The shape of the lesson
By the end of the lesson, learners can
- The oxides: Burn each element in oxygen in the simulator, then add the oxide to water and test the pH.
- Acid or base?: Insoluble oxides cannot be tested with pH paper, so test them with acids and alkalis instead.
- The chlorides: Add each chloride to water in the simulator.
- Oxides: Na2O, MgO basic; Al2O3 amphoteric; rest acidic.
- Insoluble: Al2O3 and SiO2; test with acid and alkali.
- Covalent chlorides hydrolyse, giving HCl.
- Max oxidation number = outer electrons.
One picture every idea returns to
Across Period 3 the bonding in the oxides and chlorides changes from ionic (Na, Mg) to covalent (Si, P, S). The oxides go from basic (Na2O, MgO) through amphoteric (Al2O3) to acidic (SiO2, P4O10, SO2, SO3). Ionic chlorides dissolve to give neutral or near-neutral solutions; covalent chlorides are hydrolysed by water, producing HCl and strongly acidic solutions. The highest oxidation number in the oxides and chlorides equals the number of outer electrons.
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, From basic to acidic.. Drop the oxides of Period 3 into water and the pH slides from 14 down to 1. 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: The oxides | Burn each element in oxygen in the simulator, then add the oxide to water and test the pH. Project tab 3 of the lab (Periodicity lab, the oxides) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Acid or base? | Insoluble oxides cannot be tested with pH paper, so test them with acids and alkalis instead. | Pairs |
| 21 to 30 min | Teach and check: The chlorides | Add each chloride to water in the simulator. Project tab 4 of the lab (Periodicity lab, the chlorides) and let a student drive it while the class predicts. | Anticipation / Reaction Guide |
| 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, 15 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 The chlorides, 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 |
|---|---|
| Aluminium oxide and silicon dioxide are insoluble in water, so they do not change its pH; their acid-base character is shown by their reactions with acids and alkalis. | And AlCl 3 is not neutral in water: the small, highly charged Al 3+ ion polarises water molecules, releasing H +. |
Period 3: oxides and chlorides: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Write equations for the reactions of (a) sodium, (b) magnesium and (c) phosphorus with excess oxygen.
Describe the reactions, if any, of MgO, SiO2 and SO3 with water, giving the approximate pH in each case.
Write two equations to show that aluminium oxide is amphoteric.
Compare the behaviour of NaCl and SiCl4 when added to water, including equations and pH.
Give the oxidation number of the Period 3 element in (a) SO3, (b) PCl5 and (c) Al2Cl6, and state the connection with the group number.
Explain, in terms of bonding, why Na2O is basic but SO2 is acidic.
Answer key and marking notes
- (a) 4Na + O2 → 2Na2O.
- (b) 2Mg + O2 → 2MgO.
- (c) P4 + 5O2 → P4O10.
- MgO: slightly soluble, forming Mg(OH)2; pH about 9.
- SiO2: no reaction (insoluble giant covalent); pH 7.
- SO3 + H2O → H2SO4; pH about 1.
- With acid: Al2O3 + 6HCl → 2AlCl3 + 3H2O.
- With alkali: Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4.
- NaCl is ionic; it dissolves to give Na+(aq) and Cl−(aq); pH 7.
- SiCl4 is covalent and is hydrolysed: SiCl4 + 2H2O → SiO2 + 4HCl.
- The HCl gives a strongly acidic solution, pH about 2, and white fumes.
- (a) +6; (b) +5; (c) +3.
- In each case the maximum oxidation number equals the number of outer (valence) electrons of the element.
- Na2O is ionic; the O2− ion reacts with water to form OH− ions.
- SO2 is covalent; it reacts with water to form H2SO3, which releases H+ ions.
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.
In which compound does the Period 3 element have an oxidation number of +5?
- AP4O10
- BSO2
- CSiCl4
- DAl2O3
Sodium oxide is added to water. What is the approximate pH of the solution formed, and why?
- A13 to 14, because Na2O + H2O → 2NaOH, a strong alkali
- Babout 9, because sodium oxide is only slightly soluble in water
- C7, because sodium oxide is an ionic compound that simply dissolves
- Dabout 2, because the oxide ions form an acidic solution
Which statement describes the acid-base behaviour of aluminium oxide?
- Ait is amphoteric: it reacts with hydrochloric acid and also with sodium hydroxide solution
- Bit is basic: it reacts with hydrochloric acid but not with sodium hydroxide, like magnesium oxide
- Cit is acidic: it reacts with sodium hydroxide but not with hydrochloric acid, like silicon dioxide
- Dit is neither acidic nor basic, because it is insoluble in water
Which statement about silicon dioxide is correct?
- Ait is insoluble in water, but reacts with hot concentrated sodium hydroxide to form sodium silicate, Na2SiO3, so it is an acidic oxide
- Bit dissolves in water to give an acidic solution of pH about 3
- Cit reacts with hydrochloric acid, so it is a basic oxide
- Dit reacts with both hydrochloric acid and sodium hydroxide, so it is amphoteric
The graph shows the pH when the oxides and chlorides of the Period 3 elements are added to water. Why is the pH of the aluminium chloride mixture about 3?
- Athe small, highly charged Al3+ ion polarises the water molecules bonded to it, so [Al(H2O)6]3+ releases H+ ions (hydrolysis)
- Bit is a salt of a metal, so like sodium chloride it gives a neutral solution; the point should be at pH 7
- Caluminium chloride reacts with water to release chlorine, which forms an acidic solution
- Daluminium chloride is insoluble, so the acid comes from carbon dioxide dissolved in the water
0.0100 mol of silicon tetrachloride is added to water and the mixture is made up to 1.00 dm3. The reaction is SiCl4 + 2H2O → SiO2 + 4HCl. Assuming HCl is fully dissociated, what is the pH of the solution?
- A1.40
- B2.00
- C1.70
- D1.10
Phosphorus(V) chloride reacts with excess water: PCl5 + 4H2O → H3PO4 + 5HCl. How many moles of sodium hydroxide are needed to neutralise completely the solution formed from 1.00 mol of PCl5?
- A8.00
- B5.00
- C6.00
- D4.00
Which equation shows aluminium oxide behaving as an acidic oxide?
- AAl2O3 + 2NaOH + 3H2O → 2NaAl(OH)4
- BAl2O3 + 6HCl → 2AlCl3 + 3H2O
- CAl2O3 + 3H2O → 2Al(OH)3
- D2Al2O3 → 4Al + 3O2
Quiz key
In P4O10 each O is −2, so 4x = +20 and x = +5, equal to phosphorus's five outer electrons (group 15). SO2 contains sulfur in the +4 state; sulfur only reaches its maximum +6 in SO3. In SiCl4 each Cl is −1, so Si is +4 (group 14). In Al2O3 aluminium is +3 (group 13): 2x = +6.
Sodium oxide reacts vigorously with water: Na2O(s) + H2O(l) → 2NaOH(aq). The O2− ion takes a proton from water to give OH−, so the solution is strongly alkaline, pH 13 to 14. A pH of about 9 is typical of magnesium oxide, which is only slightly soluble; sodium oxide is very soluble. An ionic oxide does not give a neutral solution: unlike NaCl, it contains the strongly basic oxide ion. Acidic solutions come from the covalent oxides of non-metals such as P4O10 and SO3, not from ionic metal oxides.
Aluminium oxide is amphoteric. As a base: Al2O3 + 6HCl → 2AlCl3 + 3H2O. As an acid: Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4. Its bonding is ionic with considerable covalent character, which matches its position between the metals and the non-metals. Saying it is only basic misses the reaction with alkali; this is the commonest error, made because aluminium is a metal. Saying it is only acidic misses the reaction with acid. Being insoluble in water does not stop it reacting with acids and alkalis, so its acid-base character is tested with HCl and NaOH, not with water.
Silicon dioxide has a giant covalent structure, so it does not dissolve in water (the pH stays at 7). It is an acidic oxide: it reacts with hot concentrated NaOH, SiO2 + 2NaOH → Na2SiO3 + H2O. Saying it dissolves in water confuses it with the molecular non-metal oxides such as P4O10 and SO2; breaking its lattice needs far more energy than hydration could supply. It does not react with hydrochloric acid, so it is not basic, and for the same reason it is not amphoteric; the amphoteric oxide in Period 3 is Al2O3.
In water, Al3+ is surrounded by six water molecules. Its high charge density draws electron density from the O−H bonds, so a hydrated ion loses a proton: [Al(H2O)6]3+ ⇌ [Al(H2O)5(OH)]2+ + H+, giving pH about 3; with a little water, steamy HCl fumes are given off. Treating AlCl3 as neutral like NaCl is the classic error: Na+ has a low charge density and does not hydrolyse, but Al3+ does. The reaction is hydrolysis, not a redox change, so no chlorine is released. Aluminium chloride is soluble (it reacts with water); dissolved carbon dioxide gives a pH of about 5.6 at most, not 3.
Each mole of SiCl4 gives 4 mol HCl, so 0.0400 mol HCl in 1.00 dm3: [H+] = 0.0400 mol dm−3 and pH = −log 0.0400 = 1.40. The SiO2 formed is an insoluble white solid and does not affect the pH. 2.00 uses a 1 : 1 ratio, as if SiCl4 gave only one HCl. 1.70 uses a 1 : 2 ratio (0.0200 mol dm−3), perhaps from the 2H2O in the equation. 1.10 uses 0.0800 mol dm−3, double the correct amount of HCl.
1.00 mol PCl5 gives 5.00 mol HCl (needs 5.00 mol NaOH) and 1.00 mol H3PO4, which is tribasic: H3PO4 + 3NaOH → Na3PO4 + 3H2O (needs 3.00 mol). Total = 8.00 mol NaOH. 5.00 ignores the phosphoric acid altogether. 6.00 treats H3PO4 as monobasic (5 + 1). 4.00 is the number of moles of water used in the hydrolysis, which has nothing to do with the amount of alkali needed.
An acidic oxide reacts with a base to form a salt. With sodium hydroxide, aluminium oxide forms sodium tetrahydroxoaluminate, Na[Al(OH)4], so here it is acting as an acid. The reaction with HCl is real but shows aluminium oxide acting as a base, which is the other half of its amphoteric behaviour. Aluminium oxide is insoluble in water and does not react with it, so the equation with water does not happen. Splitting the oxide into aluminium and oxygen is decomposition (achieved by electrolysis of the molten oxide), not acid-base behaviour.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
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.