Balancing chemical equations: a step-by-step method

1. Write the correct formulas for every reactant and product.
2. Count the atoms of each element on each side.
3. Pick an element that isn't balanced and put a coefficient in front to fix it.
4. Recount, and repeat until every element balances.
5. Leave elements that appear in several places (often H and O) until last.

Magnesium burning: Mg + O₂ → MgO

There are 2 O on the left and 1 on the right, so put 2 in front of MgO. Now there are 2 Mg on the right, so put 2 in front of Mg:

Methane burning: CH₄ + O₂ → CO₂ + H₂O

Carbon balances. There are 4 H on the left, so put 2 in front of H₂O. Now there are $2 + 2 = 4$ O on the right, so put 2 in front of O₂:

Iron(III) oxide and carbon monoxide: Fe₂O₃ + CO → Fe + CO₂

1. Iron: 2 on the left, so put 2 in front of Fe.
2. Oxygen: the left has $3 + 1 = 4$ (with one CO) and the right has 2. Each CO gains one O to become CO₂, and Fe₂O₃ gives up 3 O, so use 3 CO and 3 CO₂:

Check: Fe 2 = 2, C 3 = 3, O $3 + 3 = 6$ and $3 \times 2 = 6$.

State symbols show the state of each substance: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.

Higher and Extended students also write ionic equations, showing only the ions that react. Every neutralisation of an acid by an alkali has the same ionic equation:

Balance these:

1. H₂ + O₂ → H₂O
2. Na + Cl₂ → NaCl
3. Al + O₂ → Al₂O₃
4. C₃H₈ + O₂ → CO₂ + H₂O

Answers: 2H₂ + O₂ → 2H₂O; 2Na + Cl₂ → 2NaCl; 4Al + 3O₂ → 2Al₂O₃; C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

• Changing a formula (writing H₂O₂ instead of 2H₂O) to make it balance.
• Forgetting that a coefficient multiplies every atom in the formula.
• Not rechecking every element after the last change.

Balanced equations give the mole ratios used in moles calculations.

1. Write the word equation: magnesium + hydrochloric acid → magnesium chloride + hydrogen.
2. Replace each name with its formula: Mg + HCl → MgCl₂ + H₂.
3. Balance: there are 2 Cl and 2 H on the right, so put 2 in front of HCl:

Neutralisation of sodium hydroxide with hydrochloric acid is already balanced: NaOH + HCl → NaCl + H₂O.