This mini-lesson covers AQA 3.3.12 Polymers and 3.3.13 Amino acids, proteins and DNA: addition versus condensation polymerisation, polyesters and polyamides, biodegradability, zwitterions and the isoelectric point, protein structure and hydrolysis, DNA base pairing, and how cisplatin works.
Work through each screen, answer the questions as you go (some are wordy, most are calculations) and collect ⭐ stars. Everything here is A-level standard — the maths is done properly, not skipped. Press Start when you're ready.
In addition polymerisation, many alkene monomers join by opening the C=C double bond. There is no other product — so the atom economy is 100%.
The resulting backbone is a chain of saturated, non-polar C–C and C–H bonds — strong and chemically inert. That is exactly why addition polymers are so useful, and exactly why they are an environmental problem: they are not biodegradable, so they persist in landfill for centuries.
Dealing with waste polymers: recycling (sort, melt, remould — saves finite crude oil and reduces landfill); combustion for energy (but this releases CO₂, and PVC releases toxic HCl, which must be removed by scrubbing with a base); and feedstock recycling (cracking the polymer back into monomers and other useful chemicals).
Plasticisers are small molecules that sit between the polymer chains, forcing them apart and weakening the van der Waals forces between them, so the chains can slide over each other. This makes rigid PVC into flexible PVC.
In condensation polymerisation, each new link forms with the loss of a small molecule — usually water (or HCl if an acyl chloride is used). Each monomer needs two functional groups.
Why this matters environmentally: ester and amide links are polar and can be hydrolysed (by acid, alkali or enzymes), so condensation polymers are biodegradable. The C–C backbone of an addition polymer cannot be hydrolysed at all.
Nylon and Kevlar are strong because of hydrogen bonding. The N–H of one amide link hydrogen-bonds to the C=O of a neighbouring chain, holding the chains together. In Kevlar the rigid aromatic rings and regular H-bonding give a material stronger than steel by weight.
Tap a substance, then tap whether it is an addition polymer, a condensation polymer, or not a polymer at all.
An α-amino acid has the general formula H₂NCH(R)COOH: an amino group and a carboxyl group on the same carbon. Because that carbon carries four different groups (H, NH₂, COOH and R), every amino acid except glycine (where R = H) is chiral.
Amino acids are amphoteric: the COOH can donate a proton and the NH₂ can accept one. In fact the proton transfers internally, giving a zwitterion — a species with both a positive and a negative charge but no overall charge:
Proteins are polypeptides — amino acids joined by peptide (amide) links, formed by condensation. Primary structure is the sequence of amino acids. Secondary structure (α-helix and β-pleated sheet) is held by hydrogen bonding between the N–H and C=O of the backbone. Tertiary structure is the overall 3D fold, held by hydrogen bonds, ionic attractions, van der Waals forces and disulfide bridges.
Hydrolysis: heating a protein under reflux with 6 mol dm⁻³ HCl for 24 hours breaks every peptide link, releasing the individual amino acids — which can then be separated by chromatography or electrophoresis.
DNA is a condensation polymer of nucleotides. Each nucleotide is a phosphate group, the sugar 2-deoxyribose, and one of four bases. The sugar and phosphate form the backbone; the two strands wind into a double helix.
The strands are held together by hydrogen bonds between complementary base pairs:
Pairing is exact because the sizes and the positions of the N–H and C=O groups match only in those combinations. This complementarity is what allows DNA to be copied accurately.
Cisplatin is a square planar platinum(II) complex, Pt(NH₃)₂Cl₂ — specifically the cis isomer. Inside the cell, a chloride ligand is replaced by water, and the resulting complex forms a co-ordinate bond to a nitrogen atom on a guanine base in DNA. Because it is the cis isomer, it can bind to two such sites, cross-linking and distorting the DNA strand. Replication is blocked, so the cell cannot divide and it dies.
Evaluating cisplatin honestly: it is a highly effective anti-cancer drug, especially for testicular cancer. But it cannot distinguish cancerous from healthy cells, so it also attacks rapidly dividing healthy tissue — bone marrow, hair follicles, the gut lining — causing serious side effects. Careful dosage and targeted delivery reduce, but do not remove, this. The trans isomer is far less effective, because its chloride ligands are too far apart to form the same cross-link.
Tap a monomer on the left, then the polymer it makes on the right.
The DNA base-pair numbers: A–T is TWO hydrogen bonds, C–G is THREE. The C–G pair is therefore harder to separate, so DNA rich in C and G is more thermally stable — a genuinely useful fact, and an easy mark.
Addition polymers: from alkenes; the backbone is a saturated C–C chain — non-polar, unreactive and non-biodegradable
Condensation polymers: a small molecule (H₂O or HCl) is lost each time a link forms; contain polar ester or amide links
Biodegradability: ester and amide links can be hydrolysed, so condensation polymers break down — addition polymers do not
Amino acids: H₂NCHRCOOH — they exist as zwitterions, and at the isoelectric point the net charge is zero
Proteins: a polypeptide of amino acids joined by peptide (amide) links; hydrolysed by 6 mol dm⁻³ HCl under reflux
DNA: a phosphate–deoxyribose backbone; A pairs with T (2 H-bonds) and C pairs with G (3 H-bonds)
Cisplatin: binds to a guanine N on DNA, preventing replication in cancer cells — but it also attacks healthy cells, so there are side effects
That is AQA 3.3.12 and 3.3.13. Press Finish.
You've worked through Polymers, Amino Acids, Proteins and DNA for AQA A-level Chemistry (7405). 🎉
Your stars: 0 / 0
Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.