Balancing Chemical Equations
A step-by-step method for balancing equations, with four worked examples.
A balanced chemical equation has the same number of atoms of each element on both sides, because atoms are not created or destroyed in a chemical reaction. This is the law of conservation of mass.
The method
- Write the correct formulas for reactants and products. Never change a formula to balance it.
- Count atoms of each element on each side.
- Adjust the numbers in front of formulas (coefficients) to balance one element at a time.
- Recount everything.
- If useful, balance elements that appear in only one compound on each side first, and leave hydrogen and oxygen for last.
Example 1: water
H₂ + O₂ → H₂O
- Left: 2 H, 2 O. Right: 2 H, 1 O.
- Put 2 in front of H₂O: H₂ + O₂ → 2H₂O. Now right has 4 H, 2 O.
- Put 2 in front of H₂: 2H₂ + O₂ → 2H₂O.
- Check: 4 H and 2 O on each side. ✓
Example 2: methane burning
CH₄ + O₂ → CO₂ + H₂O
- Carbon is balanced (1 each side).
- Hydrogen: 4 on left, so put 2 in front of H₂O: CH₄ + O₂ → CO₂ + 2H₂O.
- Oxygen on right: 2 + 2 = 4, so put 2 in front of O₂.
- CH₄ + 2O₂ → CO₂ + 2H₂O. ✓
Example 3: magnesium and oxygen
Mg + O₂ → MgO
- Oxygen: 2 on left, 1 on right. Put 2 in front of MgO: Mg + O₂ → 2MgO.
- Magnesium: 2 on right, so put 2 in front of Mg.
- 2Mg + O₂ → 2MgO. ✓
Example 4: aluminium and oxygen
Al + O₂ → Al₂O₃
- Oxygen: 2 on left, 3 on right. The lowest common multiple of 2 and 3 is 6.
- 3O₂ gives 6 O on left, and 2Al₂O₃ gives 6 O on right.
- Right has 4 Al, so put 4 in front of Al.
- 4Al + 3O₂ → 2Al₂O₃. ✓
State symbols
Equations often include (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution.
Common mistakes
- Changing subscripts, which changes the substance.
- Balancing one element and forgetting to recheck the others.
- Leaving a fractional coefficient when a whole number version is required. Multiply through to clear fractions.
Quick recap
Count atoms, adjust coefficients only, recheck. Never alter chemical formulas.