How are plastics made from small molecules, and what problems do they cause?
Plastics: synthetic materials made by addition polymerisation, monomers and polymers, the repeating unit, uses of common plastics, and the problems of disposal.
An SQA National 5 Chemistry answer on plastics, covering synthetic materials, addition polymerisation, monomers and polymers, the repeating unit, the uses of common plastics, and the problems of plastic disposal.
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What this key area is asking
The SQA wants you to describe plastics as synthetic materials made by addition polymerisation, define monomer and polymer, draw or recognise the repeating unit, give uses of common plastics, and discuss the problems of disposal. It links straight back to the alkenes in Nature's Chemistry.
Plastics are synthetic
Addition polymerisation
So poly(ethene) is made from many ethene monomers, and poly(propene) from many propene monomers. The polymer is named by putting "poly" in front of the monomer name in brackets.
Monomer, polymer and the repeating unit
Uses of plastics
The problem of disposal
Worked example: naming a polymer and its monomer
Examples in context
Plastics replaced natural materials in countless everyday objects because they are cheap, light and durable, from drinks bottles to car parts. But their greatest strength, that they do not break down, is also their greatest problem: the same durability means that a plastic bottle dropped today may still exist in hundreds of years. This is the driving force behind recycling schemes and the development of newer biodegradable plastics designed to break down more readily.
Try this
Q1. Define a monomer and a polymer. [2 marks]
- Cue. A monomer is the small molecule that joins up; a polymer is the long molecule formed when many monomers join.
Q2. Name the polymer made from many ethene molecules. [1 mark]
- Cue. Poly(ethene).
Q3. Explain why most plastics cause a disposal problem. [2 marks]
- Cue. They are non-biodegradable, so they do not rot and build up as waste in landfill and the oceans.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 2018 style3 marksPoly(ethene) is made from ethene. Name the type of reaction used, explain what is meant by a monomer and a polymer, and state the feature of the ethene molecule that allows the reaction to happen.Show worked answer →
Markers reward the reaction type, the definitions, and the structural feature.
The reaction is addition polymerisation: many small molecules join to form one long molecule, with no other product made.
A monomer is the small molecule that joins up (here ethene), and a polymer is the long-chain molecule formed when many monomers join (here poly(ethene)).
The feature that allows the reaction is the carbon to carbon double bond in ethene. The double bond opens up so that the molecules can link together into a long chain, which is why only unsaturated monomers can take part in addition polymerisation.
SQA N5 2021 style3 marksMost plastics are described as non-biodegradable. Explain what this means and why it is a problem, and give one method used to dispose of plastics along with a drawback of that method.Show worked answer →
A 3 mark answer needs the meaning of non-biodegradable, the problem, and a disposal method with a drawback.
Non-biodegradable means that microorganisms in the environment cannot break the plastic down, so it does not rot away and instead remains for a very long time.
This is a problem because waste plastic builds up in landfill and in the oceans, harming wildlife and taking up space.
One disposal method is incineration (burning). A drawback is that burning plastics releases carbon dioxide and can release toxic or harmful gases, so it adds to pollution. (Landfill, with the drawback of taking up land and lasting for centuries, is also acceptable.)
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Sources & how we know this
- SQA National 5 Chemistry Course Specification — SQA (2019)