Questions on addition polymers

Question 1

Evaluate the ways in which waste poly(ethene) and poly(chloroethene), PVC, can be dealt with after use, including feedstock recycling and burning for energy.

Accepted Answer

Indicative content

both polymers have saturated chains of strong, non-polar bonds, so they are unreactive and non-biodegradable, and in landfill they last for centuries with their carbon wasted
sorted, cleaned poly(ethene) can be remelted and moulded into new objects, saving crude oil and energy, but the sorting costs money and a little PVC spoils a batch
feedstock recycling heats the waste to break the chains into small molecules, such as alkenes, that become raw materials for new polymers and other chemicals
feedstock recycling accepts mixed and dirty plastic that cannot be remelted, though the cracking plant needs energy and capital to run
burning for energy uses the energy stored in the polymer to generate electricity and cuts the volume of waste, but releases carbon dioxide, and carbon monoxide if combustion is incomplete
burning PVC also gives hydrogen chloride, which is toxic and acidic and is removed from the waste gases by a base such as calcium oxide before they are released

Levels

Level 3 (5–6 marks) remelting, feedstock recycling and burning for energy are each weighed against landfill, feedstock recycling is explained as breaking the chains into small molecules that become new raw materials, and the HCl from PVC is removed with a base; the judgement matches each method to the kind of waste it suits, such as clean poly(ethene) to remelting and mixed waste to feedstock recycling
Level 2 (3–4 marks) most of the methods are weighed on both sides and PVC is singled out for its hydrogen chloride; typically feedstock recycling is treated as remelting under another name, or the HCl is noted with no means of removing it, and the conclusion picks one method for all waste
Level 1 (1–2 marks) a few separate statements about disposal, often that the plastic rots slowly in landfill or that PVC burns as cleanly as poly(ethene); no method is set against another

Question 2

Calculate the number of repeating units in a poly(propene) chain of relative molecular mass 2.52 × 10⁵, and give its repeating unit as a condensed formula with brackets and n. Mr of propene is 42.0.

Accepted Answer

B1 the repeating unit is –[CH₂–CH(CH₃)]–, two carbons in the main chain with CH₃ as a side group B1 a bond extends out through each bracket, and n is written after the closing bracket M1 the repeating unit has the same Mr as the monomer, so number of units = 2.52 × 10⁵ ÷ 42.0 A1 6.00 × 10³ repeating units, to three significant figures

Question 3

Explain why unplasticised PVC is more rigid than poly(ethene), and give one use of unplasticised and one of plasticised PVC, with the property that suits each.

Accepted Answer

B1 the C–Cl bonds in PVC are polar, so there are permanent dipole–dipole forces between the chains as well as van der Waals forces B1 the stronger forces hold the chains more firmly in place, so they slide past each other less easily; reject 'bonds between chains' B1 unplasticised PVC is used for window frames or drainpipes, because it is rigid and does not rot B1 plasticised PVC is used for cable insulation or flooring, because it is flexible and an electrical insulator

Question 4

Identify the monomer of each polymer in the table, giving its formula and its name.

Accepted Answer

B1 polymer A repeats every two carbons as –[CH₂–C(CH₃)₂]–, so its monomer is CH₂=C(CH₃)₂ B1 the monomer of A is named 2-methylpropene; accept methylpropene B1 polymer B repeats every two carbons as –[CF₂–CFCl]–, so its monomer is CF₂=CFCl B1 the monomer of B is named chlorotrifluoroethene; allow 1-chloro-1,2,2-trifluoroethene

Question 5

Calculate the mean relative molecular mass of a sample of PTFE, –[CF₂–CF₂]ₙ–, whose chains have the lengths given in the table. Ar: C 12.0, F 19.0.

Accepted Answer

M1 Mr of the repeating unit, C₂F₄ = (2 × 12.0) + (4 × 19.0) = 100.0 M1 mean number of repeating units = (0.30 × 2000) + (0.50 × 3000) + (0.20 × 4000) = 2900 M1 mean Mr = 2900 × 100.0; error carried forward from an incorrect mean number of units A1 mean Mr = 2.90 × 10⁵, to three significant figures, with no unit

Question 6

Explain why the polymerisation of chloroethene is described as addition polymerisation, and why its atom economy is 100%.

Accepted Answer

B1 the π bond of each chloroethene molecule opens and its electrons form σ bonds to the neighbouring monomers B1 the monomers join with no other product, so the reaction is an addition B1 every atom of every monomer ends up in the polymer, so the mass of desired product equals the total mass of reactants

Question 7

Explain why poly(ethene) is unreactive and does not biodegrade.

Accepted Answer

B1 the chains are saturated, containing only C–C and C–H single bonds B1 these bonds are strong and almost non-polar, so they are not attacked by electrophiles or nucleophiles B1 there is no functional group that enzymes in microorganisms can act on, so the polymer is not broken down in the environment

Question 8

Explain how adding a plasticiser to PVC makes it flexible.

Accepted Answer

B1 the plasticiser molecules fit between the polymer chains and push them further apart B1 this weakens the intermolecular forces between the chains B1 so the chains can slide over each other more easily and the material bends

Question 9

Calculate the mass of hydrogen chloride formed when 250 g of poly(chloroethene) is burned, assuming all the chlorine forms HCl. Mr of the repeating unit is 62.5 and Mr of HCl is 36.5.

Accepted Answer

M1 amount of repeating units = 250 ÷ 62.5 = 4.00 mol M1 each repeating unit contains one chlorine atom, so 4.00 mol of HCl forms A1 mass of HCl = 4.00 × 36.5 = 146 g, to three significant figures

Question 10

Suggest why PVC waste should be incinerated only in plants fitted with gas scrubbers, and suggest a substance the scrubber could contain.

Accepted Answer

B1 burning PVC releases hydrogen chloride, from the chlorine in the polymer B1 hydrogen chloride is toxic and acidic, contributing to acid rain, so it must not be released B1 the scrubber contains a base such as calcium oxide or calcium carbonate, which neutralises the hydrogen chloride

Question 11

Compare the bonding and the reactivity of ethene with those of poly(ethene).

Accepted Answer

B1 ethene contains a C=C double bond, a σ and a π bond, while poly(ethene) has only C–C and C–H single bonds B1 ethene is unsaturated and reacts with electrophiles such as bromine, while poly(ethene) is saturated and unreactive, like an alkane B1 ethene decolourises bromine water, but poly(ethene) does not

Question 12

Describe how waste poly(propene) is recycled mechanically.

Accepted Answer

B1 the waste is collected and sorted so that only poly(propene) is present B1 it is washed and then shredded into small pieces B1 the pieces are melted and moulded or extruded into new products

Question 13

Suggest why high-density poly(ethene) is chosen for milk bottles and low-density poly(ethene) for carrier bags.

Accepted Answer

B1 high-density poly(ethene) has unbranched chains that pack closely, so the van der Waals forces between them are stronger B1 it is therefore stiffer and softens at a higher temperature, so a bottle keeps its shape B1 low-density poly(ethene) has branched chains that pack loosely, so it is flexible and can be made into thin film

Question 14

Explain how building carbonyl groups into a poly(ethene) chain makes the polymer photodegradable.

Accepted Answer

B1 the C=O groups absorb ultraviolet light, which poly(ethene) itself barely absorbs B1 the energy absorbed breaks a C–C bond next to the C=O group B1 the chains break into shorter pieces, so the plastic becomes brittle and fragments

Question 15

Calculate the energy released when 1.00 kg of ethene is polymerised, taking the enthalpy change of polymerisation as −82 kJ mol⁻¹. Mr of ethene is 28.0.

Accepted Answer

M1 amount of ethene = 1000 ÷ 28.0 = 35.7 mol M1 energy released = 35.7 mol × 82 kJ mol⁻¹ A1 2.9 × 10³ kJ released, to two significant figures, the precision of the enthalpy change given; allow 2930 kJ

Question 16

Name the monomers used to make PTFE and PVC.

Accepted Answer

B1 PTFE is made from tetrafluoroethene, CF₂=CF₂ B1 PVC is made from chloroethene, CH₂=CHCl

Question 17

Identify the monomer of the polymer whose chain reads –CH₂–CH(CN)–CH₂–CH(CN)–CH₂–CH(CN)–.

Accepted Answer

B1 the repeating unit is –[CH₂–CH(CN)]–, two main-chain carbons B1 the monomer is CH₂=CHCN, propenenitrile

Question 18

Suggest why PTFE is used to coat the inside of non-stick frying pans.

Accepted Answer

B1 its C–F and C–C bonds are very strong, so PTFE does not react with food and is stable at cooking temperatures B1 the fluorine atoms form a tight, unreactive sheath around the chain, so food has almost nothing to bond to and does not stick

Question 19

Draw the repeating unit of the addition polymer formed from but-1-ene, CH₂=CHCH₂CH₃.

Accepted Answer

B1 two carbons in the main chain joined by a single bond, with an ethyl side group on the second: –[CH₂–CH(CH₂CH₃)]– B1 a bond extended out through each bracket and n after the closing bracket; reject a C=C in the chain

Question 20

Suggest why a photodegradable carrier bag may not break down once it has been buried in landfill.

Accepted Answer

B1 photodegradation needs ultraviolet light to break bonds in the chain B1 buried under soil or other rubbish, the bag receives no light, so its chains are not broken

Question 21

Suggest how shredded poly(propene) could be separated from shredded PVC at a recycling plant, giving the reason it works.

Accepted Answer

B1 tip the mixture into water: the poly(propene) floats and the PVC sinks B1 poly(propene) has a density of about 0.90 g cm⁻³, less than water, while PVC's is about 1.4 g cm⁻³, more than water

Polymer	Section of the chain
A	–CH₂–C(CH₃)₂–CH₂–C(CH₃)₂–CH₂–C(CH₃)₂–
B	–CF₂–CFCl–CF₂–CFCl–CF₂–CFCl–

Repeating units in a chain	Percentage of chains
2000	30
3000	50
4000	20