E. Solutions and Solubility · Solutions and SolubilityTotal and Net Ionic Equations
E2.5 E2.1 A1.12About 65 minutes
Learning goals- I can write formula (molecular), total ionic and net ionic equations for precipitation reactions.
- I can identify spectator ions.
- I can write net ionic equations for neutralization reactions, including those with weak acids and gas-forming reactions.
- I can check that a net ionic equation is balanced for both atoms and charge.
Words to know
formula (molecular) equation
An equation that shows every reactant and product as a complete chemical formula.
total (complete) ionic equation
An equation that shows all high-solubility ionic compounds and strong acids as separate aqueous ions.
spectator ion
An ion that is present but does not change during the reaction; it appears unchanged on both sides of a total ionic equation.
net ionic equation
An equation that shows only the particles that actually change in a reaction.
neutralization
A reaction between an acid and a base that produces water (and a salt).
What really reacts?
When you mix silver nitrate and sodium chloride solutions, a white solid appears. Four kinds of ions were swimming around, but only two of them actually did anything. A net ionic equation cuts out the bystanders and shows the chemistry that happened. Water-treatment chemists use net ionic equations because they show exactly which ions must be added to remove a pollutant, no matter which compound they come from.
Remember from earlier lessons
Ionic compounds with high solubility dissociate in water: CaCl₂(aq) means Ca²⁺(aq) and 2 Cl⁻(aq) are present. The solubility table tells you which products are precipitates (s). Balancing equations was taught in Grade 10 and in the Chemical Reactions unit.
Three ways to write the same reaction
1. Formula equation (balanced, with states):
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
2. Total ionic equation: split every high-solubility ionic compound in solution (aq) into its ions. Leave solids, liquids, gases and molecular substances as formulas.
Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)
3. Net ionic equation: cross out spectator ions, which appear identically on both sides (here Na⁺ and NO₃⁻), and write what is left:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
This net ionic equation is true whenever any source of Ag⁺ meets any source of Cl⁻.
Rules for splitting into ions
Split into ions:
- high-solubility ionic compounds written (aq), e.g. NaCl(aq), Ba(NO₃)₂(aq), NaOH(aq);
- strong acids in solution: HCl, HBr, HI, HNO₃, HClO₄ and H₂SO₄ (in this course, write H₂SO₄(aq) as 2 H⁺ + SO₄²⁻). You will learn why these are 'strong' in the lesson on strong and weak acids.
Do not split:
- solids (s), including precipitates;
- liquids (l), especially H₂O(l);
- gases (g), such as CO₂(g);
- molecular compounds such as sugar, and weak acids such as acetic acid, CH₃COOH(aq), which stay mostly as molecules.
Check your net ionic equation: atoms balanced? Total charge the same on both sides?
Worked exampleExample 1: A precipitation reaction with coefficients
Write the formula, total ionic and net ionic equations when calcium chloride solution is mixed with sodium phosphate solution.
- Predict: swap partners → Ca₃(PO₄)₂ (low solubility, the precipitate) and NaCl (high).
- Formula equation, balanced: 3 CaCl₂(aq) + 2 Na₃PO₄(aq) → Ca₃(PO₄)₂(s) + 6 NaCl(aq).
- Total ionic: 3 Ca²⁺(aq) + 6 Cl⁻(aq) + 6 Na⁺(aq) + 2 PO₄³⁻(aq) → Ca₃(PO₄)₂(s) + 6 Na⁺(aq) + 6 Cl⁻(aq).
- Spectators: 6 Na⁺ and 6 Cl⁻. Net ionic: 3 Ca²⁺(aq) + 2 PO₄³⁻(aq) → Ca₃(PO₄)₂(s).
- Check charge: left 3(+2) + 2(−3) = 0; right 0 (a neutral solid). ✓
3 Ca²⁺(aq) + 2 PO₄³⁻(aq) → Ca₃(PO₄)₂(s)
Neutralization
A strong acid plus a strong base:
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Total ionic: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)
Net ionic: H⁺(aq) + OH⁻(aq) → H₂O(l)
Every strong acid + strong base neutralization has this same net ionic equation, unless the salt formed is a precipitate (see Example 2). (In the lesson on Arrhenius acids, you will write H⁺ more accurately as the hydronium ion, H₃O⁺, giving H₃O⁺(aq) + OH⁻(aq) → 2 H₂O(l).)
A weak acid stays as molecules, so it appears whole in the net ionic equation:
CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l)
Acids also react with carbonates to form a gas. With sodium carbonate and hydrochloric acid:
Na₂CO₃(aq) + 2 HCl(aq) → 2 NaCl(aq) + H₂O(l) + CO₂(g)
Net ionic: CO₃²⁻(aq) + 2 H⁺(aq) → H₂O(l) + CO₂(g)
Worked exampleExample 2: Neutralization that also makes a precipitate
Write the net ionic equation for sulfuric acid reacting with barium hydroxide solution.
- Formula equation: H₂SO₄(aq) + Ba(OH)₂(aq) → BaSO₄(s) + 2 H₂O(l).
- Total ionic: 2 H⁺(aq) + SO₄²⁻(aq) + Ba²⁺(aq) + 2 OH⁻(aq) → BaSO₄(s) + 2 H₂O(l).
- Look for spectators: none! Every ion ends up in either the precipitate or water.
- Net ionic = total ionic: 2 H⁺(aq) + SO₄²⁻(aq) + Ba²⁺(aq) + 2 OH⁻(aq) → BaSO₄(s) + 2 H₂O(l).
No spectator ions; the net ionic equation is the same as the total ionic equation.
Just for funSpectator ions are the chemistry version of fans at a hockey game: they're in the arena the whole time, they're very excited, but they never touch the puck, so they don't show up in the box score (the net ionic equation).
Hands-on activitySpot the spectators with baking soda
You need: baking soda (sodium hydrogen carbonate, NaHCO₃), white vinegar (acetic acid, CH₃COOH), a glass, a spoon, a notebook.
- Put 1 teaspoon of baking soda in the glass. Add 2 tablespoons of vinegar. Observe.
- Write the formula equation: NaHCO₃(s) + CH₃COOH(aq) → NaCH₃COO(aq) + H₂O(l) + CO₂(g).
- Decide what to split. Baking soda is a solid here; acetic acid is a weak acid; water and CO₂ are not split. Only the product sodium acetate is a high-solubility ionic compound in solution.
- Write the total ionic equation, then look for spectators.
What to look for: lots of fizzing (CO₂). Total ionic: NaHCO₃(s) + CH₃COOH(aq) → Na⁺(aq) + CH₃COO⁻(aq) + H₂O(l) + CO₂(g). There are no spectator ions because no ion appears on both sides, so the net ionic equation is the same. Extension: dissolve the baking soda in water first. Now NaHCO₃(aq) splits into Na⁺ and HCO₃⁻, Na⁺ becomes a spectator, and the net ionic equation is HCO₃⁻(aq) + CH₃COOH(aq) → CH₃COO⁻(aq) + H₂O(l) + CO₂(g). Keep the vinegar away from eyes.
Your turn
1Write the net ionic equation: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2 NaCl(aq).
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
2Identify the spectator ions in the last question.
3Write the total and net ionic equations for Pb(NO₃)₂(aq) + 2 KI(aq) → PbI₂(s) + 2 KNO₃(aq).
Total: Pb²⁺ + 2 NO₃⁻ + 2 K⁺ + 2 I⁻ → PbI₂(s) + 2 K⁺ + 2 NO₃⁻. Net: Pb²⁺(aq) + 2 I⁻(aq) → PbI₂(s).
4Write the net ionic equation for HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l).
H⁺(aq) + OH⁻(aq) → H₂O(l)
5Should CH₃COOH(aq) be split into ions in a total ionic equation? Why?
No. Acetic acid is a weak acid; most of it stays as molecules.
6Write the net ionic equation for CuSO₄(aq) + 2 NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq).
Cu²⁺(aq) + 2 OH⁻(aq) → Cu(OH)₂(s)
7Check the charge balance of 2 Ag⁺(aq) + CrO₄²⁻(aq) → Ag₂CrO₄(s).
Left: 2(+1) + (−2) = 0; right: 0. Balanced ✓.
More practice: warm-up
An ion that does not change; it appears unchanged on both sides.
2Is a precipitate split into ions in a total ionic equation?
No. Solids stay as formulas.
3How do you write water in an ionic equation?
4Write the ions in Mg(NO₃)₂(aq).
5What is the net ionic equation for any strong acid + strong base?
H⁺(aq) + OH⁻(aq) → H₂O(l) (as long as the salt formed stays dissolved)
More practice: on your own
1Write the formula, total ionic and net ionic equations for silver nitrate + potassium carbonate.
2 AgNO₃(aq) + K₂CO₃(aq) → Ag₂CO₃(s) + 2 KNO₃(aq). Total: 2 Ag⁺ + 2 NO₃⁻ + 2 K⁺ + CO₃²⁻ → Ag₂CO₃(s) + 2 K⁺ + 2 NO₃⁻. Net: 2 Ag⁺(aq) + CO₃²⁻(aq) → Ag₂CO₃(s).
2Write the net ionic equation for acetic acid reacting with potassium hydroxide.
CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l) (K⁺ is the spectator).
3Write the net ionic equation for calcium carbonate (s) reacting with hydrochloric acid.
CaCO₃(s) + 2 HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). Net: CaCO₃(s) + 2 H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g) (Cl⁻ is the spectator; the solid is not split).
4Two different formula equations have the net ionic equation Ag⁺(aq) + Br⁻(aq) → AgBr(s). Give one example.
Sample: AgNO₃(aq) + KBr(aq) → AgBr(s) + KNO₃(aq) (or AgNO₃ + NaBr, etc.).
5Write the net ionic equation for the removal of phosphate with alum (aluminium sulfate) at a sewage plant.
Al³⁺(aq) + PO₄³⁻(aq) → AlPO₄(s)
Challenge
1Write the net ionic equation for the reaction of aqueous iron(III) nitrate with aqueous sodium hydroxide, and explain why it would be the same if iron(III) chloride were used instead.
Fe(NO₃)₃(aq) + 3 NaOH(aq) → Fe(OH)₃(s) + 3 NaNO₃(aq). Net: Fe³⁺(aq) + 3 OH⁻(aq) → Fe(OH)₃(s). With FeCl₃, Cl⁻ replaces NO₃⁻ as a spectator, and spectators are cancelled, so the net equation is the same.
2Mixing solutions of magnesium sulfate and barium hydroxide forms two precipitates. Write the formula and net ionic equations.
MgSO₄(aq) + Ba(OH)₂(aq) → BaSO₄(s) + Mg(OH)₂(s). Both products are low solubility, so there are no spectators. Net: Mg²⁺(aq) + SO₄²⁻(aq) + Ba²⁺(aq) + 2 OH⁻(aq) → BaSO₄(s) + Mg(OH)₂(s).
3A student writes the net ionic equation for HCl(aq) + NaHCO₃(aq) as H⁺ + Na⁺ → NaH. Find two errors and write the correct one.
Errors: Na⁺ is a spectator and should cancel; NaH is not a product (the products are NaCl, H₂O and CO₂). Correct: H⁺(aq) + HCO₃⁻(aq) → H₂O(l) + CO₂(g).
Quiz
1. In AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), the spectator ions are
2. Which is NOT split into ions in a total ionic equation?
3. The net ionic equation for HCl(aq) + KOH(aq) is
4. The net ionic equation for Pb(NO₃)₂(aq) + 2 NaCl(aq) → PbCl₂(s) + 2 NaNO₃(aq) is
5. Acetic acid in a total ionic equation is written as
6. A correct net ionic equation must be balanced for
Big idea (E2.5): net ionic equations show only what changes; spectator ions are cancelled.
How to help: Have your teen highlight spectator ions in two colours on a printed total ionic equation and cross them out. Ask them to check the charges on each side.
Common difficulties:
- Splitting precipitates, water, gases or weak acids into ions.
- Forgetting coefficients when splitting (2 KI gives 2 K⁺ and 2 I⁻).
- Writing diatomic ions wrongly, e.g. Cl₂⁻ instead of 2 Cl⁻.
- Leaving out state symbols, which tell you what to split.
The vinegar and baking soda activity is safe; keep vinegar out of eyes.