction where two compounds exchange ions.
- Crisscross Method — technique for writing compound formulas using the ions’ charges as subscripts.
- Spectator Ion — ion unchanged during the reaction, present on both sides of the total ionic equation.
- Net Ionic Equation — shows only the ions and compounds directly involved in the reaction.
Action Items / Next Steps
- Review solubility rules for common ions.
- Practice predicting products and writing net ionic equations for double replacement reactions.
- Balance example equations as practice.