The shape of the lesson
By the end of the lesson, learners can
- Formulae from models and charges: A model or diagram of a molecule lets you count the atoms directly.
- Word equations to symbol equations: Write the word equation, replace each name with its correct formula, then balance by changing only the numbers in front.
- Ionic equations and lattices: Extended content.
- Ionic formula: total positive charge = total negative charge.
- Brackets round a group such as OH− or NH4+ when there is more than one.
- Balance with the numbers in front only; count O atoms last in combustion.
- Ionic equation (Extended): leave out spectator ions; atoms and charges 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 O2− combine 2 : 3 as Al2O3. A balanced symbol equation has the same number of each type of atom on both sides. Balance it by changing only the big numbers in front of formulae, never the small numbers inside them. State symbols show the state of each substance: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
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.. A formula tells you which atoms are in a substance and in what ratio. 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 models and charges | A model or diagram of a molecule lets you count the atoms directly. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Word equations to symbol equations | Write the word equation, replace each name with its correct formula, then balance by changing only the numbers in front. | Pairs |
| 21 to 30 min | Teach and check: Ionic equations and lattices | Extended content. | 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, 16 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 and lattices, 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 small number inside a formula. | Writing H 2 O 2 to make H 2 + O 2 → H 2 O balance turns water into hydrogen peroxide, a different substance. Change only the numbers in front: 2H 2 + O 2 → 2H 2 O. |
Formulae and equations: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
Write the formulae of (a) calcium hydroxide, (b) ammonium sulfate, (c) aluminium oxide, (d) sodium carbonate.
Balance these equations: (a) Fe + Cl2 → FeCl3, (b) Na + H2O → NaOH + H2.
Methane burns completely in oxygen to form carbon dioxide and water. Write the word equation, then the balanced symbol equation with state symbols.
Extended. Write the ionic equation, with state symbols, for the neutralisation of dilute hydrochloric acid by aqueous sodium hydroxide, and name the spectator ions.
Extended. Magnesium reacts with dilute hydrochloric acid to form magnesium chloride solution and hydrogen. Write the balanced symbol equation with state symbols, then the ionic equation.
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) Ca2+ and OH−: Ca(OH)2.
- (b) NH4+ and SO42−: (NH4)2SO4.
- (c) Al3+ and O2−: Al2O3.
- (d) Na+ and CO32−: Na2CO3.
- (a) 2Fe + 3Cl2 → 2FeCl3 (2 Fe and 6 Cl on each side).
- (b) 2Na + 2H2O → 2NaOH + H2 (2 Na, 4 H and 2 O on each side).
- methane + oxygen → carbon dioxide + water.
- CH4 + 2O2 → CO2 + 2H2O (1 C, 4 H and 4 O on each side).
- State symbols: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l).
- H+(aq) + OH−(aq) → H2O(l).
- The spectator ions are Na+ and Cl−.
- Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g).
- Cl− is a spectator ion, so it is left out.
- Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g). Charge: +2 on each side.
- Changing the small number changes the substance: H2O2 is hydrogen peroxide, not water. Only the numbers in front 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.
Iron(III) sulfate contains Fe3+ and SO42− ions. What is its formula?
- AFe2(SO4)3
- BFe3(SO4)2
- CFeSO4
- DFe(SO4)3
How many atoms in total are shown in one formula unit of ammonium phosphate, (NH4)3PO4?
- A20
- B10
- C18
- D12
The diagram shows a model of one molecule. What is its molecular formula?
- AC2H6O
- BC2H5O
- CCH6O
- DC3H6
Propane burns completely: C3H8 + bO2 → 3CO2 + 4H2O. What is the value of b?
- A5
- B10
- C7
- D4
Lumps of calcium carbonate are added to dilute hydrochloric acid. Which equation has the correct state symbols?
- ACaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
- BCaCO3(s) + 2HCl(l) → CaCl2(aq) + H2O(l) + CO2(g)
- CCaCO3(s) + 2HCl(aq) → CaCl2(s) + H2O(l) + CO2(g)
- DCaCO3(aq) + 2HCl(aq) → CaCl2(aq) + H2O(aq) + CO2(g)
Which equation is not balanced?
- A2Al + 3O2 → 2Al2O3
- BN2 + 3H2 → 2NH3
- C2Na + Cl2 → 2NaCl
- DCaCO3 → CaO + CO2
Aluminium (forms Al3+) reacts with dilute hydrochloric acid to give aluminium chloride and hydrogen. Which is the correct balanced equation?
- A2Al + 6HCl → 2AlCl3 + 3H2
- BAl + 3HCl → AlCl3 + H2
- CAl + 2HCl → AlCl2 + H2
- D2Al + 6HCl → 2AlCl3 + 6H2
Aqueous sodium hydroxide is added to aqueous copper(II) sulfate and a blue precipitate of copper(II) hydroxide forms. What is the ionic equation?
- ACu2+(aq) + 2OH−(aq) → Cu(OH)2(s)
- BCu2+(aq) + OH−(aq) → Cu(OH)2(s)
- CCu2+(aq) + 2OH−(aq) → Cu(OH)2(aq)
- D2Na+(aq) + SO42−(aq) → Na2SO4(s)
Quiz key
2 × (+3) = +6 and 3 × (−2) = −6, so two Fe3+ ions balance three SO42− ions: Fe2(SO4)3. Fe3(SO4)2 swaps the numbers the wrong way (+9 and −4). FeSO4 is iron(II) sulfate, and Fe(SO4)3 has +3 against −6.
The bracket holds 1 N and 4 H, and the 3 multiplies both: 3 N and 12 H. Add 1 P and 4 O: 3 + 12 + 1 + 4 = 20. Ignoring the 3 gives 10; multiplying only the H gives 1 + 12 + 1 + 4 = 18; multiplying only the N gives 3 + 4 + 1 + 4 = 12.
There are 2 grey carbon atoms, 1 red oxygen atom and 6 blue hydrogen atoms (3 on the first carbon, 2 on the second and 1 on the oxygen), so the formula is C2H6O (this is ethanol). C2H5O misses the hydrogen on the oxygen, CH6O counts only one carbon, and C3H6 counts the oxygen atom as a carbon.
Right-hand side: 3CO2 has 6 O atoms and 4H2O has 4 O atoms, 10 in total. Each O2 supplies 2 atoms, so b = 10 ÷ 2 = 5. 10 is the number of O atoms, not O2 molecules. 7 adds the numbers of product molecules (3 + 4), forgetting that CO2 has two O atoms.
Calcium carbonate lumps are solid (s). Dilute hydrochloric acid is hydrogen chloride dissolved in water (aq), not a pure liquid (l). Calcium chloride is soluble, so it stays dissolved (aq), not (s). Water is a pure liquid (l), and carbon dioxide fizzes off as a gas (g). The last option wrongly treats insoluble calcium carbonate and water as (aq).
In 2Al + 3O2 → 2Al2O3 the oxygen balances (6 and 6), but there are 2 Al on the left and 4 on the right; it should be 4Al + 3O2 → 2Al2O3. The other three balance: 2 N and 6 H each side; 2 Na and 2 Cl each side; 1 Ca, 1 C and 3 O each side.
Al3+ needs three Cl−, so the product is AlCl3. In 2Al + 6HCl → 2AlCl3 + 3H2 there are 2 Al, 6 H and 6 Cl on each side. Al + 3HCl → AlCl3 + H2 has 3 H on the left but 2 on the right. Al + 2HCl → AlCl2 + H2 balances but uses the wrong formula. 6H2 gives 12 H on the right against 6 on the left.
Na+ and SO42− stay in solution as spectator ions, so they are left out. One Cu2+ joins with two OH− (charge +2 − 2 = 0) to form the precipitate, which is a solid (s). With only one OH− the equation does not balance; (aq) is wrong for a precipitate; and sodium sulfate is soluble, so it does 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.