Three reactions that are really one
Add silver nitrate solution to sodium chloride solution, to potassium chloride solution, or to hydrochloric acid, and the same white precipitate, AgCl, forms every time. The three equations look different, but only two ions actually change: Ag⁺ and Cl⁻ combine into AgCl. The Na⁺, K⁺, H⁺, and NO₃⁻ ions just watch from the solution. A net ionic equation says exactly that.
Reactions in solution are reactions between ions
First: strong and weak electrolytes. A strong electrolyte turns almost completely into ions in water. Strong acids (HCl, H₂SO₄, HNO₃), strong bases (NaOH, KOH, Ba(OH)₂), and soluble salts are strong electrolytes: hydrochloric acid contains almost no HCl molecules, only H⁺ and Cl⁻. A weak electrolyte turns only a small fraction into ions and stays mostly as molecules; examples are acetic acid (CH₃COOH), aqueous ammonia (NH₃), and water. So when you write ionic equations, split strong electrolytes into ions and keep weak electrolytes as formulas. Lesson 2.7.5 covers electrolytes in detail.
Soluble salts, strong acids, and strong bases exist almost entirely as ions in water (they are strong electrolytes, 2.7.5). So when AgNO₃(aq) and NaCl(aq) are mixed, the solution contains Ag⁺, NO₃⁻, Na⁺, and Cl⁻. Ag⁺ and Cl⁻ leave the solution as a precipitate; Na⁺ and NO₃⁻ are unchanged.
The same reaction can be written three ways:
- The molecular equation化学方程式 writes every substance as a complete formula.
- The complete ionic equation完整的离子方程式 splits every strong electrolyte into its ions.
- The net ionic equation离子方程式 leaves out the spectator ions旁观离子, the ions that appear unchanged on both sides:
Chinese textbooks call only the last one a 离子方程式 (“ionic equation”); the complete ionic equation is just a step on the way.
Four steps
Chinese textbooks summarize the method in four words, 写、拆、删、查: write, split, cancel, check.
- Write the balanced molecular equation.
- Split strong electrolytes in solution (strong acids, strong bases, and soluble salts) into ions. Keep as formulas: solids (precipitates), gases, water, weak acids such as CH₃COOH and H₂CO₃, weak bases such as NH₃, oxides, and elements.
- Cancel the spectator ions, and reduce the coefficients to the lowest whole numbers.
- Check that each element and the total charge balance.
One net ionic equation, a whole family of reactions
H⁺(aq) + OH⁻(aq) → H₂O(l) describes not just hydrochloric acid and sodium hydroxide, but every reaction of a strong acid with a strong base that forms a soluble salt. It does not describe Ba(OH)₂ + H₂SO₄, which also forms a BaSO₄ precipitate, or acetic acid + sodium hydroxide, because acetic acid is weak and is not split.
Reactions between ions happen when they form a precipitate, a gas, or water. These are exactly the conditions for double replacement from 1.8.6. Single replacement reactions can also be written as net ionic equations, for example Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s).
Pick a reaction and step from the molecular equation to the net ionic equation. Look closely at H₂SO₄ + KOH, Ba(OH)₂ + H₂SO₄, and KNO₃ + NaCl:
Example 1: two precipitates, no spectators
Write the net ionic equation for the reaction of copper(II) sulfate solution with barium hydroxide solution.
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Write. Start with the balanced molecular equation: CuSO₄(aq) + Ba(OH)₂(aq) → BaSO₄(s) + Cu(OH)₂(s).
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Split. Strong electrolytes in solution (strong acids, strong bases, soluble salts) are written as ions; solids, weak electrolytes, gases, and water stay as formulas. CuSO₄ and Ba(OH)₂ split; both products are precipitates:
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Cancel. No ion appears on both sides, so nothing cancels; the complete ionic equation is also the net ionic equation.
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Check. The atoms balance, and the charge on the left is
the same as on the right.
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A common mistake. Writing only Ba²⁺ + SO₄²⁻ → BaSO₄ and leaving out the Cu(OH)₂ that forms at the same time. All four ions react.
Example 2: removing scale with vinegar
Kettle scale is mostly CaCO₃, and vinegar, which contains acetic acid, removes it. Write the net ionic equation.
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Write. CaCO₃(s) + 2CH₃COOH(aq) → Ca(CH₃COO)₂(aq) + H₂O(l) + CO₂(g).
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Split. Go one at a time: CaCO₃ is a solid, so it stays; CH₃COOH is a weak acid and is mostly molecules, so it stays; H₂O and CO₂ stay. Only the soluble salt Ca(CH₃COO)₂ splits, into Ca²⁺ and 2CH₃COO⁻.
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Cancel. There are no spectator ions:
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Check. The atoms balance. The charge is 0 on the left and (+2) + 2(−1) = 0 on the right.
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Compare. HCl is a strong acid, so with hydrochloric acid the equation would be CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g). With weak acetic acid, CH₃COOH has to stay as a formula.
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A common mistake. Splitting CaCO₃ into CO₃²⁻ or acetic acid into H⁺. Split a substance only if it is both soluble and a strong electrolyte.
- Marble + hydrochloric acid: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂
CaCO₃ is a solid, so it stays as a formula, and atoms and charge balance.
- Marble + hydrochloric acid: CO₃²⁻ + 2H⁺ → H₂O + CO₂
CaCO₃ is insoluble, so it cannot be split into Ca²⁺ and CO₃²⁻. This equation is for a soluble carbonate reacting with a strong acid.
- Hydrochloric acid + sodium hydroxide: H⁺ + OH⁻ → H₂O
A strong acid and a strong base form a soluble salt and water. Correct.
- Acetic acid + sodium hydroxide: H⁺ + OH⁻ → H₂O
Acetic acid is weak, so write CH₃COOH: CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O.
- Copper + silver nitrate: Cu + Ag⁺ → Cu²⁺ + Ag
The charge does not balance: +1 on the left, +2 on the right. It should be Cu + 2Ag⁺ → Cu²⁺ + 2Ag.
- Copper(II) sulfate + sodium hydroxide: CuSO₄ + 2OH⁻ → Cu(OH)₂ + SO₄²⁻
CuSO₄ is soluble and must be split; SO₄²⁻ is then a spectator. The answer is Cu²⁺ + 2OH⁻ → Cu(OH)₂.
- Iron + dilute sulfuric acid: Fe + 2H⁺ → Fe²⁺ + H₂
Iron forms Fe²⁺, and atoms and charge balance. Correct.
- Aluminum + hydrochloric acid: Al + 3H⁺ → Al³⁺ + H₂
The charge balances, but there are 3 H atoms on the left and 2 on the right. It should be 2Al + 6H⁺ → 2Al³⁺ + 3H₂.
- Silver nitrate + sodium chloride: Ag⁺ + Cl⁻ → AgCl
Na⁺ and NO₃⁻ are spectators. Correct.
Common mistakes
Mistake 1: “Anything that dissolves gets split into ions.” Split only substances that dissolve and are strong electrolytes. Acetic acid and ammonia dissolve well but only a small fraction ionizes, so they stay as formulas; sugar and ethanol dissolve without ionizing at all.
Mistake 2: “H⁺ + OH⁻ → H₂O describes every neutralization.” It only describes a strong acid and a strong base that form a soluble salt. Ba(OH)₂ and H₂SO₄ also form a BaSO₄ precipitate: Ba²⁺ + 2OH⁻ + 2H⁺ + SO₄²⁻ → BaSO₄ + 2H₂O.
Mistake 3: “If the atoms balance, the equation is right.” A net ionic equation must balance charge too. Cu + Ag⁺ → Cu²⁺ + Ag has balanced atoms but unbalanced charge; the correct equation is Cu + 2Ag⁺ → Cu²⁺ + 2Ag.
Quick check
Which reaction has the net ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l)?
KOH is a strong base, HNO₃ is a strong acid, and KNO₃ is soluble. The others form a BaSO₄ precipitate, start from insoluble Cu(OH)₂, or use weak acetic acid.
What is the net ionic equation for the reaction of Na₂CO₃(aq) with CaCl₂(aq)?
Na⁺ and Cl⁻ are spectators. Cancel them and only Ca²⁺ + CO₃²⁻ → CaCO₃(s) is left. NaCl is soluble, so it never appears as a formula.
Summary
- Reactions in solution happen between ions. Spectator ions appear unchanged on both sides; the net ionic equation shows only the particles that react.
- Molecular → complete ionic → net ionic: split only strong acids, strong bases, and soluble salts; keep solids, gases, water, weak electrolytes, oxides, and elements as formulas; cancel spectators; check atoms and charge.
- One net ionic equation stands for a family of reactions; H⁺ + OH⁻ → H₂O covers every strong acid–strong base reaction that forms a soluble salt.
On to AP (for later)
Almost every year, AP Chemistry free-response questions ask you to “write the balanced net ionic equation,” with state symbols and only the species that react. The strong acids to know are HCl, HBr, HI, HNO₃, H₂SO₄, and HClO₄; the strong bases are the Group 1 hydroxides plus Ca(OH)₂, Sr(OH)₂, and Ba(OH)₂. AP questions often write H⁺(aq) as H₃O⁺(aq).