Balancing chemical equations is really just accounting — the number of atoms of each element must match on both sides. These 10 examples walk through the process for different reaction types.
1. Balance: H₂ + O₂ → H₂O
Oxygen is unbalanced (2 on left, 1 on right). Add a coefficient of 2 to H₂O: H₂ + O₂ → 2H₂O. Now H is unbalanced (2 vs 4) — add a coefficient of 2 to H₂: 2H₂ + O₂ → 2H₂O.
2. Balance: N₂ + H₂ → NH₃
Balance N first: N₂ + H₂ → 2NH₃. Now balance H (2 vs 6): N₂ + 3H₂ → 2NH₃.
3. Balance: Mg + O₂ → MgO
Balance O first: Mg + O₂ → 2MgO. Now balance Mg: 2Mg + O₂ → 2MgO.
4. Balance: CaCO₃ → CaO + CO₂
Already balanced as written — one Ca, one C, and three O on each side. CaCO₃ → CaO + CO₂.
5. Balance: KClO₃ → KCl + O₂
Balance O: 2KClO₃ → 2KCl + 3O₂. Check K and Cl — both balanced. 2KClO₃ → 2KCl + 3O₂.
6. Balance: Zn + HCl → ZnCl₂ + H₂
Balance Cl: Zn + 2HCl → ZnCl₂ + H₂. Check H — 2 on each side. Zn + 2HCl → ZnCl₂ + H₂.
7. Balance: Fe + CuSO₄ → FeSO₄ + Cu
Already balanced — one Fe, one Cu, one S, four O on each side. Fe + CuSO₄ → FeSO₄ + Cu.
8. Balance: CH₄ + O₂ → CO₂ + H₂O
Balance C (already 1:1), then H: CH₄ + O₂ → CO₂ + 2H₂O. Now balance O (2 vs 4) — add coefficient 2 to O₂: CH₄ + 2O₂ → CO₂ + 2H₂O.
9. Balance: C₂H₆ + O₂ → CO₂ + H₂O
Balance C: → 2CO₂. Balance H: → 3H₂O. Now count O on the right (4+3=7) and set O₂ coefficient to 7/2: C₂H₆ + 7/2 O₂ → 2CO₂ + 3H₂O, or multiply everything by 2: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
10. Balance: C₃H₈ + O₂ → CO₂ + H₂O
Balance C: → 3CO₂. Balance H: → 4H₂O. Count O on right (6+4=10), so O₂ coefficient = 5: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.