Ionic equations are a powerful way to focus on the actual chemical changes that occur in aqueous reactions. In IB Chemistry Topic 1 (Stoichiometry) and Topic 19 (HL Redox), ionic equations help simplify reactions by removing ions that do not participate in the chemical change. This skill is essential for precipitation reactions, acid–base reactions, and redox processes. Mastering ionic equations makes it easier to identify reacting species, determine stoichiometry, and understand reaction mechanisms.
What Is an Ionic Equation?
An ionic equation is a chemical equation that shows only the ions and molecules directly involved in a reaction.
It removes spectator ions, which are present in solution but do not take part in the reaction.
Ionic equations help you see:
- Which ions react
- Which products form
- What changes occur at the particle level
They highlight the true chemistry happening in solution.
Three Types of Equations You Should Know
When dealing with aqueous reactions, IB students typically encounter three levels of equations:
1. Molecular Equation
Shows all reactants and products in their full, undissociated form.
Example:
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
2. Complete Ionic Equation
Shows all aqueous ionic compounds dissociated into ions.
Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)
3. Net Ionic Equation
Removes spectator ions and shows only the reacting species.
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
This final form is the ionic equation.
How to Write an Ionic Equation (Step-by-Step)
Step 1: Write the balanced molecular equation
Start with the standard balanced reaction.
Step 2: Separate aqueous ionic substances into ions
Only aqueous ionic compounds dissociate.
Solids, liquids, and gases remain intact.
Step 3: Identify spectator ions
Spectator ions appear unchanged on both sides of the complete ionic equation.
Step 4: Remove spectator ions
Eliminate them to simplify the reaction.
Step 5: Write the net ionic equation
Include only species that actually react.
Which Substances Should Be Split Into Ions?
Split (aq) ionic compounds:
- NaCl(aq) → Na⁺ + Cl⁻
- K₂SO₄(aq) → 2K⁺ + SO₄²⁻
Do NOT split:
- Solids (s)
- Liquids (l)
- Gases (g)
- Weak acids (HF, CH₃COOH)
- Weak bases (NH₃)
These remain as molecules.
Common Examples of Ionic Equations
1. Precipitation reaction
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Net ionic:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
2. Acid–base neutralization
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Net ionic:
H⁺(aq) + OH⁻(aq) → H₂O(l)
3. Carbonate reaction
2HCl(aq) + Na₂CO₃(aq) → CO₂(g) + H₂O(l) + 2NaCl(aq)
Net ionic:
2H⁺(aq) + CO₃²⁻(aq) → CO₂(g) + H₂O(l)
4. Redox (HL)
Fe²⁺(aq) + MnO₄⁻(aq) + H⁺(aq) → Fe³⁺(aq) + Mn²⁺(aq) + H₂O(l)
This is the net ionic form already.
Why Ionic Equations Matter in IB Chemistry
Ionic equations help you:
- Identify the actual reacting species
- Simplify complex reactions
- Recognize patterns (e.g., all neutralizations give H⁺ + OH⁻ → H₂O)
- Understand precipitation, acid–base, and redox reactions
- Improve accuracy in stoichiometry problems
They also appear frequently in Paper 1 multiple-choice and Paper 2 structured questions.
Common IB Misunderstandings
“All compounds split into ions.”
Only aqueous ionic substances dissociate.
“Spectator ions react but don’t appear in products.”
Spectator ions do not react at all.
“Weak acids and bases dissociate fully.”
Incorrect—they remain intact in ionic equations.
“Solids and liquids must be split.”
Never split solids, liquids, or gases.
FAQs
How do I identify spectator ions quickly?
Look for ions that appear unchanged on both sides of the complete ionic equation.
Why don’t solids dissociate?
Because ions in solids are locked in a lattice and cannot separate.
Do ionic equations need to be balanced?
Yes—balance both atoms and charges.
Conclusion
An ionic equation shows only the species that actually take part in a reaction. By removing spectator ions and focusing on the essential chemistry, ionic equations make reactions clearer and easier to understand. Mastering them is vital for precipitation, neutralization, and redox topics throughout the IB Chemistry syllabus.
