How to Write Net Ionic Equations
The step-by-step method, from molecular equation to complete ionic to canceling spectator ions.
A net ionic equation is the same reaction as a molecular equation, stripped down to only the species that actually react. Getting there is a five-step process, not a single leap, here's each step, worked through two full examples.
The Method
- Write and balance the molecular equation using full compound formulas.
- Identify which species are strong electrolytes in solution, soluble ionic compounds, strong acids, and strong bases, using solubility rules to check which ionic compounds actually dissolve.
- Write the complete ionic equation, splitting every strong electrolyte into its ions, but leaving solids, liquids, gases, and weak acids/bases written as whole formulas.
- Cancel spectator ions, any ion appearing in identical form on both sides.
- Write what's left as the net ionic equation, then check that both atoms and total charge balance on each side.
Worked Example 1: Precipitation Reaction
Aqueous lead(II) nitrate is mixed with aqueous potassium iodide.
Step 1, molecular equation:
Pb(NO₃)₂(aq) + 2 KI(aq) → PbI₂(s) + 2 KNO₃(aq)
Step 2, check solubility: nitrate and potassium salts are always soluble, so Pb(NO₃)₂, KI, and KNO₃ all split into ions. PbI₂ is a known insoluble precipitate, so it stays whole.
Step 3, complete ionic equation:
Pb²⁺(aq) + 2 NO₃⁻(aq) + 2 K⁺(aq) + 2 I⁻(aq) →
PbI₂(s) + 2 K⁺(aq) + 2 NO₃⁻(aq)
Step 4, cancel spectators: K⁺ and NO₃⁻ each appear unchanged on both sides, cross them out.
Step 5, net ionic equation:
Pb²⁺(aq) + 2 I⁻(aq) → PbI₂(s)
Charge check: left side = (+2) + 2(−1) = 0; right side = 0 (a solid has no net charge). Balanced.
Worked Example 2: Acid-Base Neutralization
Aqueous hydrochloric acid reacts with aqueous sodium hydroxide.
Molecular: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Complete ionic: HCl and NaOH are both strong electrolytes (a strong acid and a strong base),
and NaCl is soluble, so all three split into ions. Water is a liquid, so it stays whole.
H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)
Net ionic (after canceling Na⁺ and Cl⁻):
H⁺(aq) + OH⁻(aq) → H₂O(l)
This is the net ionic equation for every strong acid-strong base neutralization, the spectator ions
change depending on which acid and base you start with, but the actual reaction is always this one.
Common Mistakes
- Splitting a solid, liquid, gas, or weak acid/base into ions. Only aqueous strong electrolytes get split, everything else stays as a whole formula.
- Forgetting to check charge balance, not just atom balance. A net ionic equation with charged species has to balance charge on both sides, in addition to the usual mass balance.
- Canceling ions that aren't actually identical. Both the identity and the state (aq, s, l, g) have to match before an ion counts as a spectator.
- Writing a net ionic equation when no reaction actually happens. If nothing precipitates, no gas forms, and no water-forming neutralization occurs, mixing the solutions just leaves four spectator ions, not a new reaction.
Related Resources
- Unit 4 Review: Chemical Reactions
- AP Chemistry Solubility Rules
- AP Chemistry Naming Compounds
- AP Chemistry Oxidation Numbers
- How to Balance Redox Equations Using Half-Reactions
- AP Chemistry Study Guide
Frequently Asked Questions
What is a net ionic equation?
A net ionic equation shows only the species that actually participate in a reaction, leaving out spectator ions that appear unchanged on both sides. It's the most stripped-down of three equation forms: molecular, complete ionic, and net ionic, all of which represent the exact same reaction.
What is a spectator ion?
A spectator ion is an ion that's present in solution but doesn't actually react, it appears in the same form (same charge, same state) on both the reactant and product side, and cancels out when you write the net ionic equation.
Which species do you split into ions and which do you leave whole?
Split strong electrolytes dissolved in water: soluble ionic compounds, strong acids, and strong bases. Leave everything else whole: solids, liquids, gases, and weak acids or bases, since these don't fully dissociate into ions in solution.
How do you know a reaction actually happens (so there's a real net ionic equation to write)?
Check for one of three outcomes: a precipitate forms (using solubility rules), a gas is produced, or water forms from a strong acid-base neutralization. If none of those happen, mixing the solutions just gives you four spectator ions in solution and no real reaction, no net ionic equation to write beyond "no reaction."
Does a net ionic equation need to be charge-balanced, not just mass-balanced?
Yes, both. Unlike a molecular equation, a net ionic equation has charged species on it, so the total charge on the reactant side must equal the total charge on the product side, in addition to the usual atom-count balance.
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