AQA Chemistry (8462) 4.2.2.5 · Classify
Polymers
Ethene is a gas. Poly(ethene), made of nothing but ethene joined up, is a solid carrier bag. Can you tell a polymer from every other structure at a glance, and say why it is solid?
Triple Science · Foundation tier
Start here
Same atoms. One is a gas, one is a carrier bag.
An ethene molecule, C₂H₄, has six atoms. It boils at −104 °C. A single molecule of poly(ethene) in a carrier bag is a chain of many thousands of carbon atoms, each with two hydrogens. The bag stays solid until it softens at over 100 °C. Both substances contain only strong covalent bonds inside their molecules.
Commit first
Why is poly(ethene) a solid at room temperature when ethene is a gas?
A polymer is a very large molecule made of many small units joined in a long chain. Inside each chain, atoms are linked by strong covalent bonds. Between chains there are only intermolecular forces, just as between small molecules, but because the molecules are so large those forces are relatively strong. That is why polymers are solids at room temperature. An exam diagram rarely names the structure. You have to recognise it.
Structure decider · round 1
What structure does the diagram show?
0 of 8 decided
Think again
“A polymer chain is so huge that it must be a giant covalent structure, like diamond.”
Spot the flaw
ComparePoly(ethene) softens at about 120 °C. Diamond survives above 3500 °C. What explains the gap?
Addition polymerisation · AQA 4.7.3.1, 4.10.3.3 (chemistry only)
Open the double bond. Join the chain.
Ethene has a C=C double bond. In addition polymerisation one bond of each double bond opens and the monomers link into a chain. Before you build one, commit.
Eight ethene molecules (48 atoms) join into one chain. How many atoms are lost?
Thermosoftening polymers, like poly(ethene), are separate chains: heat overcomes the forces between chains and they soften and melt. Thermosetting polymers have cross-links, covalent bonds between chains, so they do not melt when heated.
Command words in this lesson
- Identify
- Name the structure from the diagram. Look for brackets and n, or bonds running off the edge.
- Explain
- Large molecules, relatively strong forces between them, so more energy to overcome.
- Compare
- Name what is overcome in each: forces between molecules, or covalent bonds.
Key fact
Polymers are very large molecules. The intermolecular forces between them are relatively strong, so they are solids at room temperature, but they are molecules, not giant covalent structures.
Examiner tip
Key note · AQA 4.2.2.5 (8462)
Polymers
- A polymer is a very large molecule: many small units joined in a long chain.
- Atoms in the chain are linked by strong covalent bonds.
- Intermolecular forces between polymer molecules are relatively strong, so polymers are solids at room temperature.
- A polymer is a molecule, not a giant covalent structure.
- Recognise from diagrams: brackets with n, or long separate chains.
- Triple: addition polymerisation opens the C=C bond; no atoms are lost. n CH₂=CH₂ → [—CH₂—CH₂—]ₙ
Photograph this card. Tomorrow, cover it and say each line first.
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Ask Mr Badmus AI
Stuck telling a polymer from a giant covalent structure?
Diagrams show a few repeat units or a small region of each structure; a real poly(ethene) chain has thousands of carbon atoms and a real crystal has billions of particles. Bonds drawn running off the edge of a diagram mean the structure continues. Addition polymerisation, thermosoftening and thermosetting polymers are separate-science content and show only on Triple routes.