Overview
This lecture covers how to write ionic and net ionic equations for precipitation reactions, emphasizing the importance of phases, charges, and the identification of spectator ions.
Ionic vs. Molecular Equations
- Ionic equations show how compounds, atoms, and ions actually exist in solution.
- Soluble ionic compounds and strong acids are written as ions in ionic equations.
- Insoluble compounds and weak acids/bases are written as full formula units.
- Molecular equations write all compounds as if they were intact molecules.
Writing Ionic Equations
- Split all aqueous ionic compounds and strong acids into their constituent ions.
- Leave solids, liquids, gases, and weak electrolytes as whole units.
- Always indicate phases (aq, s, l, g) next to each substance.
- Include ion charges for all separated ions.
- Balance both atoms and charge across the equation.
- Do not invent ions—only split compounds into recognized ions you have memorized.
Net Ionic Equations & Spectator Ions
- Spectator ions are those unchanged on both sides of the ionic equation.
- To write the net ionic equation, cancel out spectator ions and show only species involved in forming the precipitate.
- Alternatively, split the precipitate into its component ions for the reactants, and show the solid as the product.
Step-by-Step Example Workflow
- Write the complete molecular equation with correct product prediction and phases.
- Write the full ionic equation by splitting all soluble (aq) compounds into ions.
- Identify and cancel spectator ions (ions present in identical form on both sides).
- The remaining species form the net ionic equation with the precipitate as the product.
Key Terms & Definitions
- Ionic Equation — Shows dissolved ionic compounds as separated ions.
- Molecular Equation — Conventional chemical equation with compounds written together.
- Spectator Ions — Ions not involved in the actual chemical change; unchanged on both sides.
- Net Ionic Equation — Equation showing only species actually involved in the reaction, omitting spectator ions.
- Precipitate — An insoluble solid formed during a reaction in solution.
Action Items / Next Steps
- Practice writing molecular, ionic, and net ionic equations for given reactions.
- Remember to include correct phases and charges in all equations.
- Identify precipitates and spectator ions in each problem.
- Review nomenclature and solubility rules for common ions.