The shape of the lesson
By the end of the lesson, learners can
- Formulae from ionic charges: Learn the charges of the common polyatomic ions.
- Balanced equations with state symbols: Write the correct formula of every reactant and product first, then balance by changing only the coefficients.
- Ionic equations: Split every soluble ionic compound in solution into its ions, cross out the ions that appear unchanged on both sides (the spectator ions), and write what is left.
- Ionic formula: total positive charge = total negative charge.
- Brackets round a polyatomic ion when there is more than one.
- Balance with coefficients only; leave O and H to last in combustion.
- Ionic equation: drop spectators; atoms and charge must both balance.
One picture every idea returns to
In an ionic compound the total positive charge equals the total negative charge, so Al3+ and SO42− combine 2 : 3 as Al2(SO4)3. A balanced equation has the same number of each atom on both sides; balance only with the large numbers in front, never by changing a formula. An ionic equation shows only the species that change, and its charges must balance as well as its atoms.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Balance the charges, then the atoms.. An ionic formula balances positive and negative charge. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Formulae from ionic charges | Learn the charges of the common polyatomic ions. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Balanced equations with state symbols | Write the correct formula of every reactant and product first, then balance by changing only the coefficients. | Pairs |
| 21 to 30 min | Teach and check: Ionic equations | Split every soluble ionic compound in solution into its ions, cross out the ions that appear unchanged on both sides (the spectator ions), and write what is left. | Rally Robin |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Ionic equations, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Rally Robin
Pairs take turns to say one item each, back and forth: a definition, a unit, a step of the method. Keep it fast and oral; the point is retrieval, not writing.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Never balance an equation by changing a subscript. | Writing H 2 O 2 to make H 2 + O 2 → H 2 O balance turns water into hydrogen peroxide, a different substance. And an ionic equation that balances atoms but not charge is still wrong: check the total charge on each side. |
Formulae and equations: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Write the formulae of (a) ammonium sulfate, (b) iron(III) hydroxide, (c) sodium hydrogencarbonate, (d) copper(II) nitrate.
Write a balanced equation for the complete combustion of butane, C4H10.
Write balanced equations, with state symbols, for (a) the thermal decomposition of calcium carbonate and (b) the reaction of magnesium with dilute hydrochloric acid.
Aqueous silver nitrate is added to aqueous sodium chloride. Write the ionic equation, with state symbols, and name the spectator ions.
Write the ionic equation, with state symbols, for the reaction of solid calcium carbonate with dilute hydrochloric acid. Explain why calcium carbonate is not written as separate ions.
A student balances H2 + O2 → H2O by writing H2 + O2 → H2O2. Explain why this is wrong and give the correct equation.
Answer key and marking notes
- (a) NH4+ and SO42−: (NH4)2SO4.
- (b) Fe3+ and OH−: Fe(OH)3.
- (c) Na+ and HCO3−: NaHCO3.
- (d) Cu2+ and NO3−: Cu(NO3)2.
- C4H10 + 6½O2 → 4CO2 + 5H2O (4 C, 10 H, then 8 + 5 = 13 O atoms).
- Doubling to clear the fraction: 2C4H10 + 13O2 → 8CO2 + 10H2O.
- (a) CaCO3(s) → CaO(s) + CO2(g).
- (b) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g).
- Ag+(aq) + Cl−(aq) → AgCl(s).
- The spectator ions are Na+ and NO3−.
- CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g). Charge: +2 on each side.
- Calcium carbonate is an insoluble solid, so its ions are not free in solution; only dissolved ionic compounds are split into ions.
- Changing the subscript changes the substance: H2O2 is hydrogen peroxide, not water. Only coefficients may be changed.
- 2H2 + O2 → 2H2O.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Magnesium forms Mg2+ ions and nitrogen forms nitride ions, N3−. What is the formula of magnesium nitride?
- AMg3N2
- BMgN
- CMg2N3
- DMg3N
Ethane burns completely in oxygen: aC2H6 + bO2 → cCO2 + dH2O. When the equation is balanced with the smallest whole numbers, what is b?
- A7
- B3.5
- C14
- D6
Aqueous sodium hydroxide is added to aqueous copper(II) sulfate and a pale blue precipitate forms. What is the ionic equation?
- ACu2+(aq) + 2OH−(aq) → Cu(OH)2(s)
- BCu2+(aq) + OH−(aq) → CuOH(s)
- CCu2+(aq) + 2OH−(aq) → Cu(OH)2(aq)
- D2Na+(aq) + SO42−(aq) → Na2SO4(s)
Which equation for the reaction of zinc with dilute sulfuric acid has correct state symbols?
- AZn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
- BZn(s) + H2SO4(aq) → ZnSO4(s) + H2(g)
- CZn(s) + H2SO4(l) → ZnSO4(aq) + H2(g)
- DZn(s) + H2SO4(aq) → ZnSO4(aq) + H2(aq)
Element X forms an ion X3+. What is the formula of the carbonate of X? (carbonate ion: CO32−)
- AX2(CO3)3
- BX3(CO3)2
- CXCO3
- DX2CO3
Zinc reduces iron(III) ions to iron(II) ions in aqueous solution. Which ionic equation is balanced for both atoms and charge?
- A2Fe3+ + Zn → 2Fe2+ + Zn2+
- BFe3+ + Zn → Fe2+ + Zn2+
- C2Fe3+ + 2Zn → 2Fe2+ + 2Zn2+
- D3Fe3+ + Zn → 3Fe2+ + Zn2+
Iron pyrite, FeS2, is roasted in air to form iron(III) oxide and sulfur dioxide: aFeS2 + bO2 → cFe2O3 + dSO2. When the equation is balanced with the smallest whole numbers, what is b?
- A11
- B5.5
- C8
- D22
Aqueous ammonium chloride is warmed with aqueous sodium hydroxide and a gas that turns damp red litmus blue is given off. What is the ionic equation?
- ANH4+(aq) + OH−(aq) → NH3(g) + H2O(l)
- BNH4Cl(aq) + NaOH(aq) → NaCl(aq) + NH3(g) + H2O(l)
- CNH4+(aq) + OH−(aq) → NH4OH(s)
- DNa+(aq) + Cl−(aq) → NaCl(s)
Quiz key
Three Mg2+ give +6 and two N3− give −6, so the formula is Mg3N2. MgN would carry an overall charge of −1. Mg2N3 swaps the numbers the wrong way round (+4 and −9). Mg3N gives +6 and only −3.
One C2H6 gives 2CO2 + 3H2O, which needs 4 + 3 = 7 O atoms, that is 3.5 O2. Doubling everything to clear the half gives 2C2H6 + 7O2 → 4CO2 + 6H2O, so b = 7. 3.5 is correct for one ethane molecule but is not a whole number. 14 is the number of oxygen atoms, not O2 molecules. 6 is the coefficient of water.
The precipitate is copper(II) hydroxide. One Cu2+ needs two OH− to make a neutral solid, Cu(OH)2(s); Na+ and SO42− are spectator ions. CuOH ignores the 2+ charge on copper. Writing (aq) for the product contradicts the observation that a precipitate forms. The sodium sulfate equation shows the spectator ions, which stay dissolved and do not react.
Dilute sulfuric acid is a solution in water, so it is (aq); zinc sulfate is soluble and stays dissolved, (aq); hydrogen is given off as bubbles of gas, (g). Writing ZnSO4(s) wrongly suggests a precipitate. (l) would mean pure liquid sulfuric acid, not the dilute solution. H2(aq) ignores the effervescence.
Two X3+ ions give +6 and three CO32− ions give −6, so the formula is X2(CO3)3. X3(CO3)2 swaps the numbers (+9 and −4). XCO3 would suit a 2+ ion. X2CO3 would suit a 1+ ion, such as in sodium carbonate.
Left: 2 × (+3) = +6. Right: 2 × (+2) + 2 = +6. Atoms and charge both balance, and it matches electrons: each Zn loses 2 electrons and each Fe3+ gains 1, so 2 Fe3+ are needed per Zn. The 1 : 1 equation has +3 on the left and +4 on the right. Doubling it keeps the mistake (+6 and +8). The 3 : 1 version gives +9 and +8, too many Fe3+ for the electrons zinc supplies.
Try 2FeS2 → Fe2O3 + 4SO2: the right has 3 + 8 = 11 O atoms, which is 5.5 O2. Doubling gives 4FeS2 + 11O2 → 2Fe2O3 + 8SO2, so b = 11. 5.5 is the fractional version, not whole numbers. 8 comes from balancing only the oxygen in the sulfur dioxide and forgetting the oxygen in the iron oxide. 22 is the number of oxygen atoms, not O2 molecules.
The alkaline gas is ammonia. The ammonium ion transfers H+ to the hydroxide ion, forming NH3 and water; Na+ and Cl− are spectators. The full equation with NH4Cl and NaOH is balanced but is not an ionic equation because it still contains the spectator ions. NH4OH(s) is not formed, and it would not explain the gas. Sodium chloride is soluble, so Na+ and Cl− do not precipitate.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.