Balancing Chemical Equations

Key Principle

• Conservation of Matter/Mass: Matter is not created or destroyed, it is only changed. This means the number of type of atoms on the reactant side must equal the number on the product side.

Steps to Balance Equations

1. Identify the Components: List all elements involved in the reaction.
2. Write the Unbalanced Equation: Use the chemical formulae for the reactants and products.
3. Count Atoms: Determine the number of each type of atom on both sides.
4. Add Coefficients: Adjust coefficients to balance the number of atoms for each element.
5. Check Your Work: Ensure the equation is balanced by verifying atom counts.

Example 1: Basic Balancing

• Reaction: CS₂ (liquid) + O₂ (gas) → CO₂ (gas) + SO₂ (gas)
• Initial Count:
  • Reactants: 1 Carbon, 2 Sulfurs, 2 Oxygens
  • Products: 1 Carbon, 1 Sulfur, 4 Oxygens
• Steps:
  • Balance Sulfur: Add coefficient 2 before SO₂.
  • Adjust Oxygen: 3 O₂ molecules needed to balance the 6 Oxygens in products.
• Balanced Equation: CS₂ + 3 O₂ → CO₂ + 2 SO₂

Example 2: Polyatomic Ions

• Reaction: K₂CrO₄ + Pb(NO₃)₂ → KNO₃ + PbCrO₄
• Approach:
  • Identify and keep polyatomic ions (CrO₄ and NO₃) intact when they appear unchanged on both sides.
  • Balance Potassium: Adjust by placing coefficients to match number of ions.
• Balanced Equation: 2 K₂CrO₄ + Pb(NO₃)₂ → 2 KNO₃ + PbCrO₄

Example 3: Hydrocarbon Combustion

• Reaction: C₃H₆ + O₂ → CO₂ + H₂O
• Initial Count:
  • Reactants: 3 Carbons, 6 Hydrogens, 2 Oxygens
  • Products: 1 Carbon, 2 Hydrogens, 3 Oxygens
• Steps:
  • Balance Carbon: Place 3 as coefficient of CO₂.
  • Balance Hydrogen: Place 3 as coefficient of H₂O.
  • Balance Oxygen: Calculate required O₂ and address fractional coefficients by scaling the entire equation.
• Final Balanced Equation: 2 C₃H₆ + 9 O₂ → 6 CO₂ + 6 H₂O

Key Points

• Only coefficients are changed to balance equations, not the compounds.
• Never use fractions in final balanced equations; scale to whole numbers if necessary.
• Keep polyatomic ions together when they appear unchanged on both sides of the equation to simplify balancing.