What are alcohols, how is ethanol made, and how do alkenes form polymers?
Alcohols as a homologous series with the OH functional group, the production of ethanol by fermentation and by hydration of ethene, addition polymerisation of alkenes, drawing the repeating unit, and the problems of plastic disposal.
A focused CCEA GCSE Double Award Science (Chemistry Unit C2) answer on alcohols and polymers, covering alcohols and the OH group, making ethanol by fermentation and by hydration of ethene, addition polymerisation of alkenes, the repeating unit, and plastic disposal.
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What this dot point is asking
CCEA Double Award wants alcohols and the OH functional group, the two ways to make ethanol, addition polymerisation of alkenes with the repeating unit, and the problems of disposing of plastics. The two routes to ethanol and the polymer repeating unit are the high-mark parts.
Alcohols
Alcohols burn (combustion) to release energy and are used as fuels and solvents. The OH group gives them their characteristic reactions.
Making ethanol
Ethanol can be made in two ways:
- Fermentation: sugar is broken down by yeast at a warm temperature, with no oxygen, to make ethanol and carbon dioxide. The raw material (sugar from plants) is renewable, but the process is a slow batch process and gives a dilute, impure ethanol that needs distilling.
- Hydration of ethene: ethene (from crude oil) reacts with steam over a catalyst to make ethanol. This is a fast, continuous process that gives pure ethanol, but it uses a finite resource (crude oil).
Addition polymers
For example, many ethene molecules join to make poly(ethene). The repeating unit is the part of the polymer that repeats, drawn from the monomer with the double bond changed to a single bond and bonds extending out each side. Other examples are poly(propene) and poly(chloroethene) (PVC).
Disposing of polymers
Addition polymers are very unreactive and not biodegradable, so microorganisms cannot break them down and they last for a very long time in landfill. Burning them can release toxic gases, and recycling is limited because different plastics must be sorted. These disposal problems are a major environmental issue.
Examples in context
Example 1. Biofuel from sugar cane. Some countries ferment sugar cane to make ethanol that is blended with petrol as a renewable fuel. This uses the fermentation route because the crop is renewable, even though it is slower and the ethanol needs purifying.
Example 2. Why plastic bags last for centuries. Poly(ethene) bags are so unreactive that they do not rot, so a bag dropped today could still be in the environment in hundreds of years. This durability, useful in the product, is exactly what makes disposal such a problem.
Try this
Q1. What is the functional group of an alcohol? [1 mark]
- Cue. The OH group.
Q2. Name the polymer made from ethene. [1 mark]
- Cue. Poly(ethene).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style4 marksCompare making ethanol by fermentation with making it by the hydration of ethene.Show worked answer →
Two contrasts for four marks.
Fermentation uses sugar (a renewable resource) and yeast at a warm temperature; it is a batch process and makes a dilute, impure ethanol that must be distilled.
Hydration of ethene reacts ethene (from crude oil) with steam over a catalyst; it is continuous and makes pure ethanol.
So fermentation uses renewable materials but is slower and less pure, while hydration is fast and pure but uses a finite resource. Markers reward the raw materials and one further contrast such as batch versus continuous or purity.
CCEA-style3 marksExplain why addition polymers are difficult to dispose of.Show worked answer →
Link the structure to disposal for three marks.
Addition polymers are unreactive and not biodegradable, so microorganisms cannot break them down.
They take a very long time to decay in landfill.
Burning them can release toxic gases. Markers want not biodegradable, lasting in landfill, and a disposal problem such as toxic fumes.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)