AQA GCSE Combined Science (8464), Higher tier · Chemistry › Bonding, structure, and the properties of matter › How bonding and structure are related to the properties of
Practise Polymers. 12 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Why polymers such as poly(ethene) are solids at room temperature, and how to recognise a polymer from a diagram. Questions are usually short: explain the state of a polymer in terms of forces, or pick out the polymer from diagrams of different structures.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State that polymers have very large moleculesPolymer molecules are long chains made of many repeating units.
4
State that polymers are solids at room temperatureThe forces between their large molecules are strong enough to keep them solid.
5
Describe the bonds in a polymer chainThe atoms in each chain are linked to each other by strong covalent bonds.
5
Recognise a polymer from a diagramLook for a repeating unit in brackets with bonds through the brackets and n after them.
6
Explain why polymers are solids at room temperatureTheir large molecules have relatively strong intermolecular forces, which need more energy to overcome than those between small molecules.
7
Tell polymers from giant covalent structuresPolymers are separate chain molecules with forces between them; a giant covalent structure is one continuous network of covalent bonds.
Notes
What a polymer is
Polymers have very large molecules: long chains that can contain thousands of atoms.
Each chain is made of many copies of the same small section, called the repeating unit.
The atoms in each chain are joined to each other by strong covalent bonds.
Example: poly(ethene), used for plastic bags and bottles, is a long chain of carbon atoms, each carbon also bonded to two hydrogen atoms.
Why polymers are solids
Between the polymer molecules there are intermolecular forces.
Because the molecules are so large, these intermolecular forces are relatively strong: much stronger than the forces between small molecules.
So more energy is needed to overcome them, and polymers are solids at room temperature.
When a polymer such as poly(ethene) melts, it is the intermolecular forces between the chains that are overcome; the covalent bonds in the chains do not break.
Recognising a polymer in a diagram
A polymer is usually drawn as its repeating unit inside brackets, with a small n after the brackets (n is a large number).
The bonds at each end of the repeating unit go through the brackets, showing that the chain carries on.
Poly(ethene): two carbon atoms joined by a single bond, each with two hydrogen atoms, inside brackets with n.
Compare: a small molecule is drawn complete, with no bonds going off it; a giant covalent structure is a network of bonds going off in all directions, with no brackets or n; an ionic structure shows + and − ions.
Cheatsheet
Polymer = very large molecule made of many repeating units
Strong covalent bonds between the atoms in each chain
Relatively strong intermolecular forces between chains → solid at room temperature
Diagram: repeating unit in brackets, bonds through the brackets, n outside
Poly(ethene) repeating unit: (–CH2–CH2–)n
Melting a polymer overcomes intermolecular forces, not covalent bonds
How to answer each type of question
Identify a polymer from diagrams
1 to 2 marks5
Look for brackets with n and bonds that go through the brackets: that is a polymer.
A complete small unit is a small molecule; a network of bonds in all directions is giant covalent; + and − ions are ionic.
Example. Four structures are described. A: a molecule of one oxygen atom bonded to two hydrogen atoms. B: two carbon atoms, each bonded to two hydrogen atoms, inside brackets, with bonds through the brackets and n after them. C: a regular lattice of + and − ions. D: carbon atoms each bonded to four other carbon atoms in a network that continues in all directions. (a) Which structure is a polymer? (b) Which structure is a giant covalent structure?
Show the model answer
(a) B (1) (b) D (1)
Explain why a polymer is a solid at room temperature
2 to 3 marks6
Say the polymer molecules are very large.
Say the intermolecular forces between them are relatively strong.
Link this to the energy needed to overcome them, and compare with small molecules if asked.
Example. Ethene, C2H4, is a gas at room temperature. Poly(ethene) is a solid. Explain why, in terms of the forces between molecules.
Show the model answer
Poly(ethene) molecules are much larger than ethene molecules (1). So the intermolecular forces between poly(ethene) molecules are much stronger and need more energy to overcome (1). Ethene has only weak intermolecular forces, which are overcome at a low temperature (1).
Compare a polymer with a giant covalent substance
3 marks7
Say the polymer is made of separate large molecules.
Say melting it overcomes intermolecular forces.
Say melting the giant covalent substance means breaking many strong covalent bonds, which needs much more energy.
Example. Poly(ethene) softens and melts at about 120 °C. Diamond does not melt until well over 3000 °C. Explain this difference.
Show the model answer
Poly(ethene) is made of separate (very large) molecules (1). When it melts, only the intermolecular forces between the chains are overcome (1). To melt diamond, many strong covalent bonds must be broken, which needs much more energy (1).
Shortcuts and memory tricks
Poly means many: a polymer is many repeating units joined in a chain.
Spot it: brackets + n + bonds sticking out through the brackets = polymer.
Melting point ladder: small molecules (low) < polymers (solids, but melt fairly easily) < giant covalent (very high).
Where marks are lost
Saying polymers are solid because of strong covalent bonds between the chains. The chains are held together by intermolecular forces; the covalent bonds are inside the chains.
Calling a polymer a giant covalent structure. Polymers are made of separate, very large molecules.
Drawing a repeating unit without the bonds through the brackets, or leaving out the n.
Calling the intermolecular forces in polymers 'strong' without a comparison. They are relatively strong compared with those between small molecules, but much weaker than covalent bonds.
Exam technique
Use comparisons: 'the molecules are larger, so the intermolecular forces are stronger'. Examiners want the link between size and the strength of the forces.
In 'which diagram' questions, check every option: usually one is a small molecule, one ionic and one giant covalent.
Name the right forces: intermolecular forces between polymer chains, covalent bonds within them.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What name is given to the small molecules that join together to make a polymer?
monomers
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Poly(ethene) is a polymer.
(a) Complete the sentence. Tick (✓) one box. Polymers have ................ molecules.[1]
very large
very small
charged
single-atom
(b) What type of bond joins the atoms within a polymer molecule? Tick (✓) one box.[1]
Covalent
Ionic
Metallic
Intermolecular
(c) Why is poly(ethene) a solid at room temperature? Tick (✓) one box.[1]
The intermolecular forces between the polymer molecules are relatively strong.
The polymer molecules are held together by ionic bonds.
Poly(ethene) contains delocalised electrons.
Poly(ethene) molecules are very small.
(d) What name is given to the small molecules that join together to make a polymer?[1]
Show the answer and mark scheme
(a)Answer: very large
(b)Answer: Covalent
(c)Answer: The intermolecular forces between the polymer molecules are relatively strong.
(d)Answer: monomers
monomers
Question 2Medium2 marks
A student says: 'Poly(ethene) is a solid at room temperature because its covalent bonds are very strong.'
Evaluate the student's statement.[2]
Show the answer and mark scheme
Answer: The covalent bonds in the chains are strong, but they are not broken when poly(ethene) melts. Poly(ethene) is a solid because its molecules are very large, so the intermolecular forces between them are relatively strong.
the covalent bonds within the chains are strong, but they are not broken when poly(ethene) melts / softens
it is a solid because the intermolecular forces between the (very large) polymer molecules are relatively strong
Question 3Hard6 marks
Poly(ethene) is made from ethene. The repeating unit of poly(ethene) is –CH2–CH2–, which has a relative formula mass of 28.
(a) A poly(ethene) molecule contains 4000 repeating units. Calculate the relative formula mass of the molecule.[2]
(b) A sample of poly(ethene) contains molecules with different numbers of repeating units. Suggest why the sample softens over a range of temperatures, rather than melting at one temperature.[2]
(c) Explain why the covalent bonds in the poly(ethene) chains are not broken when poly(ethene) melts.[2]
Show the answer and mark scheme
(a)Answer: 112 000
4000 × 28
112 000
(b)Answer: The molecules have different lengths, so the intermolecular forces between them have different strengths and are overcome at different temperatures.
the molecules have different lengths / sizes
so the intermolecular forces have different strengths / need different amounts of energy to overcome
(c)Answer: Only the intermolecular forces between the chains are overcome; the covalent bonds are much stronger.
only the intermolecular forces between the chains are overcome
covalent bonds are much stronger / need much more energy to break