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Balancing Chemical Equations

Obeying the Law of Conservation of Mass in chemical reactions

Fundamental Principles

Conservation of Mass

Matter is not created or destroyed. The number of atoms of each element must stay the same before and after the reaction.

Coefficients vs Subscripts

Change only coefficients (numbers in front). Never change subscripts in a chemical formula while balancing.

Skeletal vs. Balanced

  • Skeletal: Shows only the formulas (unbalanced)
    H2 + O2 → H2O
  • Balanced: Same number of each atom on both sides
    2H2 + O2 → 2H2O

How to Balance

The Golden Rules

  • 1. Write correct formulas first
  • 2. Count atoms on both sides
  • 3. Balance one element at a time
  • 4. Final check: same count on both sides

Why it Matters

A balanced equation isn't just a math problem—it's a requirement of physics. Without it, calculations for laboratory experiments would be wrong, suggesting that atoms were magically created or lost during the reaction.

Step-by-Step: Propane Combustion

C3H8 + O2 → CO2 + H2O
1

Balance Carbon

3 Carbons on left, need 3 on right.

C3H8 + O2 → 3CO2 + H2O
2

Balance Hydrogen

8 Hydrogens on left, need 8 on right (4 × 2).

C3H8 + O2 → 3CO2 + 4H2O
3

Balance Oxygen

Right side now has 10 Oxygens (6 from CO2, 4 from H2O). Need 5O2 on left.

C3H8 + 5O2 → 3CO2 + 4H2O

Common Examples

Reactants Balanced Equation
Iron + Oxygen 4Fe + 3O2 → 2Fe2O3
Magnesium + HCl Mg + 2HCl → MgCl2 + H2
Sodium + Water 2Na + 2H2O → 2NaOH + H2
Aluminium + Oxygen 4Al + 3O2 → 2Al2O3

Mastery Challenges

Challenge 1: Phosphoric Acid + Calcium Hydroxide

3Ca(OH)2 + 2H3PO4 → Ca3(PO4)2 + 6H2O

Challenge 2: Bonus Bonus Mastery

4FeS2 + 11O2 → 2Fe2O3 + 8SO2

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