Rules for Writing Ionic Reaction Equations
Write net ionic equations step by step: split aqueous strong electrolytes, cancel spectator ions, and check atoms and charge with a worked example.
Syllabus path: Properties of Substances and Reactions › Ionic Reactions & Testing
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CSCA Chemistry Concept & Principle Guide
Concept, principle, and explanation
How to write a net ionic equation
Direct answer: A net ionic equation keeps the species that change during a reaction and removes spectator ions present in the same form and amount on both sides. It need not contain only ions: a reacting weak acid, a precipitate, a gas or water may appear as a chemical formula. For an aqueous reaction, represent dissolved strong electrolytes as ions and conserve both atoms and total charge.
From the molecular equation to the net ionic equation
- Establish what reacts, then write a balanced molecular equation with states: aq for aqueous, s for solid, l for liquid and g for gas. Swapping ions between formulas does not by itself establish that a reaction occurs.
- Write dissolved strong electrolytes as separate ions. Normally keep weak acids, weak bases, nonelectrolytes, solids, gases and liquid water in formula form. Being soluble is not the same as ionizing completely.
- Cancel equal amounts of spectator ions in identical forms on both sides of the complete ionic equation. An ion bound in a solid is not the same species as its free aqueous form.
- Check each element's atom count and the total charge, then reduce the coefficients to the smallest whole-number ratio. In a redox reaction, electron loss and gain must also balance.
Worked example: mixing barium chloride and sodium sulfate solutions
Assume the solutions form a barium sulfate precipitate when mixed. In this aqueous reaction, barium chloride, sodium sulfate and sodium chloride are represented by their ions. Keep solid BaSO₄ intact. The three forms of the equation are shown below.
| Form | Equation |
|---|---|
| Molecular | BaCl₂(aq)+Na₂SO₄(aq) → BaSO₄(s)+2NaCl(aq) |
| Complete ionic | Ba²⁺(aq)+2Cl⁻(aq)+2Na⁺(aq)+SO₄²⁻(aq) → BaSO₄(s)+2Na⁺(aq)+2Cl⁻(aq) |
| Net ionic | Ba²⁺(aq)+SO₄²⁻(aq) → BaSO₄(s) |
Cancel 2Na⁺ and 2Cl⁻ from each side. The net ionic equation has one Ba atom, one S atom and four O atoms on each side. Its total charge is (+2)+(−2)=0 on the left and 0 on the right.
Common mistakes and a weak-acid counterexample
- Writing every acid as H⁺. Acetic acid is weak, so its reaction with hydroxide is CH₃COOH(aq)+OH⁻(aq) → CH₃COO⁻(aq)+H₂O(l). Do not split the acetic acid term into hydrogen and acetate ions.
- Splitting liquid water into H⁺ and OH⁻, or splitting BaSO₄(s) into free ions. Keep water and the precipitate in formula form here.
- Balancing atoms but overlooking charge. Omitting the charge on SO₄²⁻ would leave unequal total charges in this example.
- Assuming that any mixture of ions must react. If every species is unchanged and all terms cancel, there is no net ionic change to write.
References and example notes
- OpenStax Chemistry 2e: molecular and net ionic equations
- University of Texas at Austin: electrolytes and net ionic equations
Last reviewed: . The references explain the rules. AICSCA wrote the reaction walkthrough and checks; these are not official exam questions.
Related tutorial and examples
Ionic Reactions & Testing
Continue with ionic reactions and testing to study precipitates, gases and ion identification, choosing the appropriate species from their physical states.
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