The shape of the lesson
By the end of the lesson, learners can
- Colours and states: Warm each halogen in the simulator and compare the colours and how easily each turns to vapour.
- Displacement reactions: Add each halogen solution to each halide solution in the simulator.
- Cl2 green gas, Br2 red-brown liquid, I2 grey-black solid.
- Down the group: darker, denser, less reactive.
- Displacement: only a more reactive halogen reacts.
- Colours: bromine orange, iodine brown in solution.
One picture every idea returns to
The Group VII halogens are diatomic non-metals: chlorine (Cl2) is a pale yellow-green gas, bromine (Br2) a red-brown liquid and iodine (I2) a grey-black solid. Down the group the colour gets darker, the density increases and the reactivity decreases. A more reactive halogen displaces a less reactive halogen from a solution of its halide: Cl2 + 2KBr → 2KCl + Br2.
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, Coloured, toxic and competitive.. The halogens exist as diatomic molecules and change from a green gas to a brown liquid to a black solid down the group. 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: Colours and states | Warm each halogen in the simulator and compare the colours and how easily each turns to vapour. Project tab 1 of the lab (Group 17 lab, colour and volatility) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Displacement reactions | Add each halogen solution to each halide solution in the simulator. Project tab 2 of the lab (Group 17 lab, displacement) and let a student drive it while the class predicts. | 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, 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 Displacement reactions, 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 a displacement reaction the colour change comes from the halogen that is formed, not the one added. | Aqueous bromine is orange and iodine solution is brown, so "the solution turns brown" with potassium iodide means iodine has been made. |
Group VII: the halogens: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Give the colour and state at room temperature of chlorine, bromine and iodine.
Chlorine water is added to potassium bromide solution. Describe what is seen, name the products and write the balanced equation.
Explain why iodine does not react with potassium chloride solution.
Predict three properties of astatine, the element below iodine in Group VII.
(Extended) Write the ionic equation for the reaction between bromine and potassium iodide solution, and state which species is reduced.
A student has three unlabelled solutions: potassium chloride, potassium bromide and potassium iodide. Describe how chlorine water could be used to identify them.
Answer key and marking notes
- Chlorine: pale yellow-green gas.
- Bromine: red-brown liquid.
- Iodine: grey-black solid.
- The solution turns orange (from colourless).
- Products: potassium chloride and bromine.
- Cl2 + 2KBr → 2KCl + Br2
- Iodine is less reactive than chlorine.
- A halogen can only displace a less reactive halogen from its compounds.
- It is a (black) solid at room temperature.
- It is denser than iodine.
- It is less reactive than iodine (it cannot displace iodine from iodides).
- Br2 + 2I− → 2Br− + I2
- Bromine is reduced (it gains electrons).
- Add chlorine water to each: no change with potassium chloride.
- Orange with potassium bromide (bromine formed); brown with potassium iodide (iodine formed).
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.
Which statement correctly describes the trend in the halogens from chlorine to iodine?
- AThe melting point increases and the colour gets darker.
- BThe reactivity increases and the colour gets darker.
- CThe melting point decreases and the colour gets lighter.
- DThe density decreases and the reactivity decreases.
Fluorine is the element above chlorine in Group VII. Which prediction about fluorine is most likely correct?
- AIt is a gas at room temperature and is more reactive than chlorine.
- BIt is a solid at room temperature and is less reactive than chlorine.
- CIt is a liquid at room temperature and is more reactive than chlorine.
- DIt is a gas at room temperature and is less reactive than chlorine.
Aqueous bromine is added to colourless potassium iodide solution and the mixture turns brown. What causes the brown colour?
- Aiodine formed in the reaction
- Bthe bromine that was added
- Cpotassium bromide formed in the reaction
- Dunreacted potassium iodide
A solution of halogen X is added to three potassium halide solutions. With potassium chloride there is no change; with potassium bromide the solution turns orange; with potassium iodide it turns brown. What is X?
- Achlorine
- Bbromine
- Ciodine
- Dastatine
Chlorine is bubbled into potassium iodide solution. Which half-equation shows the oxidation that takes place?
- A2I− → I2 + 2e−
- BCl2 + 2e− → 2Cl−
- CI2 + 2e− → 2I−
- DK+ + e− → K
Which ionic equation shows a reaction that actually takes place?
- ACl2 + 2At− → 2Cl− + At2
- BI2 + 2Br− → 2I− + Br2
- CBr2 + 2Cl− → 2Br− + Cl2
- DAt2 + 2I− → 2At− + I2
In the ionic equation for the reaction between chlorine and potassium bromide solution, why are potassium ions not shown?
- AThey are spectator ions and do not change during the reaction.
- BThey are oxidised to potassium atoms.
- CThey form a precipitate that is removed.
- DThey gain electrons from the chlorine.
Why is the reaction Cl2 + 2Br− → 2Cl− + Br2 described as a redox reaction?
- AChlorine gains electrons and bromide ions lose electrons.
- BChlorine loses electrons and bromide ions gain electrons.
- COxygen is added to the bromide ions.
- DBoth chlorine and bromide ions gain electrons.
Quiz key
Down Group VII the halogens change from a pale yellow-green gas (chlorine) to a red-brown liquid (bromine) to a grey-black solid (iodine), so the melting point rises and the colour darkens. Reactivity decreases down Group VII; an increase is the Group I trend carried over by mistake. Density increases down the group, it does not decrease.
Going up Group VII the melting and boiling points fall and the reactivity rises. Chlorine is already a gas, so fluorine is also a gas, and it is the most reactive halogen. A solid or a liquid would mean higher melting points than chlorine, and lower reactivity than chlorine would follow the Group I pattern, where reactivity increases down the group.
Bromine is more reactive than iodine, so it displaces iodine: Br2 + 2KI → 2KBr + I2. Iodine solution is brown, and that is the colour seen. Aqueous bromine is orange, so the colour of the halogen added would not explain brown. Potassium bromide and potassium iodide both form colourless solutions.
X displaces bromine (orange) and iodine (brown), so it is more reactive than both, yet it leaves potassium chloride unchanged. Only chlorine fits. Bromine cannot displace bromine from potassium bromide, so there would be no orange colour. Iodine and astatine are less reactive than bromine, so neither would change the potassium bromide solution.
Iodide ions lose electrons to become iodine molecules, and loss of electrons is oxidation: 2I− → I2 + 2e−. Chlorine gaining electrons is the reduction half of the same reaction. Iodine gaining electrons is the reverse of what happens, and potassium ions are spectator ions that do not change.
A halogen displaces only a less reactive halogen from its halide ions. Chlorine is far more reactive than astatine, which is at the bottom of Group VII, so chlorine displaces astatine. In each of the other equations the halogen molecule is less reactive than the halogen in the halide ion (iodine below bromine, bromine below chlorine, astatine below iodine), so no reaction occurs.
The potassium ions are K+(aq) at the start and still K+(aq) at the end, so they take no part and are left out: Cl2 + 2Br− → 2Cl− + Br2. Turning K+ into K atoms would be reduction, not oxidation, and does not happen here. Potassium chloride is soluble, so there is no precipitate, and chlorine gains electrons rather than giving them away.
Each chlorine atom gains one electron to become Cl− (reduction), while each bromide ion loses one electron to become part of Br2 (oxidation). Both processes happen together, so it is redox. The reverse electron transfer would turn chloride into chlorine, which does not happen. No oxygen is involved, and electrons cannot be gained by both species at once because they must come from somewhere.
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.