Overview
This lecture covers key concepts in stoichiometry, specifically focusing on word and symbol equations, naming compounds, ionic equations, balancing chemical equations, and example reactions.
Word and Symbol Equations
- Word equations describe chemical reactions using names of substances.
- Reactants are starting materials; products are the substances formed.
- State symbols: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous (dissolved in water).
- Symbol equations use chemical formulas (e.g., NaOH for sodium hydroxide).
Naming Compounds
- For metal + non-metal, the metal is named first and non-metal ends in “-ide” (e.g., potassium bromide).
- For two non-metals, hydrogen comes first if present (e.g., hydrogen bromide).
- When both are non-metals (not hydrogen), the lower group number element is named first.
- Compounds with metal and common ion group list the metal first.
Ionic and Balanced Equations
- Ionic equations show only ions involved in the reaction, omitting spectator ions.
- Split aqueous ionic compounds into individual ions in equations.
- Cancel spectator ions to write the net ionic equation (e.g., H⁺ + OH⁻ → H₂O).
Deducing and Balancing Symbol Equations
- Use valencies (combining powers) to determine chemical formulas.
- Crisscross method: swap valency numbers between elements to write formulas.
- Balancing equations ensures equal numbers of each atom/type on both sides.
- Use coefficients (numbers in front) to balance atoms by trial and error.
- Treat common ion groups (e.g., SO₄²⁻) as single units when balancing.
Worked Examples
- Calcium reacts with chlorine: Ca + Cl₂ → CaCl₂ (using valencies to derive formula).
- Potassium hydroxide and sulfuric acid: Use dissociation into ions, combine and balance to get K₂SO₄ + H₂O.
- Iron reacts with oxygen: Derive and balance Fe₂O₃ using crisscross and coefficients.
- Calcium oxide with nitric acid: Pay attention to brackets for polyatomic ions when writing and balancing.
Key Terms & Definitions
- Stoichiometry — calculation of reactants and products in chemical reactions.
- Reactant — substance present at the start of a reaction.
- Product — substance formed by a chemical reaction.
- State symbols — notations for phase: (s), (l), (g), (aq).
- Ionic equation — equation showing only ions that participate in a reaction.
- Spectator ion — ion not directly involved in the chemical change.
- Valency — combining power of an element.
Action Items / Next Steps
- Practice writing and balancing chemical equations.
- Review definitions and rules for naming compounds.
- Attempt similar questions on stoichiometry for mastery.