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General Chemistry · Reactions in solution

Net ionic equations practice, with worked answers

A net ionic equation shows only the particles that actually change. You get it by splitting every strong electrolyte into its ions, then crossing out the ions that look the same on both sides. Those are the spectator ions. The hard part is deciding what to split, and that comes down to a short list of rules.

Already know the basics? Jump to the 8 practice questions or the worked examples.

Five steps to a net ionic equation

  1. Write the balanced molecular equation with states: (s), (l), (g), (aq). Use the solubility rules to decide whether each product is (aq) or (s).
  2. Find the strong electrolytes. Only three kinds get split into ions: soluble ionic compounds (aq), strong acids, and strong bases.
  3. Write the complete ionic equation. Split each strong electrolyte. Keep the coefficients: 2 NaCl(aq) becomes 2 Na+(aq) + 2 Cl−(aq). Everything else stays whole: solids, liquids like water, gases, weak acids, and weak bases.
  4. Cancel the spectator ions, the ions that appear unchanged on both sides.
  5. Write what is left, reduced to the smallest whole-number coefficients. Check that atoms and charge both balance. If everything cancels, there is no net reaction.

Never split a solid, even an ionic one. CaCO3(s) and AgCl(s) stay together because their ions are locked in a crystal, not floating free in the solution.

What splits into ions: solubility rules, strong acids, strong bases

Solubility guidelines for ionic compounds in water (OpenStax style)
Ion in the compoundUsuallyExceptions
Group 1 cations (Li+, Na+, K+, Rb+, Cs+), NH4+SolubleNone
NO3−, C2H3O2− (acetate), HCO3−, ClO3−SolubleNone
Cl−, Br−, I−SolubleWith Ag+, Hg22+, Pb2+
F−SolubleWith group 2 cations, Pb2+, Fe3+
SO42−SolubleWith Ag+, Ba2+, Ca2+, Hg22+, Pb2+, Sr2+
CO32−, CrO42−, PO43−, S2−InsolubleWith group 1 cations or NH4+
OH−InsolubleWith group 1 cations or Ba2+

When two rules conflict, the "always soluble" rule for group 1, NH4+, and nitrate wins. Na2CO3 is soluble, for example.

Strong acids (OpenStax Chemistry 2e lists six): HCl, HBr, HI, HNO3, HClO4, and H2SO4. Sulfuric acid loses only its first proton completely; HSO4− is a weak acid. Some courses also count HClO3 as strong, so check your instructor's list. Every other acid is weak and stays molecular: HF, HC2H3O2 (acetic acid), HNO2, H2CO3, H3PO4, and so on.

Strong bases: the group 1 hydroxides (LiOH, NaOH, KOH, and so on) and the heavier group 2 hydroxides Ca(OH)2, Sr(OH)2, and Ba(OH)2. Ammonia, NH3, is a weak base and stays molecular.

Three reaction types and their usual net ionic equations

  • Precipitation: two soluble salts swap partners and one product is insoluble. The net ionic equation is just the ions forming the solid, for example Ag+(aq) + Cl−(aq) → AgCl(s).
  • Strong acid + strong base: always H+(aq) + OH−(aq) → H2O(l), whichever strong acid and base you use. (H3O+ is also fine in place of H+; then the product is 2 H2O.) If the acid is weak, the acid molecule appears in the net equation.
  • Gas forming: a carbonate or bicarbonate with an acid makes H2CO3, which falls apart into H2O(l) and CO2(g). With a soluble carbonate and a strong acid: CO32−(aq) + 2 H+(aq) → H2O(l) + CO2(g).

Worked examples

1. Precipitation: lead(II) nitrate and potassium iodide

Write the net ionic equation for mixing Pb(NO3)2(aq) and KI(aq).

  1. Swap partners and check solubility. KNO3 is soluble (K+, NO3−). PbI2 is not: iodides are soluble except with Ag+, Hg22+, and Pb2+.
  2. Molecular: Pb(NO3)2(aq) + 2 KI(aq) → PbI2(s) + 2 KNO3(aq).
  3. Complete ionic: Pb2+(aq) + 2 NO3−(aq) + 2 K+(aq) + 2 I−(aq) → PbI2(s) + 2 K+(aq) + 2 NO3−(aq).
  4. Cancel 2 K+ and 2 NO3−, which are the same on both sides.
  5. Check: one Pb and two I on each side. Charge: +2 + 2(−1) = 0 on the left, 0 on the right.

Answer: Pb2+(aq) + 2 I−(aq) → PbI2(s)

2. Neutralization with a weak acid: acetic acid and sodium hydroxide

Write the net ionic equation for HC2H3O2(aq) + NaOH(aq).

  1. Molecular: HC2H3O2(aq) + NaOH(aq) → NaC2H3O2(aq) + H2O(l).
  2. Acetic acid is weak, so it stays together. NaOH is a strong base and sodium acetate is a soluble salt, so both split.
  3. Complete ionic: HC2H3O2(aq) + Na+(aq) + OH−(aq) → Na+(aq) + C2H3O2−(aq) + H2O(l).
  4. Only Na+ cancels. Acetate does not cancel, because on the left it is part of the acid molecule.
  5. Check charge: −1 on each side.

Answer: HC2H3O2(aq) + OH−(aq) → C2H3O2−(aq) + H2O(l)

3. Gas forming: sodium carbonate and hydrochloric acid

Write the net ionic equation for Na2CO3(aq) + HCl(aq).

  1. The products are NaCl and H2CO3, and H2CO3 breaks down to H2O and CO2.
  2. Molecular: Na2CO3(aq) + 2 HCl(aq) → 2 NaCl(aq) + H2O(l) + CO2(g).
  3. Complete ionic: 2 Na+(aq) + CO32−(aq) + 2 H+(aq) + 2 Cl−(aq) → 2 Na+(aq) + 2 Cl−(aq) + H2O(l) + CO2(g).
  4. Cancel 2 Na+ and 2 Cl−.
  5. Check: C 1, O 3, H 2 on each side. Charge: −2 + 2 = 0 on the left, 0 on the right.

Answer: CO32−(aq) + 2 H+(aq) → H2O(l) + CO2(g)

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Practice questions

Pick an answer, then check it. A wrong answer tells you what went wrong, so you can try again. Every question also has a full solution.

Question 1What is the net ionic equation for BaCl2(aq) + Na2SO4(aq)?

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Molecular: BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2 NaCl(aq). Complete ionic: Ba2+ + 2 Cl− + 2 Na+ + SO42− → BaSO4(s) + 2 Na+ + 2 Cl−. Cancel 2 Na+ and 2 Cl−: Ba2+(aq) + SO42−(aq) → BaSO4(s).

Question 2Which ions are spectators when AgNO3(aq) and KCl(aq) are mixed?

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Products: AgCl and KNO3. Chlorides of Ag+ are insoluble, so AgCl(s) forms. KNO3 stays dissolved. The spectators are K+ and NO3−, and the net ionic equation is Ag+(aq) + Cl−(aq) → AgCl(s).

Question 3What is the net ionic equation for HNO3(aq) + KOH(aq)?

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Complete ionic: H+ + NO3− + K+ + OH− → K+ + NO3− + H2O(l). Cancel K+ and NO3−: H+(aq) + OH−(aq) → H2O(l).

Question 4What is the net ionic equation for HF(aq) + NaOH(aq)?

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Complete ionic: HF(aq) + Na+ + OH− → Na+ + F− + H2O(l). HF stays together because it is a weak acid, even though F− is dissolved on the right. Cancel Na+: HF(aq) + OH−(aq) → F−(aq) + H2O(l).

Question 5Solid calcium carbonate is added to hydrochloric acid. What is the net ionic equation?

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Molecular: CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g). Complete ionic: CaCO3(s) + 2 H+ + 2 Cl− → Ca2+ + 2 Cl− + H2O(l) + CO2(g). Only Cl− cancels. Charge check: +2 on each side.

Question 6Solutions of KNO3 and NaCl are mixed. What is the net ionic equation?

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Swapping partners gives KCl and NaNO3. Both are soluble (group 1 cations and nitrate have no exceptions). The complete ionic equation has the same four ions on each side, so everything cancels. There is no net ionic equation.

Question 7What is the net ionic equation when Fe(NO3)3(aq) is mixed with NaOH(aq)?

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Molecular: Fe(NO3)3(aq) + 3 NaOH(aq) → Fe(OH)3(s) + 3 NaNO3(aq). Cancel 3 Na+ and 3 NO3−. Net: Fe3+(aq) + 3 OH−(aq) → Fe(OH)3(s). Charge: +3 − 3 = 0 on the left, 0 on the right.

Question 8Which of these is written as separate ions in a complete ionic equation?

Show solution

Split only strong electrolytes: soluble ionic compounds, strong acids, and strong bases. HBr is a strong acid. Acetic acid and NH3 are weak, and PbCl2 is an insoluble solid, so those three stay whole.

Answer key

  1. A: Ba2+ + SO42− → BaSO4(s).
  2. B, K+ and NO3−: KNO3 stays dissolved.
  3. C: H+ + OH− → H2O(l).
  4. D: HF stays molecular because it is a weak acid.
  5. B: CaCO3(s) + 2 H+ → Ca2+ + H2O + CO2(g).
  6. A, no reaction: all products stay dissolved.
  7. C: Fe3+ + 3 OH− → Fe(OH)3(s).
  8. D, HBr: a strong acid.

Frequently asked questions

What is the difference between a complete ionic equation and a net ionic equation?

The complete ionic equation shows every strong electrolyte split into its ions, including spectators. The net ionic equation is what is left after you cancel the spectator ions.

Why don't weak acids get split into ions?

In solution, a weak acid such as HF or acetic acid stays mostly as intact molecules. Only a small fraction ionizes, so the molecule is the main species and that is what you write.

Is H2SO4 split into 2 H+ and SO4 2-?

Many intro courses do write it that way. Strictly, only the first proton ionizes completely; HSO4- is a weak acid. Follow the convention your instructor uses.

Are net ionic equations on the MCAT?

The AAMC content outline includes solution chemistry, including ions in solution and solubility, and acid-base reactions. You should be comfortable identifying spectator ions and precipitates, though full equation writing is rarely the whole question.

Sources

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