Part 1: Ionic formulae and balanced equations

Build electrically neutral formulae, balance atoms, and remove spectator ions without changing the reacting species.

Recall the common ions

For simple ions, Group 1 metals form +1 ions, Group 2 +2, aluminium +3, Group 17 halides −1 and oxygen commonly −2. Do not apply this mechanically to transition metals with variable charges: use the name or information given.

Cancel total charge with whole ions

A formula unit must be electrically neutral. Aluminium sulfate needs two Al³⁺ ions and three SO₄²⁻ ions: total charges +6 and −6 give Al₂(SO₄)₃. Zinc nitrate is Zn(NO₃)₂; ammonium carbonate is (NH₄)₂CO₃. Brackets preserve a polyatomic ion when more than one is present.

Roman numerals specify oxidation state: iron(III) chloride is FeCl₃, whereas iron(II) chloride is FeCl₂. Do not assume every metal is +2 or put ion charges on the overall neutral salt.

Build a formula from the smallest neutral combination

Use the smallest common total charge, not an unexplained “swap the numbers” trick. Aluminium is Al³⁺ and oxide is O²⁻. The smallest shared magnitude is six: two aluminium ions contribute +6 and three oxide ions contribute −6. The formula is Al₂O₃. For Mg²⁺ and O²⁻ the smallest total is two, so MgO is sufficient; Mg₂O₂ is not the simplest formula ratio.

A polyatomic ion remains a group. Three nitrates contain N₃O₉, but the salt is written Al(NO₃)₃ to preserve that grouping. The 3 outside brackets multiplies every atom inside. A leading coefficient instead multiplies the entire formula: 2Al(NO₃)₃ represents two Al, six N and eighteen O atoms in the formula count.

Change coefficients, never chemical identities

Balance one element at a time using coefficients in front of formulae. Changing H₂O into H₂O₂ changes the substance. Check the final smallest whole-number ratio, all elements and total charge. State symbols identify solid (s), liquid (l), gas (g) and aqueous (aq), meaning dissolved in water, not simply liquid.

Worked example: aluminium reacts with hydrochloric acid to form aluminium chloride and hydrogen. Start Al + HCl → AlCl₃ + H₂, balance chlorine and hydrogen, then aluminium.

Balance a reaction while preserving the substances

Take Fe₂O₃ + CO → Fe + CO₂. Two iron atoms on the left require 2Fe on the right. If x CO molecules react, oxygen balance is 3 + x = 2x, so x = 3. The balanced equation is Fe₂O₃ + 3CO → 2Fe + 3CO₂: two Fe, three C and six O on both sides.

Now consider Al₂(SO₄)₃ + NaOH → Al(OH)₃ + Na₂SO₄. Balance Al to give 2Al(OH)₃, retain the sulfate groups to give 3Na₂SO₄, then use 6NaOH. Check H and O at the end. Treating unchanged polyatomic groups as blocks simplifies the bookkeeping without changing any chemical formula.

A coefficient is an amount ratio, not a licence to alter a subscript. Replacing MgO with MgO₂ balances a different chemical identity. A balanced equation also needs chemically plausible products; atom conservation alone cannot prove that a proposed reaction actually occurs.

Keep only the species that change

Separate soluble strong electrolytes in aqueous solution into their ions. Keep solids, liquids, gases and weakly ionised molecules intact. Cancel identical aqueous ions on both sides. Spectator ions remain chemically unchanged.

For AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), Na⁺ and NO₃⁻ cancel. The remaining equation shows the formation of a precipitate. A neutralisation between a strong acid and strong alkali similarly reduces to hydrogen ions reacting with hydroxide ions.

Show the full ionic equation before cancelling

For barium nitrate and sodium sulfate, begin Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq). Split the aqueous soluble salts: Ba²⁺ + 2NO₃⁻ + 2Na⁺ + SO₄²⁻ → BaSO₄(s) + 2Na⁺ + 2NO₃⁻. The nitrate and sodium ions are identical on both sides, so cancel them.

The net equation is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). Check atoms and charge: the left sums to zero and the solid is neutral. Do not split the precipitate into free ions, because its formation is the change the equation describes.

A solid carbonate behaves differently from dissolved carbonate ions. For calcium carbonate use CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + CO₂(g) + H₂O(l). Cancelling a “spectator calcium ion” would be wrong: the calcium begins in a solid and ends dissolved. Keep weak acids mainly molecular when writing a more detailed net equation; H⁺ + OH⁻ is the usual strong-acid/strong-alkali model.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources