Year 9 Science · Unit 2 · Lesson 16
Challenge Worksheet
Learning Goals
1. Explain why ethane, C₂H₆, cannot take part in the reaction that makes polyethylene.
2. A chain of PVC, (–CH₂–CHCl–)ₙ, has n = 2000. How many chlorine atoms, carbon atoms and hydrogen atoms does the chain contain? Ignore the two ends of the chain.
3. Three of the four lines are wrong. Explain each mistake and give a corrected version.
Find the mistakes: four lines from a student’s notes
(a) Polypropylene: n CH₂=CHCH₃ → (–CH₂–CH₂–CH₃–)ₙ
(b) PVC: n CH₂=CHCl → (–CH₂=CHCl–)ₙ
(c) PTFE (non-stick coating): n CF₂=CF₂ → (–CF₂–CF₂–)ₙ
(d) Nylon forms by addition polymerisation, so nothing is given off as the monomers join.
Someone claims…
A start-up company says: “Our factory will turn 90 tonnes of lactic acid into 90 tonnes of the plastic PLA, because no atoms are lost when a polymer forms.”
PLA forms when lactic acid molecules, C₃H₆O₃, join into a chain. Each time two molecules link, one water molecule, H₂O, is given off. The repeat unit of PLA is –O–CH(CH₃)–CO–, which has the formula C₃H₄O₂.
| Substance | Formula | Relative mass |
|---|---|---|
| Lactic acid (monomer) | C₃H₆O₃ | |
| PLA repeat unit | C₃H₄O₂ | |
| Water (by-product) | H₂O | 18 |
4. Complete Table 1. Show that the mass of one lactic acid molecule equals the mass of one repeat unit plus one water molecule.
5. Calculate the mass of PLA and the mass of water made from 90 tonnes of lactic acid. (Treat the chain as very long, so ignore its two ends.) Then explain the error in the company’s reasoning.
PLA tonnes
Water tonnes
6. If the same factory instead made polypropylene from 90 tonnes of propene, what mass of polymer would it make? Explain why the company’s sentence would be true for this polymer but not for PLA.
7. Evaluate the statement: “A polymer’s properties are decided by its monomer alone.” Use at least three pairs of polymers from Lesson 16 as evidence.