The shape of the lesson
By the end of the lesson, learners can
- Hydrolysis breaks the links: Hydrolyse the polyester chain in the simulator with water, acid and then alkali.
- Sunlight, landfill and disposal: Not every polymer can be made hydrolysable.
- Poly(alkenes): non-polar C–C backbone, inert, non-biodegradable.
- Polyesters, polyamides: hydrolysed by acid, alkali or enzymes.
- Acid gives COOH (and NH3+); alkali gives COO− (and NH2).
- Photodegradable: C=O absorbs UV.
One picture every idea returns to
Poly(alkenes) such as poly(ethene) contain only strong, non-polar C–C and C–H bonds, so they are chemically inert and non-biodegradable: they persist in landfill and the oceans. Polyesters and polyamides contain polar C=O groups in their ester and amide links, so they can be hydrolysed by hot aqueous acid, alkali or by enzymes in microorganisms; they are biodegradable. Acid hydrolysis gives the free acid and the alcohol or the ammonium salt of the amine; alkaline hydrolysis gives the carboxylate salt and the alcohol or free amine. Photodegradable polymers contain C=O groups that absorb ultraviolet radiation; the energy breaks nearby bonds in the chain so the polymer crumbles. Burning plastics releases CO2 and can release toxic gases such as HCl from PVC.
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, Built to last, or built to break down.. Poly(alkenes) are chemically inert, which is why a plastic bag can outlive the person who used it. 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: Hydrolysis breaks the links | Hydrolyse the polyester chain in the simulator with water, acid and then alkali. Project tab 5 of the lab (Condensation polymers lab, degradable polymers) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Sunlight, landfill and disposal | Not every polymer can be made hydrolysable. Project tab 5 of the lab (Condensation polymers lab, degradable polymers) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 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 Sunlight, landfill and disposal, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 |
|---|---|
| Poly(alkenes) are not biodegradable because their C–C and C–H bonds are non-polar and strong, not because they are "too big". | Biodegradable and photodegradable are different: one needs hydrolysis (water, acid, alkali or enzymes) at ester or amide links, the other needs UV light absorbed by C=O groups. In alkaline hydrolysis the acid monomer appears as its carboxylate salt, and in acid hydrolysis of a polyamide the amine appears as its ammonium salt. |
Degradable polymers: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why poly(lactic acid) is biodegradable but poly(propene) is not.
Give the structures of the organic products when nylon-6,6 is heated with excess NaOH(aq).
Give the structures of the organic products when PET is heated with dilute hydrochloric acid.
Explain how a photodegradable polymer breaks down, and state one limitation.
Compare incineration and landfill as ways of disposing of poly(chloroethene), PVC.
1.92 g of PET (repeat unit Mr 192) is completely hydrolysed by acid. Calculate the mass of benzene-1,4-dicarboxylic acid (Mr 166) formed.
Answer key and marking notes
- Poly(lactic acid) is a polyester with ester links containing a polar C=O; the δ+ carbon is attacked by water (catalysed by acid, alkali or enzymes).
- Hydrolysis breaks the chain back to lactic acid.
- Poly(propene) has only non-polar C–C and C–H bonds, so there is no site for hydrolysis or enzyme attack.
- Sodium hexanedioate, NaOOC(CH2)4COONa.
- Hexane-1,6-diamine, H2N(CH2)6NH2.
- Benzene-1,4-dicarboxylic acid, HOOCC6H4COOH.
- Ethane-1,2-diol, HOCH2CH2OH.
- Carbonyl (C=O) groups in the chain absorb ultraviolet radiation, and the energy breaks bonds near them, splitting the chain into small pieces.
- Limitation: it needs light, so buried or covered waste does not degrade (and fragments may persist as microplastics).
- Landfill: PVC is non-biodegradable, so it persists for centuries and uses up land.
- Incineration destroys it and can recover energy, but produces CO2 (greenhouse gas) and toxic HCl.
- HCl must be removed from flue gases (for example with a base); incomplete burning can form dioxins.
- n(repeat units) = 1.92 ÷ 192 = 0.0100 mol, each giving one molecule of the acid.
- Mass = 0.0100 × 166 = 1.66 g.
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.
Why are poly(alkenes) such as poly(ethene) non-biodegradable?
- Atheir chains contain only strong, non-polar C–C and C–H bonds, so there is no site for hydrolysis or enzyme attack
- Btheir molecules are too large to fit into the soil
- Cthey contain ester links that resist hydrolysis
- Dthey dissolve in water and are washed away before bacteria can act
Which type of reaction makes polyesters and polyamides biodegradable?
- Ahydrolysis of the ester or amide links
- Baddition across C=C bonds
- Cfree-radical substitution of C–H bonds
- Ddehydration of the chain
Which polymer would be expected to biodegrade most easily?
- Apoly(lactic acid), –[OCH(CH3)CO]n–
- Bpoly(propene), –[CH2CH(CH3)]n–
- Cpoly(chloroethene), –[CH2CHCl]n–
- Dpoly(phenylethene), –[CH2CH(C6H5)]n–
Some polymers are designed to be photodegradable. Which feature allows this?
- AC=O groups in the chain absorb ultraviolet radiation, and the energy breaks nearby bonds
- BC=C groups absorb infrared radiation and polymerise further
- Cester links react with oxygen in the dark
- DC–H bonds absorb visible light and release hydrogen
What is the main environmental problem when PVC, poly(chloroethene), is incinerated?
- Atoxic, acidic hydrogen chloride gas is produced, as well as carbon dioxide
- Bit produces no energy, so it is wasteful
- Cit releases chlorofluorocarbons
- Dit gives off ammonia
Polymer R is hydrolysed separately with hot HCl(aq) and with hot NaOH(aq). Which row gives products X and Y correctly?
- AX: HOOC(CH2)2COOH and +H3N(CH2)4NH3+; Y: −OOC(CH2)2COO− and H2N(CH2)4NH2
- BX: −OOC(CH2)2COO− and H2N(CH2)4NH2; Y: HOOC(CH2)2COOH and +H3N(CH2)4NH3+
- CX and Y: both HOOC(CH2)2COOH and H2N(CH2)4NH2
- DX and Y: both HO(CH2)4OH and HOOC(CH2)2COOH
2.26 g of nylon-6,6 (repeat unit Mr 226) is completely hydrolysed by hot dilute acid. What mass of hexanedioic acid (Mr 146) can be obtained?
- A1.46 g
- B2.26 g
- C0.730 g
- D2.92 g
A company replaces poly(ethene) carrier bags with photodegradable bags containing C=O groups. Which is the strongest criticism of this as a solution to plastic waste?
- Abags buried in landfill receive no UV light, so they do not degrade, and those that do can break into persistent small fragments
- BC=O groups make the bags biodegradable by hydrolysis, so they dissolve in rain
- Cphotodegradable bags cannot be made because C=O groups cannot be put into a polymer chain
- DUV light turns the bags back into ethene monomer, which is flammable
Quiz key
Polyesters and polyamides have polar C=O groups in the chain whose δ+ carbon can be attacked by water, acids, alkalis or enzymes. Poly(alkenes) have only non-polar C–C and C–H bonds, so they are inert and persist for centuries. Size is not the reason: natural polymers such as proteins are large but biodegradable. Poly(alkenes) have no ester links. They are insoluble in water.
Water, catalysed by acid, alkali or enzymes from microorganisms, breaks ester and amide links, returning the chain to its monomers. These polymers have no C=C bonds for addition. Free-radical substitution needs UV and halogens and does not break the chain. Dehydration removes water, which is how the polymer formed, the opposite of degradation.
Poly(lactic acid) is a polyester with ester links in the backbone, so it can be hydrolysed by enzymes back to lactic acid; it is used for compostable packaging. Poly(propene), PVC and poly(phenylethene) are addition polymers with all-carbon backbones and no hydrolysable link. The C–Cl bond in PVC is polar, but it is a side group, and breaking it does not cut the chain.
Carbonyl groups absorb UV radiation; the absorbed energy breaks bonds near the C=O, cutting the chain into fragments. Infrared only makes bonds vibrate and does not break chains, and further polymerisation would make the plastic more durable, not less. Photodegradation needs light by definition. C–H bonds do not absorb visible light.
Burning PVC releases HCl, which is toxic and acidic and must be removed from the flue gases, along with CO2; incomplete burning can also form dioxins. Incineration does recover energy. PVC contains no fluorine, so no CFCs form. There is no nitrogen, so no ammonia.
Hydrolysis breaks each amide link to give butanedioic acid and butane-1,4-diamine. In excess HCl(aq) the amine is protonated to its ammonium salt while the acid stays as COOH (X). In excess NaOH(aq) the acid becomes the carboxylate ion while the amine stays free (Y). The second option swaps the conditions. The third ignores the effect of excess acid or alkali on the products. The fourth gives a diol, but R contains NH, not O, so it is not a polyester.
n(repeat units) = 2.26 ÷ 226 = 0.0100 mol, each giving one hexanedioic acid: 0.0100 × 146 = 1.46 g. 2.26 g assumes the whole polymer becomes the acid, ignoring the diamine part. 0.730 g halves the moles, as if two repeat units gave one acid. 2.92 g doubles them, as if each amide link gave a diacid.
Photodegradation needs UV light, which does not reach buried waste, and the chain breaks into smaller pieces (microplastic fragments) rather than being fully broken down by organisms. C=O groups in a C–C backbone (for example from a little CO copolymerised with ethene) do not create hydrolysable links, so the bag does not dissolve. Such copolymers are manufactured. Chain scission gives fragments, not ethene monomer.
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.