Overview
This lecture covers strategies and step-by-step methods for balancing different types of chemical equations, including combustion, synthesis, and double replacement reactions.
General Steps for Balancing Equations
- A balanced equation has equal numbers of each type of atom on both sides.
- Balance in this order: metals → nonmetals → hydrogen → oxygen (for combustion).
- When fractions occur, multiply all coefficients by a common multiple to get whole numbers.
Balancing Combustion Reactions
- For C₃H₈ + O₂ → CO₂ + H₂O: Balance C first (3 CO₂), then H (4 H₂O), finally O (5 O₂).
- For C₄H₁₀ + O₂ → CO₂ + H₂O: Balance C (4 CO₂), H (5 H₂O), then O (13/2 O₂), then multiply all by 2 to get whole numbers.
Balancing Single Replacement and Synthesis Reactions
- Al + HCl → AlCl₃ + H₂: Balance Cl (6 HCl), then Al (2 Al), then H (3 H₂).
- Ga + CuBr₂ → GaBr₃ + Cu: Balance Br (3 CuBr₂ and 2 GaBr₃), then Cu (3 Cu), then Ga (2 Ga).
Balancing with Odd-Even Atom Situations
- Use least common multiples to balance atoms that appear in odd and even numbers.
- Multiply reactants or products to achieve the same number of atoms on both sides.
Balancing Equations with Polyatomic Ions
- Treat polyatomic ions (like PO₄³⁻ or SO₄²⁻) as units when they remain unchanged on both sides.
- For Na₃PO₄ + MgCl₂ → NaCl + Mg₃(PO₄)₂: Balance PO₄ units first, then Na, then Mg, then Cl.
Special Cases
- If a molecule like S₈ is present, balance sulfur first, then adjust other elements.
- When fractions appear, multiply all coefficients to get whole numbers in the final balanced equation.
Key Terms & Definitions
- Coefficient — the number in front of a molecule or atom in a chemical equation indicating quantity.
- Subscript — the small number in a chemical formula showing the number of atoms of an element.
- Combustion Reaction — reaction of a hydrocarbon with O₂ producing CO₂ and H₂O.
- Polyatomic Ion — a charged group of covalently bonded atoms acting as a unit.
Action Items / Next Steps
- Practice balancing provided sample reactions.
- Review molecular formulas and common polyatomic ions.
- Prepare for assignment on balancing equations with increasing complexity.