Biodegradability and disposal of polymers (A-level only) — AQA A-Level Chemistry
Test yourself on Biodegradability and disposal of polymers (A-level only) with AQA A-Level practice questions.
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Biodegradability and disposal of polymers (A-level only) explained
An addition polymer made from alkenes has a backbone built only from carbon and hydrogen.
Read the full explanation
Carbon and hydrogen have almost the same electronegativity, so the C-C and C-H bonds are essentially non-polar and no carbon atom in the chain carries a δ⁺ charge. A nucleophile such as water or a hydroxide ion has nothing to attack, and the C-C bonds are strong, so acids, alkalis, water and the enzymes that micro-organisms use simply leave the chain alone. That inertness is exactly why polyalkenes are useful for packaging and pipes, and exactly why they are a problem afterwards: buried in landfill, a polyalkene article stays essentially unchanged for hundreds of years.
Polyesters and polyamides can be broken down by hydrolysis and are biodegradable.
The backbone of a polyester or a polyamide is interrupted, at every link, by a carbonyl carbon bonded to an oxygen or a nitrogen. Both of those are more electronegative than carbon, so that carbon carries a δ⁺ charge, which leaves it open to attack by a nucleophile. Water, especially with an acid or an alkali present, attacks it and cuts the chain, reversing the condensation and returning the monomers. Acid hydrolysis of a polyamide gives the dicarboxylic acid and the diamine as its salt, because excess acid protonates the amine; alkaline hydrolysis gives the free diamine and the carboxylate salt. Micro-organisms use enzymes to do the same chemistry slowly in soil and water, which is what makes these polymers biodegradable.
The advantages and disadvantages of different methods of disposal of polymers, including recycling.
Landfill is cheap and needs no sorting, but it takes up land, polyalkenes stay there for centuries and decomposing waste releases methane and leachate. Incineration destroys the polymer, cuts the volume sharply and the heat released can generate electricity, but carbon dioxide is produced, and burning PVC releases hydrogen chloride, which must be removed by passing the flue gases through a base; too low a combustion temperature also produces toxic dioxins. Recycling conserves crude oil and uses less energy than making new polymer, but the waste has to be collected, sorted by type and cleaned, and each reprocessing shortens the chains so the quality falls. Feedstock recycling instead cracks the waste back into useful small molecules.
Students should be able to: explain why polyesters and polyamides can be hydrolysed but polyalkenes cannot.
The difference is in the backbone, not in how the polymer was made. A polyester or polyamide chain has a carbonyl carbon at every link, bonded to an oxygen or a nitrogen. Those atoms are more electronegative than carbon, so they draw electron density away and leave that carbon with a δ⁺ charge. A nucleophile, water or a hydroxide ion, is attracted to it, bonds to it and the chain is cut. A polyalkene chain has no such site: every bond in the backbone is C-C or C-H, the electronegativities are almost equal, so the bonds are non-polar and no carbon is electron deficient. With nothing to attract a nucleophile, hydrolysis never starts, whatever the conditions.
Your focus
- Explain, in terms of bond polarity, why a polyalkene chain resists attack by water, acid and alkali.
- State one advantage and one disadvantage that follow from a polyalkene being chemically inert.
- Identify from a structure whether a polymer chain contains a site open to nucleophilic attack.
Show all 12 objectives
- Draw the products formed when a given polyester or polyamide is hydrolysed in acid and in alkali.
- Explain why the carbonyl carbon of the link is the site attacked during hydrolysis.
- Account for the biodegradability of a condensation polymer in terms of the bonds in its backbone.
- Give one gain and one cost for landfill, for incineration and for recycling, each one naming a substance, a resource or a practical difficulty rather than saying it harms the environment.
- Explain how the hydrogen chloride produced by burning PVC is dealt with, and write an equation for it.
- Evaluate which disposal route is most appropriate for a given waste stream and justify the choice.
- Give the products of hydrolysing a named polyamide under acid conditions, and state what happens to poly(ethene) treated the same way.
- Label the partial charges on the carbonyl carbon and the neighbouring oxygen of a polyester chain.
- Predict from an unfamiliar polymer structure whether it will be biodegradable, and justify the prediction.
Biodegradability and disposal of polymers (A-level only) exam tips
Marking Points
- one mark for the backbone containing only carbon and hydrogen atoms
- one mark for those bonds being non-polar, so there is no δ⁺ carbon atom
- one mark for stating that no nucleophile can attack the chain, so it is not hydrolysed
- one mark for linking that inertness to persistence in landfill
- one mark for the carbonyl carbon of the link carrying a δ⁺ charge, so a nucleophile can attack it
- one mark for water or the hydroxide ion identified as the attacking nucleophile
- one mark for the monomers named as the products of hydrolysis
- one mark for those products being obtained as salts under acidic or alkaline conditions
- one mark for an advantage and a matching disadvantage given for the same disposal method
- one mark for hydrogen chloride from burning PVC being removed by reaction with a base
- one mark for recycling conserving finite crude oil or saving energy
- one mark for a practical difficulty of recycling, such as sorting mixed polymers or falling quality
- one mark for identifying the ester or amide link as the polar part of the backbone
- one mark for the carbonyl carbon being δ⁺ because oxygen or nitrogen is more electronegative than carbon
- one mark for a nucleophile attacking that carbon and breaking the chain
- one mark for the polyalkene backbone being non-polar, with no δ⁺ carbon for a nucleophile to attack
Examiner Tips
- 💡The word the examiner wants is 'non-polar'. Write it explicitly rather than implying it.
- 💡Break the argument into two halves: no polar site, therefore no nucleophilic attack, therefore no hydrolysis.
- 💡Decide first whether the conditions are acidic or alkaline, then adjust each product for it before you write anything down.
- 💡Number the products: a polymer of n repeating units gives n of each monomer, so say 'the dicarboxylic acid and the diamine', not 'the monomer'.
- 💡Give balanced answers in pairs; a question worth four marks usually wants two methods with one point each way.
- 💡Name the gas and name the remedy: hydrogen chloride, removed by a base such as calcium oxide.
- 💡This is a compare-and-contrast answer: make the same point twice, once for each polymer, rather than describing one and stopping.
- 💡Write δ⁺ on the carbon in your diagram; a labelled structure often earns the mark faster than a sentence.
Common Mistakes
- saying polyalkenes are unreactive because they still contain C=C double bonds
- explaining inertness by chain length rather than by the polarity of the bonds
- confusing chemical inertness with being insoluble in water
- claiming that because some plastics break up in sunlight they are therefore biodegradable
- giving the free diamine as the product of acid hydrolysis, when the excess acid protonates it
- giving the free dicarboxylic acid as the product of alkaline hydrolysis rather than its salt
- describing biodegradation as the polymer melting or dissolving
- claiming polyamides hydrolyse but polyesters do not, when both links are attacked
- writing 'it harms the environment' without naming a substance or an effect
- claiming incineration produces no carbon dioxide because the energy is recovered
- assuming all polymers can be recycled together without separation
- forgetting that collecting, sorting and reprocessing polymers themselves use energy
- saying the polyalkene chain is too strong rather than saying it is non-polar
- putting the δ⁺ charge on the oxygen and the δ⁻ charge on the carbon
- describing the attack in a polyester as happening at a C-C bond in the chain
- answering only that polyalkenes are addition polymers, with no reference to bonding