Polymers — AQA GCSE Chemistry
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Polymers explained
Polymers covers 4 specification points, including Polymers have very large molecules.
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and The atoms in the polymer molecules are linked…. Open a point below for its explanation and revision guidance.
Polymers have very large molecules.
Polymers are large molecules made from many small repeating units called monomers joined together by covalent bonds. Because each polymer molecule contains many atoms, it is described as a macromolecule. It is important not to confuse these discrete macromolecules with continuous giant covalent structures like diamond. The atoms within a polymer molecule are held by strong covalent bonds, while separate polymer molecules are held together by intermolecular forces. The large size of polymer molecules means the intermolecular forces between them are stronger than those between small molecules, so polymers are often solid at room temperature. An example is poly(ethene), formed from ethene monomers.
The atoms in the polymer molecules are linked to other atoms by strong covalent bonds.
Polymers form when many small monomer molecules join together. Within each polymer molecule, the atoms are held together by strong covalent bonds. These bonds form the long carbon backbone and any side groups, so the polymer molecule behaves as one very large molecule. For example, in poly(ethene), each carbon atom is covalently bonded to two other carbon atoms and to two hydrogen atoms, giving a long chain. The covalent bonds are strong, so breaking the polymer molecule into smaller molecules requires a lot of energy. However, the covalent bonds are not the forces between separate polymer molecules; those intermolecular forces are weaker. This distinction matters when explaining melting, flexibility and strength.
The intermolecular forces between polymer molecules are relatively strong and so these substances are solids at room temperature.
Polymer molecules are long chains. Between one polymer molecule and its neighbours there are intermolecular forces. These forces are relatively strong compared with those between small molecules such as methane, although they are weaker than the covalent bonds inside each polymer molecule. Because the chains are long, many intermolecular forces act along each chain, so a large amount of energy is needed to separate the molecules. As a result, polymers are usually solids at room temperature. For example, poly(ethene) is a solid at room temperature, while ethene is a gas. The intermolecular forces also help explain why polymers are flexible and can be moulded when heated: heating weakens the forces between chains so the chains can slide, but the covalent bonds within the chains remain intact.
Students should be able to recognise polymers from diagrams showing their bonding and structure.
A diagram of a polymer shows a large molecule built from repeating units. The key feature is a continuous chain of covalently bonded atoms, often carbon atoms, with side groups or hydrogen atoms attached. The repeating unit is the part of the chain that repeats many times. To recognise a polymer, look for a long chain rather than a small discrete molecule, and identify the repeating unit by finding the shortest section that repeats along the chain. For example, in poly(ethene) the repeating unit is a two-carbon section with hydrogen atoms attached. The diagram may also show intermolecular forces between neighbouring chains as dashed lines. Recognising these features allows you to distinguish a polymer from a simple molecule such as ethene or from a giant covalent structure such as diamond.
Your focus
- Define polymer and monomer.
- Describe the structure of a polymer in terms of repeating monomer units.
- Explain why polymers have large molecules and how this affects their properties.
Show all 12 objectives
- Identify covalent bonds within a polymer molecule from a diagram or description.
- Distinguish between covalent bonds within polymer molecules and intermolecular forces between polymer molecules.
- Explain how strong covalent bonds affect the behaviour of a polymer molecule.
- Describe the intermolecular forces between polymer molecules.
- Explain why polymers are solids at room temperature.
- Compare the strength of intermolecular forces with the strength of covalent bonds in a polymer.
- Identify the repeating unit in a polymer diagram.
- Distinguish a polymer from a small molecule using a diagram.
- Interpret dashed lines between polymer chains as intermolecular forces.
Polymers exam tips
Marking Points
- Polymers are macromolecules made from many monomers joined together.
- Monomers are small repeating units that covalently bond to form a polymer.
- The atoms within a polymer molecule are held by strong covalent bonds.
- Intermolecular forces act between separate polymer molecules.
- Large polymer molecules have stronger intermolecular forces than small molecules, so many polymers are solid at room temperature.
- Polymer molecules are made from many monomers joined together.
- Atoms within a polymer molecule are joined by covalent bonds.
- The covalent bonds are strong, so the polymer molecule is difficult to break apart.
- The covalent bonds form the long chain, including the carbon backbone and any side groups.
- The covalent bonds within a polymer molecule are distinct from the intermolecular forces between separate polymer molecules.
- Intermolecular forces act between separate polymer molecules.
- These intermolecular forces are relatively strong compared with those between small molecules.
- The intermolecular forces are weaker than the covalent bonds within each polymer molecule.
- The relatively strong intermolecular forces mean polymers are solids at room temperature.
- Long polymer chains have many intermolecular forces along their length, so more energy is needed to separate them.
- A polymer diagram shows a large molecule made from repeating units.
- The atoms in the chain are joined by covalent bonds.
- The repeating unit is the smallest part of the chain that repeats.
- Side groups or hydrogen atoms may be attached to the main chain.
- Dashed lines between chains represent intermolecular forces, not covalent bonds.
Examiner Tips
- 💡Define monomer and polymer clearly before describing polymer structure.
- 💡State that covalent bonds join atoms within a polymer molecule, while intermolecular forces act between molecules.
- 💡Use a named example, such as poly(ethene) from ethene, to show understanding.
- 💡When a diagram of a polymer is given, trace the continuous covalent bonds along the chain rather than assuming every line represents an intermolecular force.
- 💡Use the phrase 'strong covalent bonds within the polymer molecule' to make the location of the bonds clear.
- 💡If asked why a polymer melts at a higher temperature than simple molecules, link the answer to overcoming the relatively strong intermolecular forces between the large polymer chains, not breaking covalent bonds.
- 💡Use the term 'intermolecular forces' specifically for forces between molecules, not for bonds within a molecule.
- 💡When comparing a polymer with a small molecule, refer to the length of the polymer chain and the total effect of many intermolecular forces.
- 💡Link the solid state at room temperature to the energy needed to overcome the intermolecular forces between chains.
- 💡Trace the continuous line of covalent bonds to identify the main chain before looking at side groups.
- 💡Count the atoms in the repeating unit and check that the same pattern repeats along the chain.
- 💡If dashed lines are shown between chains, label them as intermolecular forces rather than bonds.
Common Mistakes
- Confusing monomers with polymers; the correction is that monomers are the small repeating units and polymers are the large molecules made from them.
- Thinking polymers are held together only by intermolecular forces; the correction is that covalent bonds hold the atoms within each polymer molecule.
- Assuming all polymers are identical; the correction is that different monomers produce different polymers with different properties.
- Saying that polymer molecules are held to each other by covalent bonds; correct this by stating that covalent bonds are within each polymer molecule, while intermolecular forces act between molecules.
- Confusing strong covalent bonds with strong intermolecular forces; correct this by explaining that the covalent bonds are strong but the forces between polymer molecules are relatively weak.
- Describing a polymer as a small molecule; correct this by stating that a polymer molecule is a large molecule made from many monomer units.
- Stating that polymers are solids because of strong covalent bonds between molecules; correct this by saying that the intermolecular forces between polymer molecules are relatively strong, while covalent bonds are within each molecule.
- Saying that polymers have no intermolecular forces; correct this by stating that intermolecular forces exist between polymer molecules and are relatively strong.
- Confusing the strength of intermolecular forces with the strength of covalent bonds; correct this by explaining that intermolecular forces are weaker than covalent bonds but strong enough to keep polymers solid at room temperature.
- Mistaking a small molecule such as ethene for a polymer; correct this by checking whether the diagram shows a long chain with repeating units.
- Treating dashed lines between chains as covalent bonds; correct this by identifying them as intermolecular forces.
- Choosing a repeating unit that does not repeat along the chain; correct this by selecting the shortest section that repeats.