The shape of the lesson
By the end of the lesson, learners can
- Monomer to polymer: Add reagents across the double bond of an alkene in the simulator, then link many alkene molecules together to make a polymer.
- Polymer to monomer: Extended.
- The problem with plastics: Plastics are useful because they are unreactive and durable.
- Polymer: large molecule made of many monomers.
- Addition: alkene C=C opens; polymer is the only product.
- Extended, monomer: two backbone carbons, put the C=C back.
- Disposal: landfill, oceans, toxic gases on burning; recycle.
One picture every idea returns to
Polymers are large molecules built up from many small molecules called monomers. Many plastics are polymers made from chemicals obtained from crude oil. In addition polymerisation, alkene monomers join when their C=C double bonds open; the polymer is the only product. Ethene forms poly(ethene), whose repeat unit is –CH2–CH2–. Extended: draw the polymer from a given alkene, and deduce the monomer from a section of polymer. Plastics are hard to dispose of: they are non-biodegradable and fill landfill sites, they accumulate in oceans, and burning them can release toxic gases. Recycling reduces these problems.
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, Thousands of alkenes, one chain.. Open the C=C double bond of ethene and the molecules link into long chains of poly(ethene), the plastic in bags and bottles. 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: Monomer to polymer | Add reagents across the double bond of an alkene in the simulator, then link many alkene molecules together to make a polymer. Project tab 5 of the lab (Hydrocarbons lab, alkene reactions) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Polymer to monomer | Extended. | Pairs |
| 21 to 30 min | Teach and check: The problem with plastics | Plastics are useful because they are unreactive and durable. | Cooperative Concept Mapping |
| 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, 13 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 problem with plastics, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| The repeat unit has single bonds only: the double bond belongs to the monomer, not the polymer. | Do not include the side group in the backbone (poly(propene) has a two-carbon repeat unit with a CH 3 branch, not three carbons in a row). Plastics are made from crude oil, so they are not renewable; "non-biodegradable" means microorganisms cannot break them down. |
Addition polymers: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Define the terms polymer and monomer.
Write an equation for the formation of poly(ethene) from ethene, showing the repeat unit.
(Extended) Draw (or write) the repeat unit of the polymer made from tetrafluoroethene, CF2=CF2, and name the polymer.
(Extended) A section of polymer is –CH2–CH(CN)–CH2–CH(CN)–. Deduce the structure of the monomer.
Describe three problems caused by the disposal of plastics.
A poly(propene) molecule has Mr 126 000. Calculate the number of propene, C3H6, monomers in the molecule. (Ar: C 12, H 1)
Answer key and marking notes
- Monomer: a small molecule that can join with many others to form a polymer.
- Polymer: a large molecule built up from many monomers.
- n CH2=CH2 → –[CH2–CH2]n–
- The repeat unit has single C–C bonds, in brackets with continuation bonds and subscript n.
- Repeat unit: –CF2–CF2– (single bond between the carbons).
- Poly(tetrafluoroethene), PTFE.
- The repeat unit is –CH2–CH(CN)–.
- Put the double bond back between the two carbons: CH2=CH(CN).
- In landfill: they are non-biodegradable and take up space for hundreds of years.
- In oceans: they accumulate and harm animals that swallow them or are trapped.
- Burning: releases toxic gases (such as hydrogen chloride from PVC, carbon monoxide) and carbon dioxide.
- Mr of propene = 3 × 12 + 6 × 1 = 42.
- Number of monomers = 126 000 ÷ 42 = 3000 (no atoms are lost in addition polymerisation).
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 monomer is used to make poly(ethene)?
- Aethene
- Bethane
- Cethanol
- Dethanoic acid
Which statement describes addition polymerisation?
- Amany small unsaturated monomers join to form one long chain and no other product
- Bmany monomers join and a small molecule such as water is lost each time
- Ca long chain molecule is broken into smaller molecules
- Dan alkane reacts with chlorine in ultraviolet light
Which is the repeat unit of poly(ethene)?
- A−CH2−CH2−
- BCH2=CH2
- C−CH3−CH3−
- D−CH=CH−
Which is a problem caused by disposing of plastics?
- Athey are non-biodegradable, so they build up in landfill sites and oceans
- Bthey decompose quickly, releasing all their energy
- Cburning them only releases oxygen
- Dthey dissolve in rain water and form acids
Why can burning waste plastics be harmful?
- Atoxic gases can be produced, and carbon dioxide adds to climate change
- Bit produces only water vapour
- Cit makes the plastics biodegradable
- Dit releases the monomers, which are all toxic
The diagram shows the repeat unit of an addition polymer. What is the monomer?
- Apropene
- Bpropane
- Cethene
- Dpoly(propene)
Chloroethene, CH2=CHCl, forms an addition polymer. Which is the repeat unit?
- A−CH2−CHCl−
- B−CH2=CHCl−
- C−CHCl−CHCl−
- D−CH2−CH2−Cl−
A sample of poly(ethene) has a relative molecular mass of 28 000. How many ethene monomers (Mr = 28) joined to make one chain?
- A1000
- B2000
- C500
- D14 000
Quiz key
Poly(ethene) is made by addition polymerisation of ethene, CH2=CH2, whose C=C bonds open and link up. Ethane is saturated and cannot polymerise this way. Ethanol and ethanoic acid have no C=C double bond either.
In addition polymerisation, monomers containing C=C bonds join together, and the polymer is the only product. Losing a small molecule such as water is condensation polymerisation. Breaking long chains is cracking. Alkane plus chlorine in UV light is substitution.
When ethene polymerises, the C=C opens to a single bond and each carbon forms a new bond to the next unit, giving the repeat unit −CH2−CH2−. CH2=CH2 is the monomer, still with its double bond. −CH3−CH3− would give carbon five bonds. −CH=CH− has lost two hydrogens and kept a double bond.
Most addition polymers are non-biodegradable: microorganisms cannot break them down, so they persist in landfill and accumulate in oceans, where they harm wildlife. They do not decompose quickly. Burning plastics releases carbon dioxide and can form toxic gases, not oxygen. Plastics are insoluble in water.
Burning plastics gives carbon dioxide, a greenhouse gas, and can produce toxic gases such as carbon monoxide or hydrogen chloride. Water vapour is not the only product, because the plastics contain carbon. Burning destroys plastic rather than making it biodegradable. Burning does not simply release the monomers.
Restoring the double bond between the two carbons gives CH2=CHCH3, propene. Propane, CH3CH2CH3, has no C=C, so it cannot form an addition polymer. Ethene would give a repeat unit with four hydrogens and no CH3 group. Poly(propene) is the polymer, not the monomer.
The repeat unit has the same atoms as the monomer with a single C−C bond: −CH2−CHCl− (poly(chloroethene), PVC). Keeping the double bond would give carbon too many bonds. −CHCl−CHCl− has an extra Cl and too few H. Putting Cl in the main chain is wrong because Cl forms only one bond.
No atoms are lost in addition polymerisation, so n = 28 000 / 28 = 1000. 2000 comes from dividing by 14, the mass of one CH2 group, instead of the whole C2H4 monomer. 14 000 comes from dividing by 2, the number of carbon atoms, instead of the monomer mass. 500 comes from using Mr = 56, two monomers instead of one.
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.