The shape of the lesson
By the end of the lesson, learners can
- In alkali and in water: Bubble chlorine into cold dilute and hot concentrated sodium hydroxide in the simulator, then into water, and track the oxidation number of chlorine in each product.
- Treating water: Adding a small amount of chlorine to drinking water saves millions of lives, but it is not without risk.
- Cold dilute NaOH: NaCl + NaClO (−1 and +1).
- Hot conc. NaOH: NaCl + NaClO3 (−1 and +5).
- Water: HCl + HClO; HClO kills bacteria.
- Risk: chlorinated organic compounds.
One picture every idea returns to
Disproportionation is a redox reaction in which the same element is simultaneously oxidised and reduced. Chlorine (oxidation number 0) disproportionates in sodium hydroxide: in cold, dilute NaOH it forms NaCl (−1) and NaClO (+1), the mixture used as bleach; in hot, concentrated NaOH it forms NaCl (−1) and NaClO3 (+5). With water, Cl2 + H2O ⇌ HCl + HClO, and the chloric(I) acid kills bacteria, which is why chlorine is used to treat drinking 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, Oxidised and reduced at once.. In sodium hydroxide, some chlorine atoms gain electrons while others lose them. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: In alkali and in water | Bubble chlorine into cold dilute and hot concentrated sodium hydroxide in the simulator, then into water, and track the oxidation number of chlorine in each product. Project tab 4 of the lab (Group 17 lab, chlorine in water and alkali) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Treating water | Adding a small amount of chlorine to drinking water saves millions of lives, but it is not without risk. | Jigsaw |
| 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 Treating water, 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.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
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 |
|---|---|
| In disproportionation the same element is both oxidised and reduced, so chlorine appears in two products with different oxidation numbers. | Do not mix up the two NaOH conditions: cold dilute gives chlorate(I), NaClO; hot concentrated gives chlorate(V), NaClO 3. |
Chlorine: disproportionation and water treatment: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Write equations for the reaction of chlorine with (a) cold, dilute and (b) hot, concentrated aqueous sodium hydroxide.
Use oxidation numbers to explain why the reaction of chlorine with hot, concentrated NaOH is disproportionation.
Write the equation for the reaction of chlorine with water and name the product responsible for killing bacteria.
Discuss one benefit and two risks of adding chlorine to drinking water.
Calculate the mass of NaClO (Mr 74.5) formed when 1.42 g of Cl2 (Mr 71.0) reacts with excess cold, dilute NaOH.
Explain how the electrons balance in the reaction of chlorine with hot, concentrated NaOH.
Answer key and marking notes
- (a) Cl2 + 2NaOH → NaCl + NaClO + H2O.
- (b) 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O.
- Chlorine in Cl2 is 0.
- In NaCl it is −1 (reduced); in NaClO3 it is +5 (oxidised).
- The same element is oxidised and reduced in the same reaction.
- Cl2 + H2O ⇌ HCl + HClO.
- Chloric(I) acid, HClO.
- Benefit: it kills pathogenic bacteria, preventing waterborne diseases such as cholera.
- Risk: chlorine is toxic at high concentrations.
- Risk: it can react with organic compounds to form chlorinated organic compounds that may be harmful.
- n(Cl2) = 1.42 ÷ 71.0 = 0.0200 mol = n(NaClO).
- Mass = 0.0200 × 74.5 = 1.49 g.
- Five Cl atoms each gain one electron (0 → −1): five electrons gained.
- One Cl atom loses five electrons (0 → +5), so electrons lost equal electrons gained.
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.
Chlorine is bubbled into cold, dilute aqueous sodium hydroxide. Which equation is correct?
- ACl2 + 2NaOH → NaCl + NaClO + H2O
- B3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O
- CCl2 + 2NaOH → 2NaCl + H2O + ½O2
- DCl2 + NaOH → NaCl + HClO
In the reaction Cl2 + 2NaOH → NaCl + NaClO + H2O, what happens to the chlorine?
- Ait is both oxidised (0 to +1) and reduced (0 to −1), so this is a disproportionation
- Bit is only oxidised, from 0 to +1
- Cit is only reduced, from 0 to −1
- Dnothing, because chlorine has oxidation number −1 throughout
Chlorine is passed into hot, concentrated aqueous sodium hydroxide. Which equation is correct?
- A3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O
- BCl2 + 2NaOH → NaCl + NaClO + H2O
- C3Cl2 + 6NaOH → 3NaCl + 3NaClO + 3H2O
- D3Cl2 + 6NaOH → 6NaCl + 3H2O + O3
Chlorine is added to drinking water. Cl2(aq) + H2O(l) ⇌ HCl(aq) + HClO(aq). Which species is mainly responsible for killing bacteria?
- AHClO, chloric(I) acid
- BHCl, hydrochloric acid
- CCl− ions
- DH2O, because chlorine makes the water more polar
Which statement best describes the benefits and risks of chlorinating drinking water?
- Ait prevents water-borne diseases such as cholera, but chlorine can react with organic matter to form chlorinated organic compounds that may be harmful
- Bit has no risks at all, because chlorine is completely removed from the water before drinking
- Cit is harmful and has no benefit, because chlorine makes the water acidic
- Dits only purpose is to improve the taste and colour of the water
In the reaction 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O, what is the oxidation number of chlorine in NaClO3, and what is the ratio of chlorine atoms reduced to chlorine atoms oxidised?
- A+5; 5 : 1
- B+5; 1 : 5
- C+3; 5 : 1
- D+1; 1 : 1
Household bleach contains sodium chlorate(I) and sodium chloride. Why is it dangerous to mix bleach with an acidic toilet cleaner?
- Athe acid causes ClO− and Cl− to react, releasing toxic chlorine gas: ClO− + Cl− + 2H+ → Cl2 + H2O
- Bthe acid oxidises the chloride ions to chlorate(V), which explodes
- Cthe mixture releases hydrogen gas, which is flammable: ClO− + H+ → HCl + ½H2
- Dit is safe, because an acid and an alkali simply neutralise each other
What volume of chlorine, measured at room conditions, is needed to make 7.45 g of sodium chlorate(I), NaClO, by reaction with cold dilute sodium hydroxide? (Mr NaClO = 74.5; molar gas volume = 24.0 dm3 mol−1)
- A2.40 dm3
- B4.80 dm3
- C1.20 dm3
- D7.20 dm3
Quiz key
With cold dilute alkali, chlorine forms a chloride and a chlorate(I): Cl2 + 2NaOH → NaCl + NaClO + H2O (ionic: Cl2 + 2OH− → Cl− + ClO− + H2O). This is how bleach is made. The equation forming NaClO3 is the reaction with HOT CONCENTRATED alkali, not cold dilute. Chlorine does not liberate oxygen from cold alkali in this reaction. The last equation is unbalanced in hydrogen and oxygen; in excess alkali the HClO would in any case be neutralised to ClO−.
Chlorine starts at 0 in Cl2. In NaCl it is −1 (reduced) and in NaClO it is +1 (Na +1, O −2, so Cl = +1; oxidised). The same element is simultaneously oxidised and reduced, which is disproportionation. Saying chlorine is only oxidised ignores the NaCl; this is the classic error. Saying it is only reduced ignores the NaClO. Elemental chlorine is 0, not −1, and the oxidation numbers clearly change.
Hot concentrated alkali gives sodium chloride and sodium chlorate(V): 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O. Five Cl atoms are reduced (0 to −1) for every one oxidised (0 to +5), so the electrons balance. The NaClO equation is for cold dilute alkali. Tripling the cold equation still gives chlorate(I), not the product formed when hot. Ozone is not formed; the oxygen ends up in ClO3− and water.
Chlorine disproportionates in water, forming HCl (Cl −1) and HClO, chloric(I) acid (Cl +1). HClO (and ClO−) is a powerful oxidising agent that kills bacteria and other microorganisms, which is why chlorine is used in water treatment. HCl is formed in only tiny amounts and at very low concentration it has no useful disinfecting effect. Cl− ions are not oxidising agents, since chlorine in them is already in its lowest state. Water is the solvent; chlorine does not disinfect by changing its polarity.
Chlorination kills the microorganisms that spread diseases such as cholera and typhoid, a huge public-health benefit. The risk is that chlorine reacts with organic matter in the water to form chlorinated organic compounds (for example trichloromethane), some of which may be carcinogenic; chlorine gas itself is also toxic to handle. The benefits are judged to outweigh these risks at low concentrations. A small residual concentration of chlorine is deliberately left in the water to keep it safe in the pipes, so it is not 'completely removed', and there are risks. The tiny amount of acid formed has no significant effect, and the benefit is real. Chlorine is added to disinfect water, not to improve its taste (it often makes the taste worse).
In NaClO3, +1 + Cl + 3(−2) = 0, so Cl = +5. Of the six Cl atoms in 3Cl2, five go to NaCl (0 to −1, reduced, gaining 5 electrons in total) and one goes to NaClO3 (0 to +5, oxidised, losing 5 electrons), so the ratio reduced : oxidised is 5 : 1 and the electrons balance. 1 : 5 reverses the ratio. +3 would require only two oxygen atoms (chlorate(III)). +1 and 1 : 1 describe the reaction with cold dilute alkali, which forms NaClO, not NaClO3.
Adding H+ removes OH− and drives the reverse of the bleach-forming reaction: chlorine in ClO− (+1) and in Cl− (−1) comproportionate to Cl2 (0), ClO− + Cl− + 2H+ → Cl2 + H2O, releasing toxic chlorine gas. An acid such as HCl is not an oxidising agent that could make chlorate(V). No hydrogen is formed, and the equation given is not balanced for charge. Although neutralisation of excess alkali does happen, the redox reaction that releases chlorine makes the mixture dangerous, not safe.
n(NaClO) = 7.45 / 74.5 = 0.100 mol. The ratio Cl2 : NaClO is 1 : 1 (each Cl2 gives one NaCl and one NaClO), so n(Cl2) = 0.100 mol and V = 0.100 × 24.0 = 2.40 dm3. 4.80 dm3 uses a 2 : 1 ratio, confusing the 2NaOH with the chlorine. 1.20 dm3 halves the chlorine, as if each Cl2 gave two NaClO; one Cl atom goes to NaCl. 7.20 dm3 uses the 3 : 1 ratio of the hot concentrated reaction, which makes NaClO3, not NaClO.
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.