Equations are the grammar of Chemistry. If you can balance confidently and write ionic equations cleanly, you unlock marks across the whole Pure and Combined Chemistry syllabus: acids and bases, salt preparation, redox, electrolysis, stoichiometry and qualitative analysis. If you cannot, you lose easy marks in Paper 2 for reasons that have nothing to do with understanding the chemistry.
Here is the method our tutors use, plus the mistakes we see most often in marked scripts.
Step 1: Get the formulae right before you balance
You cannot balance a wrong equation. Most "balancing errors" are actually formula errors.
- Diatomic elements: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. Metals and carbon are written as single atoms.
- Ionic compounds: balance the charges first. Calcium hydroxide is Ca(OH)₂, not CaOH₂. Aluminium sulfate is Al₂(SO₄)₃.
- Know your polyatomic ions cold: NO₃⁻, SO₄²⁻, CO₃²⁻, OH⁻, NH₄⁺, HCO₃⁻, MnO₄⁻.
- Never change a subscript to balance. Only coefficients in front of formulae may change.
Step 2: A reliable balancing routine
- Count atoms of each element on both sides in a small table.
- Balance metals first, then non-metals, then hydrogen, then oxygen last.
- Treat polyatomic ions that appear unchanged on both sides as one unit.
- If you get stuck with a fraction, multiply the whole equation by 2.
- Re-count everything at the end, including charges if the equation involves ions.
Worked example: combustion of propane
C₃H₈ + O₂ → CO₂ + H₂O
Carbon: 3 on the left, so 3CO₂. Hydrogen: 8 on the left, so 4H₂O. Oxygen on the right is now 6 + 4 = 10, so we need 5O₂.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Worked example: aluminium and dilute sulfuric acid
Al + H₂SO₄ → Al₂(SO₄)₃ + H₂
Treat SO₄²⁻ as a block. Three blocks on the right means 3H₂SO₄. That gives 6 hydrogens, so 3H₂. Two aluminiums on the right means 2Al.
2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
Step 3: State symbols are marks, not decoration
Cambridge examiners frequently allocate a separate mark for correct state symbols, and questions often say "including state symbols".
- (s) solid, (l) pure liquid, (g) gas, (aq) dissolved in water.
- Concentrated or dilute acids in solution are (aq). Molten lead(II) bromide is (l), not (aq).
- Water formed in neutralisation is (l).
- A precipitate is (s), even though it sits in a beaker of solution.
Step 4: Writing ionic equations correctly
An ionic equation shows only the species that actually change. Follow four steps.
- Write the balanced full equation with state symbols.
- Split every aqueous ionic substance into its separate ions. Do not split solids, liquids, gases, or covalent molecules such as H₂O, CO₂, NH₃ and CH₃COOH.
- Cancel the spectator ions, the ones identical on both sides.
- Check that atoms and total charge balance.
Worked example: precipitation of barium sulfate
Full: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Split: Ba²⁺ + 2Cl⁻ + 2Na⁺ + SO₄²⁻ → BaSO₄(s) + 2Na⁺ + 2Cl⁻
Cancel Na⁺ and Cl⁻:
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Worked example: neutralisation
Any strong acid with any strong alkali reduces to the same thing:
H⁺(aq) + OH⁻(aq) → H₂O(l)
That is why the enthalpy change of neutralisation is roughly constant. Examiners love this link.
Worked example: metal carbonate with acid
CaCO₃ is insoluble, so it stays whole:
CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g)
Step 5: Half equations for electrolysis and redox
At the cathode, reduction happens, so electrons are added on the left. At the anode, oxidation happens, so electrons appear on the right.
- Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)
- Cathode in dilute solutions: 2H⁺(aq) + 2e⁻ → H₂(g)
- Anode: 2Cl⁻(aq) → Cl₂(g) + 2e⁻
- Anode with water: 4OH⁻(aq) → 2H₂O(l) + O₂(g) + 4e⁻
The number of electrons lost must equal the number gained when you combine half equations. Multiply one up if needed.
Common mistakes to stop making today
- Splitting an insoluble precipitate or a solid metal into ions.
- Splitting weak acids such as ethanoic acid, or splitting water and ammonia.
- Forgetting to cancel spectator ions, which loses the ionic equation mark.
- Leaving charges unbalanced, for example writing Fe²⁺ → Fe³⁺ + e⁻ but forgetting the electron.
- Changing subscripts, for example writing H₂O₂ to balance oxygen in a combustion equation.
- Omitting state symbols when the question explicitly asks for them.
Practise the right way
Do ten equations a day for a week: three balancing, three ionic, two half equations and two with state symbols only. Mark them against the answer immediately, and write down the specific reason for every error. Patterns show up fast, and so does confidence.
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