Ionic Equations & Half Equations
Summary: Ionic equations show only the particles that actually change in a reaction. Spectator ions are omitted. Half equations show electron transfer in oxidation and reduction separately. Both are cross-cutting skills tested across ALL topics. Tags: igcse chemistry equations ionic redox cross-cutting Created: 2026-07-14 Last Updated: 2026-07-14
Why Ionic Equations?
Full equations show ALL ions present, but many don’t participate in the reaction. Ionic equations strip out spectator ions to show only what actually changes.
Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Full ionic: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l) Net ionic: H⁺(aq) + OH⁻(aq) → H₂O(l) ← Na⁺ and Cl⁻ are spectator ions — they don’t change
How to Write Ionic Equations (Step by Step)
Step 1: Write the Balanced Full Equation
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Step 2: Split All Aqueous Substances into Ions
- AgNO₃(aq) → Ag⁺(aq) + NO₃⁻(aq)
- NaCl(aq) → Na⁺(aq) + Cl⁻(aq)
- NaNO₃(aq) → Na⁺(aq) + NO₃⁻(aq)
- AgCl(s) → AgCl(s) — solid stays as formula (not split)
Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)
Step 3: Cancel Spectator Ions
- Na⁺(aq) appears on both sides → cancel
- NO₃⁻(aq) appears on both sides → cancel
Step 4: Write the Net Ionic Equation
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
State Symbols — When to Split
| State Symbol | Split into Ions? | Reason |
|---|---|---|
| (aq) — aqueous solution | YES | Ionic compounds dissociate into free ions in water |
| (s) — solid | NO | Ions are locked in a lattice, not free to move |
| (l) — pure liquid | NO | No dissociation into ions |
| (g) — gas | NO | Molecules, not ions |
Key examples of (l) vs (aq):
- H₂O(l) — liquid water, NOT split: stays as H₂O
- NaCl(aq) — aqueous sodium chloride, SPLIT: Na⁺ + Cl⁻
- Br₂(l) — liquid bromine, NOT split: stays as Br₂
- HCl(aq) — aqueous hydrochloric acid, SPLIT: H⁺ + Cl⁻
Essential Ionic Equations to Memorize
These appear repeatedly in IGCSE exams:
Neutralisation
H⁺(aq) + OH⁻(aq) → H₂O(l)
- This is the ionic equation for ALL acid + alkali reactions
- The spectator ions change with each acid/alkali, but the net equation is always the same
Precipitation
| Precipitate | Ionic Equation | Colour |
|---|---|---|
| Silver chloride | Ag⁺(aq) + Cl⁻(aq) → AgCl(s) | White |
| Barium sulfate | Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) | White |
| Lead(II) iodide | Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s) | Yellow |
| Calcium carbonate | Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s) | White |
| Iron(II) hydroxide | Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s) | Green |
| Iron(III) hydroxide | Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) | Brown |
| Copper(II) hydroxide | Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) | Blue |
Acid + Carbonate
2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g)
Acid + Metal
2H⁺(aq) + Mg(s) → Mg²⁺(aq) + H₂(g)
Acid + Metal Oxide
2H⁺(aq) + CuO(s) → Cu²⁺(aq) + H₂O(l)
Half Equations (Electron Transfer)
Half equations show either oxidation (loss of electrons) or reduction (gain of electrons) separately.
OIL RIG
- Oxidation Is Loss of electrons
- Reduction Is Gain of electrons
How to Write Half Equations
- Balance the atom being oxidised/reduced
- Balance oxygen by adding H₂O
- Balance hydrogen by adding H⁺
- Balance charge by adding electrons (e⁻)
Key Half Equations to Memorize
Simple Metal Ion Reductions
| Half Equation | Process | Electrons |
|---|---|---|
| Na⁺ + e⁻ → Na | Reduction | 1 e⁻ gained |
| Mg²⁺ + 2e⁻ → Mg | Reduction | 2 e⁻ gained |
| Al³⁺ + 3e⁻ → Al | Reduction | 3 e⁻ gained |
| 2H⁺ + 2e⁻ → H₂ | Reduction | 2 e⁻ gained |
| Cl₂ + 2e⁻ → 2Cl⁻ | Reduction | 2 e⁻ gained |
| Cu²⁺ + 2e⁻ → Cu | Reduction | 2 e⁻ gained (at cathode) |
| Fe³⁺ + e⁻ → Fe²⁺ | Reduction | 1 e⁻ gained |
Electrolysis Half Equations (Aqueous Solutions)
| Electrode | Half Equation | Process |
|---|---|---|
| Cathode (−) | 2H⁺ + 2e⁻ → H₂ | Reduction (from water) |
| Anode (+) | 4OH⁻ → O₂ + 2H₂O + 4e⁻ | Oxidation (from water) |
| Anode (+) | 2Cl⁻ → Cl₂ + 2e⁻ | Oxidation (halide discharge) |
| Anode (+) | 2Br⁻ → Br₂ + 2e⁻ | Oxidation (halide discharge) |
| Anode (+) | 2I⁻ → I₂ + 2e⁻ | Oxidation (halide discharge) |
| Cathode (−) | Cu²⁺ + 2e⁻ → Cu | Reduction (copper plating) |
Displacement Reactions (as Half Equations)
Zinc displaces copper: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
- Oxidation: Zn → Zn²⁺ + 2e⁻ (zinc loses electrons)
- Reduction: Cu²⁺ + 2e⁻ → Cu (copper ions gain electrons)
Rusting of Iron
- Oxidation (at anode): Fe → Fe²⁺ + 2e⁻
- Reduction (at cathode): O₂ + 2H₂O + 4e⁻ → 4OH⁻
- Overall: Fe²⁺ + 2OH⁻ → Fe(OH)₂ → oxidised further to Fe₂O₃·xH₂O (rust)
Combining Half Equations
To get the full ionic equation from two half equations:
- Make the number of electrons equal in both half equations
- Add the two half equations
- Cancel the electrons (they appear on both sides)
Example: Electrolysis of molten NaCl
- Cathode: Na⁺ + e⁻ → Na
- Anode: 2Cl⁻ → Cl₂ + 2e⁻
Make electrons equal: multiply cathode equation by 2:
- 2Na⁺ + 2e⁻ → 2Na
- 2Cl⁻ → Cl₂ + 2e⁻
Add and cancel e⁻: 2Na⁺ + 2Cl⁻ → 2Na + Cl₂
Common Question Types
Type 1: Write the Ionic Equation for… (2-3 marks)
- Frequency: ~50% of papers
- Write full equation → split (aq) → cancel spectators → give net equation
- Most common: neutralisation (H⁺ + OH⁻ → H₂O) and precipitation
Type 2: Identify Spectator Ions (1-2 marks)
- Given full ionic equation, write which ions are spectators
- Spectators appear unchanged on both sides as (aq)
Type 3: Write Half Equations (2-3 marks)
- Frequency: ~30% of papers (electrolysis, redox topics)
- Balance atoms, then balance charge with e⁻
- Specify whether it’s oxidation or reduction
Type 4: Combine Half Equations (2-3 marks)
- Make electrons equal, add, cancel
Common Mistakes
- Splitting solids, liquids, or gases into ions: Only (aq) substances split
- Forgetting to balance charge: Half equations MUST have charges balanced
- Writing H₂O as H⁺ + OH⁻: H₂O(l) is covalently bonded — only split if told it’s (aq) electrolyte
- Forgetting state symbols: Ionic equations without (aq)/(s)/(l)/(g) lose marks
- Electrons on wrong side: Oxidation = electrons on RIGHT (lost), Reduction = electrons on LEFT (gained)
- Not cancelling ALL spectator ions: Check both sides carefully
Key Facts to Memorize
- H⁺(aq) + OH⁻(aq) → H₂O(l) — the single most-tested ionic equation
- Only aqueous (aq) substances are split into ions
- Spectator ions: appear identical on both sides of the equation
- OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
- Half equations show electron transfer
- A complete redox equation combines two half equations (electrons cancel)
Related Notes
- Acids and Bases — Neutralisation ionic equation
- Redox Reactions — Oxidation and reduction half equations
- Electrolysis — Half equations at electrodes (molten)
- Electrolysis of Aqueous Solutions — Half equations with water competition
- Qualitative Analysis — Precipitation ionic equations
- Rusting of Iron — Rusting half equations
- Displacement Reaction — Redox half equations in displacement
- Chemical Equations and Calculations — Writing balanced equations
- IGCSE-Chem-Index
IGCSE Chemistry (0620/0971) wiki. Cross-cutting skill — ionic equations are tested in every topic, not as a standalone subject.