AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/4/4-2)
Unit 4 · Topic 4.2
4.2 Net Ionic Equations
A balanced equation has the same number of each kind of atom, and the same total charge, on both sides. Reactions in water can be written as molecular, complete ionic or net ionic equations; the net ionic equation shows only the particles that actually change.
Key terms
- balanced equation
- molecular equation
- complete ionic equation
- net ionic equation
- spectator ion
- conservation of mass
Balanced equations conserve atoms and charge
A chemical equation shows atoms rearranging. Atoms can't be created or destroyed, so every element must have the same number of atoms on each side. That's why mass is conserved in a reaction. Charge is conserved too: the total charge on the left must equal the total charge on the right.
Balance equations by changing coefficients (the numbers in front of formulas), never subscripts. Changing a subscript changes the substance: H₂O₂ is hydrogen peroxide, not water.
State symbols tell you each substance's phase: (s) solid, (l) liquid, (g) gas and (aq) dissolved in water.
A balancing routine
- Write correct formulas for every reactant and product first.
- Balance elements that appear in only one compound on each side before elements that appear in several.
- If a polyatomic ion such as SO₄²⁻ or NO₃⁻ appears unchanged on both sides, balance it as a single unit.
- Leave elements that appear by themselves (like O₂ or Fe) for last, since changing their coefficient affects nothing else.
- Use the smallest whole-number coefficients, then recount every atom and the total charge.
Three ways to write a reaction in water
Take silver nitrate solution mixed with sodium chloride solution.
- Molecular equation: shows full formulas, as if nothing were ionized. AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
- Complete ionic equation: shows every strong electrolyte (a dissolved substance that breaks completely into ions) as separate ions. Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)
- Net ionic equation: removes spectator ions, the ions that appear unchanged on both sides (here Na⁺ and NO₃⁻). Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
What to split into ions, and what to keep together
Weak acids and bases are written whole because only a small fraction of their molecules ionize in water; most of the particles really are whole molecules.
| Write as separate ions | Keep as a whole formula |
|---|---|
| Soluble ionic compounds, like NaCl(aq) and KNO₃(aq) | Solids and precipitates, like AgCl(s) |
| Strong acids, like HCl(aq) and HNO₃(aq) | Weak acids, like CH₃COOH(aq) and HF(aq) |
| Strong bases, like NaOH(aq) | Weak bases, like NH₃(aq) |
| Soluble salts of weak acids, like NaF(aq) and CH₃COONa(aq) | Liquids, gases and molecular compounds, like H₂O(l), CO₂(g) and C₆H₁₂O₆(aq) |
Choosing the right form
Each form is useful for something. A molecular equation tells you which compounds to measure out in the lab. A net ionic equation shows what really reacts, and it reveals that different reactions can be the same at the particle level: mixing any soluble silver salt with any soluble chloride gives Ag⁺ + Cl⁻ → AgCl.
Physical changes can be written as equations too. Dissolving salt is NaCl(s) → Na⁺(aq) + Cl⁻(aq), and boiling water is H₂O(l) → H₂O(g).
Worked examples
Try each one yourself first, then open the solution.
- Example 1
A gas-forming reaction
Write the net ionic equation for the reaction of aqueous sodium carbonate with hydrochloric acid, which produces carbon dioxide gas and water.
Show the solutionHide the solution
- Step 1: Balanced molecular equation: Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g).
- Step 2: Complete ionic: 2Na⁺(aq) + CO₃²⁻(aq) + 2H⁺(aq) + 2Cl⁻(aq) → 2Na⁺(aq) + 2Cl⁻(aq) + H₂O(l) + CO₂(g). HCl is a strong acid, so it's written as ions; H₂O and CO₂ stay whole.
- Step 3: Cancel the spectators, 2Na⁺ and 2Cl⁻.
- Step 4: Check: 1 C, 3 O and 2 H on each side; charge is −2 + 2 = 0 on the left and 0 on the right.
Answer: CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g)
- Example 2
Don't split a weak acid (classic trap)
Write the net ionic equation for the reaction of acetic acid, CH₃COOH(aq), with aqueous sodium hydroxide.
Show the solutionHide the solution
- Step 1: Molecular: CH₃COOH(aq) + NaOH(aq) → NaCH₃COO(aq) + H₂O(l).
- Step 2: Acetic acid is a weak acid, so most of it is present as whole molecules: keep it together. NaOH is a strong base: split it into Na⁺ and OH⁻. Sodium acetate is soluble: split it into Na⁺ and CH₃COO⁻.
- Step 3: Complete ionic: CH₃COOH(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + CH₃COO⁻(aq) + H₂O(l).
- Step 4: Cancel Na⁺. Writing H⁺ + OH⁻ → H₂O here would be wrong, because there's very little free H⁺ in acetic acid solution.
Answer: CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l)
- Example 3
Checking charge balance
Is this equation balanced? Al(s) + Cu²⁺(aq) → Al³⁺(aq) + Cu(s). If not, balance it.
Show the solutionHide the solution
- Step 1: Atoms: 1 Al and 1 Cu on each side, so atoms balance.
- Step 2: Charge: left side is +2; right side is +3. Charge doesn't balance.
- Step 3: The least common multiple of 2 and 3 is 6: use 2 Al (making +6 on the right) and 3 Cu²⁺ (+6 on the left).
- Step 4: 2Al(s) + 3Cu²⁺(aq) → 2Al³⁺(aq) + 3Cu(s). Check: 2 Al, 3 Cu each side; charge +6 each side.
Answer: 2Al(s) + 3Cu²⁺(aq) → 2Al³⁺(aq) + 3Cu(s)
Common mistakes
- Splitting precipitates, weak acids, weak bases or gases into ions.
- Balancing atoms but not charge in net ionic equations.
- Leaving spectator ions in a net ionic equation, or cancelling ions that actually change phase.
- Changing subscripts to balance an equation.
On the exam
- Free-response questions frequently ask for a balanced net ionic equation. Include correct state symbols if asked, leave out spectators and make sure both atoms and charges balance.
- Precipitation questions will tell you which product is insoluble, or give you enough information to work it out. The only rule to memorize: compounds containing Na⁺, K⁺, NH₄⁺ or NO₃⁻ always dissolve.
Connected topics
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Check yourself
4 questions on 4.2 Net Ionic Equations. Pick an answer to see if you got it, and why.
When aqueous solutions of AgNO₃ and KCl are mixed, a white precipitate of AgCl forms. Which of the following is the net ionic equation for the reaction?
Which of the following is the net ionic equation for the reaction between aqueous acetic acid, CH₃COOH (a weak acid), and aqueous sodium hydroxide?
When aqueous Ba(NO₃)₂ is mixed with aqueous Na₂SO₄, a white precipitate of BaSO₄ forms. Which ions are spectator ions in this reaction?
Magnesium hydroxide, Mg(OH)₂, is an insoluble solid used in some antacids. Which of the following is the net ionic equation for its reaction with excess hydrochloric acid?
0 of 4 answered