Worked example 1
Write the net ionic equation for the reaction of BaCl₂(aq) with Na₂SO₄(aq).
Try it first: Decide which product is the precipitate before writing anything out in ionic form.
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Chemical ReactionsNet Ionic EquationsContent level: Core 22 min
Writing Net Ionic Equations
What you'll be able to do: Convert a molecular equation into complete ionic and net ionic form by removing spectator ions.
Introduction
A molecular equation lists the bottles you poured. A net ionic equation shows the chemistry that actually happened. Learning to strip one down to the other is the single most useful skill in this unit.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Distinguish molecular, complete ionic and net ionic equations
- Decide which species to split into ions and which to leave whole
- Identify and cancel spectator ions
- Check that a net ionic equation balances in both mass and charge
Lesson
Three levels of detail
The molecular equation shows compounds as intact formulas. The complete ionic equation shows every dissolved strong electrolyte as separate ions. The net ionic equation shows only the species that change. All three describe the same reaction, but the net ionic form is the one that generalises across different starting salts.
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
What gets split
Split only strong electrolytes that are dissolved: soluble ionic compounds, strong acids and strong bases, all marked (aq). Keep intact anything marked (s), (l) or (g), plus weak acids, weak bases and water. A solid never dissociates on paper even if its ions are the whole point of the reaction.
Split (aq) strong electrolytes only. Solids, liquids, gases and weak electrolytes stay whole.
Spectator ions
An ion that appears in identical form and identical amount on both sides took no part in the change. Cancel it exactly as you would cancel a common factor. In a precipitation reaction the spectators are usually the always-soluble ions such as Na⁺, K⁺ and NO₃⁻.
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Charge must balance too
A correct net ionic equation balances atoms and total charge. In the silver chloride example the left side carries 1+ and 1-, a net of zero, and the right side is a neutral solid, also zero. If the charges do not match, a coefficient or a species has been dropped.
Sum the charges on each side as a final check. Unequal totals always mean an error.
Why the net form is powerful
The same net ionic equation describes many different experiments. Silver nitrate with sodium chloride, silver nitrate with potassium chloride and silver acetate with calcium chloride all reduce to the same one line, because the spectators were never the story.
Solubility rules and precipitationKeep weak bases molecular in ionic equations
Split soluble strong electrolytes into ions, but keep weak acids and weak bases as intact species. A weak base such as NH₃ reacts with H⁺ to make its conjugate acid; representing all NH₃ as NH₄⁺ and OH⁻ before it reacts would falsely treat it as fully dissociated. Cancel only ions that truly appear unchanged on both sides.
Weak electrolyte formulas stay together in the complete ionic line.
Key ideas
Definition
Complete ionic equation
An equation showing every dissolved strong electrolyte as its separate ions.
Definition
Spectator ion
An ion present in the same form on both sides of an equation, taking no part in the reaction.
Definition
Strong electrolyte
A substance that dissociates essentially completely into ions in water.
Rule
Keep it whole
Solids, pure liquids, gases, weak acids and weak bases are never split into ions.
Rule
Double balance
A net ionic equation must balance both the atoms and the total electric charge.
Worked examples
Worked example 2
Write the net ionic equation for HCl(aq) reacting with NaOH(aq).
Try it first: Both reactants are strong, but one product is not an electrolyte at all.
0 of 3 steps revealed.
Worked example 3
Write the net ionic equation for acetic acid, CH₃COOH(aq), reacting with KOH(aq).
Try it first: Ask whether every reactant here is a strong electrolyte.
0 of 3 steps revealed.
Worked example 4
Write the net ionic equation when aqueous NH₃ reacts with HCl(aq).
Try it first: HCl is a strong acid; NH₃ is a weak base.
0 of 3 steps revealed.
Common mistakes
Splitting the precipitate into ions because it is ionic.
Why it's wrong: The solid has left the solution, its ions are locked in a lattice.
Check instead: Only species marked (aq) may be split.
Writing a weak acid as separated ions.
Why it's wrong: Weak acids ionise only slightly, so almost all of the substance is present as molecules.
Check instead: Memorise the short list of strong acids, and treat everything else as weak.
Cancelling an ion that appears on both sides in different amounts.
Why it's wrong: Only the amount common to both sides is a spectator, the remainder is genuinely consumed or produced.
Check instead: Cancel the smaller amount and leave the difference in the equation.
Forgetting to check total charge.
Why it's wrong: An equation can balance atoms and still be impossible if charge is not conserved.
Check instead: Add up the charges on each side before finishing.
After the lesson · Question bank
Independent practice
Work through new problems on this topic on your own. The lesson's quick checks and worked examples stay above if you need to review a step.
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What you should now know
To write a net ionic equation, first balance the molecular equation, then split every strong electrolyte labelled (aq) into its ions while leaving solids, liquids, gases and weak electrolytes intact. Ions that appear unchanged on both sides are spectators and are cancelled. What remains is the net ionic equation, which must be balanced in both atoms and total charge.
- Split only aqueous strong electrolytes into ions
- Solids, liquids, gases and weak electrolytes stay whole
- Spectator ions are identical on both sides and are cancelled
- Net ionic equations must balance atoms and total charge
- One net ionic equation can describe many different experiments
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.