What are alcohols, how are they made, and what are they used for?
Alcohols as a homologous series with the OH functional group, the production of ethanol by fermentation and by hydration of ethene, and the uses and combustion of alcohols.
A CCEA GCSE Chemistry answer on alcohols, covering the OH functional group and the alcohol homologous series, the two ways of making ethanol (fermentation and the hydration of ethene), the combustion of alcohols, and their uses as fuels, solvents and drinks.
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What this dot point is asking
CCEA wants you to describe alcohols as a homologous series with the OH functional group, explain the two ways of making ethanol (fermentation and hydration of ethene), and describe the combustion and uses of alcohols.
The alcohol homologous series
The functional group is what matters: because every alcohol has the OH group, they react in similar ways, just as every alkene reacts through its double bond. Ethanol, , is the most important alcohol at GCSE.
Making ethanol by fermentation
Fermentation uses a renewable plant raw material, but it is a slow batch process that gives a dilute, impure solution of ethanol that must be distilled to concentrate it.
Making ethanol by hydration of ethene
This is a fast, continuous process that gives pure ethanol, but it uses non-renewable crude oil as its source. So the two methods trade off renewability against speed and purity.
Combustion and uses
Alcohols burn in oxygen to give carbon dioxide and water, releasing energy, which is why ethanol is used as a fuel (sometimes added to petrol). Ethanol is also used as a solvent (in perfumes, inks and cleaning products) and is the alcohol in alcoholic drinks.
Worked example
Examples in context
Example 1. Biofuel from crops. Ethanol made by fermenting sugar cane is blended with petrol as a renewable biofuel in some countries. The renewable raw material that makes fermentation attractive is exactly why it is chosen for fuel despite being slower.
Example 2. Ethanol as a solvent. Many perfumes and hand sanitisers use ethanol because it dissolves substances that water cannot and evaporates quickly. Its use as a solvent draws on the same OH group that links it to water-like behaviour.
Try this
Q1. Name the functional group present in all alcohols. [1 mark]
- Cue. The OH (hydroxyl) group.
Q2. State one advantage of making ethanol by fermentation rather than from ethene. [1 mark]
- Cue. It uses a renewable raw material (sugar from plants).
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 20194 marksDescribe how ethanol is made by fermentation, including the raw material, the conditions and the equation.Show worked answer →
Markers want the raw material, conditions and a correct equation.
Ethanol is made by fermentation of a sugar (glucose) from plant material, using yeast. The yeast provides enzymes that convert the sugar to ethanol.
The conditions are a warm temperature (about 30 to 40 degrees C) and the absence of air (anaerobic), so the ethanol is not oxidised further.
The equation is:
Markers reward glucose and yeast as raw materials, a warm anaerobic temperature, and the balanced equation giving ethanol and carbon dioxide.
CCEA 20214 marksEthanol can also be made by the hydration of ethene. State the conditions, write the equation, and give one advantage of this method over fermentation.Show worked answer →
The marks are for the conditions, the equation and a comparison.
Ethanol is made by reacting ethene with steam in the presence of a catalyst (phosphoric acid) at a high temperature and pressure. This is the hydration of ethene.
The equation is:
An advantage over fermentation is that it is a continuous process giving pure ethanol quickly, whereas fermentation is a slow batch process giving a dilute, impure product.
Markers reward ethene plus steam with a catalyst, the balanced equation, and a sensible advantage (continuous, fast, pure).
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)