To balance an equation, change only the coefficients in front of formulas until each element has the same number of atoms on both sides. The formulas themselves never change.
This lesson is part of moles, formulas and equations. The mole ratios you get from a balanced equation drive solving reacting-mass and gas-volume calculations.
What is the method?
Write an atom-count table with one row per element, left and right. Then change one coefficient, update the table and repeat.
- Write the correct formulas first and do not alter them.
- Count atoms of each element on both sides.
- Balance the element in the fewest substances first, and hydrogen and oxygen last.
- Recount everything after each change.
Worked example 1: aluminium and oxygen
Unbalanced: Al + O₂ → Al₂O₃
| Element | Left | Right |
|---|---|---|
| Al | 1 | 2 |
| O | 2 | 3 |
There are only two elements, so start with oxygen: 2 on the left against 3 on the right. Use the lowest common multiple, 6. Put 3 in front of O₂ and 2 in front of Al₂O₃, which gives 6 oxygen atoms on each side and 4 aluminium atoms on the right.
Then put 4 in front of Al. 4Al + 3O₂ → 2Al₂O₃. Check: Al is 4 and 4, O is 6 and 6.
Worked example 2: iron(III) oxide and carbon monoxide
Unbalanced: Fe₂O₃ + CO → Fe + CO₂
- Fe: 2 on the left, so write 2Fe.
- Fe₂O₃ already has 3 oxygen atoms, and each CO adds 1 more. Each CO₂ on the right needs 2 oxygen atoms and 1 carbon atom.
- Try 3CO and 3CO₂: carbon is 3 and 3, and oxygen is 3 + 3 = 6 on the left and 3 × 2 = 6 on the right.
Fe₂O₃ + 3CO → 2Fe + 3CO₂. Check: Fe 2 and 2, C 3 and 3, O 6 and 6.
Worked example 3: propane burning
Unbalanced: C₃H₈ + O₂ → CO₂ + H₂O
- Carbon: 3 on the left, so 3CO₂.
- Hydrogen: 8 on the left, so 4H₂O.
- Oxygen on the right is 6 + 4 = 10, so 5O₂ on the left.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Left oxygen is 10, right oxygen is 10.
The slip that unbalances a formula
A student balances Al + O₂ → Al₂O₃ by changing O₂ to O₃ or Al₂O₃ to AlO. The atom counts may match, but the equation now describes substances that do not exist.
Only the number in front of a formula changes, and you write it as a plain number beside the whole formula. If you run out of ideas, recount the atoms and change a different coefficient. Never change a subscript.
Check yourself
Balance: Mg + HCl → MgCl₂ + H₂ and then N₂ + H₂ → NH₃.
Answer
Mg + 2HCl → MgCl₂ + H₂. Chlorine is 2 on the right, so 2HCl. Hydrogen is then 2 and 2.
N₂ + 3H₂ → 2NH₃. Nitrogen is 2 on the left, so 2NH₃. Hydrogen on the right is then 6, so 3H₂.
What to study next
Continue with calculating concentration from supplied data. Then practise the whole chapter using the moles, formulas and equations practice set.
For a teacher to coach your first moves on unfamiliar equations, see online one-to-one Chemistry tuition.