4.2 Net Ionic Equations

Net Ionic Equations

Step through molecular, complete ionic, and net ionic equations for 8 reactions. Toggle spectator ion crossouts and see which solubility rules apply at each stage.

Spectator IonsPrecipitationAcid-BaseSolubility Rules
Topic 4.2

Net Ionic Equations

Represent changes in matter with a balanced chemical or net ionic equation: (i) for physical changes, (ii) for given information about the identity of the reactants and/or products, (iii) for ions in a given chemical reaction.

All physical and chemical processes can be represented symbolically by balanced equations — including phase changes, which are perfectly legitimate equations: H₂O(s) → H₂O(l).

Balancing is not a bookkeeping ritual; EK 4.2.A.2 explains why it is required. A chemical change rearranges atoms into new combinations, so any representation of it must contain equal numbers of atoms of every element before and after. Equations thereby demonstrate that both mass and charge are conserved.

EK 4.2.A.3 names three symbolic forms, all valid, chosen by context:

  1. Molecular (formula) equation — every species written as a complete neutral formula.
    AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
  2. Complete ionic equation — every strong electrolyte split into its ions.
    Ag⁺ + NO₃⁻ + Na⁺ + Cl⁻ → AgCl(s) + Na⁺ + NO₃⁻
  3. Net ionic equation — spectator ions removed, showing only what changes.
    Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

What gets split apart? Only strong electrolytes in aqueous solution: soluble ionic compounds, strong acids, and strong bases. Everything else stays written as a whole formula — solids, liquids, gases, weak acids, weak bases, and molecular solutes.

Spectator ions appear identically on both sides. Cancelling them is what makes the net ionic equation general: Ag⁺ + Cl⁻ → AgCl(s) describes every reaction that precipitates silver chloride, regardless of the counterions.

Net ionic equations must balance for charge as well as for atoms. Check both.

Key points

  • Balancing expresses conservation of both mass and charge.
  • Split only strong electrolytes into ions; leave solids, gases, liquids, and weak acids/bases intact.
  • Spectator ions appear unchanged on both sides and are cancelled.
  • A net ionic equation must balance charge as well as atoms.

Common mistakes

  • Splitting a precipitate into ions. AgCl(s) is a solid and stays whole.
  • Splitting a weak acid. CH₃COOH is written as a molecule; it barely ionizes.
  • Changing subscripts to balance. Only coefficients may change — altering a subscript changes the substance.
  • Forgetting to cancel the ion coefficients properly. If 2 Na⁺ appears on both sides, both cancel completely.

Worked example

Solid barium hydroxide is added to a solution of sulfuric acid. Write the balanced molecular, complete ionic, and net ionic equations.

Molecular:
Ba(OH)₂(s) + H₂SO₄(aq) → BaSO₄(s) + 2 H₂O(l)

Complete ionic: H₂SO₄ is a strong acid, so it is split. Ba(OH)₂ is written as a solid in this problem, so it is not split, and BaSO₄ is an insoluble product, so it is not split either.
Ba(OH)₂(s) + 2 H⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2 H₂O(l)

Net ionic: there are no spectator ions — every species changes form.
Ba(OH)₂(s) + 2 H⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2 H₂O(l)

Charge check: left = 2(+1) + (−2) = 0; right = 0 ✓

Full notes for topic 4.2 →