Edexcel IGCSE Chemistry · Spec 4.29C-4.33C
Alcohols
The alcohols — their structure, properties and reactions, including combustion.
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Alcohols
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Explained
Alcohols, and the two ways to make ethanol
The alcohols are a homologous series defined by their functional group, the hydroxyl group written as O H, an oxygen joined to a hydrogen. Their general formula is C n H 2n plus 1 O H, and their names end in ol.
Methanol has one carbon, ethanol two, propanol three, butanol four. The functional group is what decides the chemistry, so all four react in the same ways, with the reactions becoming slightly less vigorous as the chain gets longer.
Reactions
Alcohols burn in a good supply of air to give carbon dioxide and water, releasing energy. This is why ethanol is used as a fuel, and why it is described as flammable rather than simply dangerous.
They can be oxidised to carboxylic acids, either by an oxidising agent such as potassium dichromate or by bacteria in the air, which is what turns an open bottle of wine into vinegar. Ethanol becomes ethanoic acid.
They react with carboxylic acids, in the presence of an acid catalyst, to form esters and water. Esters have distinctive sweet or fruity smells and are used in perfumes and flavourings.
The two routes to ethanol
Fermentation uses yeast to convert glucose from a plant source into ethanol and carbon dioxide. It runs at around 30 to 40 degrees Celsius, without air, and it uses a renewable raw material. It is a batch process and produces an impure, dilute product that must be distilled.
Hydration of ethene reacts ethene with steam over a catalyst, at high temperature and pressure. It is a continuous process and gives pure ethanol, but ethene comes from crude oil, which is finite.
Comparison questions want that trade off stated: renewable and cheap equipment but slow, impure and batch, against fast, pure and continuous but dependent on a non renewable feedstock.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme gives the conditions for hydrating ethene as a pressure of 60 to 70 atmospheres and phosphoric acid as the catalyst, allowing sulfuric acid as an alternative. The examiner report on an earlier paper says plainly that these conditions are often tested and should be learned.
For fermentation the same mark scheme asks for the condition and credits anaerobic, no oxygen, or absence of air, then instructs the examiner to ignore respiration. The word names a process rather than the condition asked for.
On temperature it is precise in both directions: enzymes become inactive below 30 degrees Celsius, and yeast cells or enzymes denature above 40. If neither of those is given, saying that the enzymes in yeast need an optimum temperature scores one mark instead of two, which again shows the general answer being worth less than the specific one.
Two smaller notes are worth having. On a safety question the mark scheme wants ethanol described as flammable or able to ignite. And on identifying an ester it credits a distinctive, fruity or sweet smell while explicitly ignoring nice smell.
A note on the word alcohol
In everyday use, alcohol means ethanol specifically. In chemistry it means the whole family, so a question about the alcohols is asking about the homologous series.
Read which is meant from the context. If the question names a drink or a fuel it means ethanol; if it gives a general formula or asks about a trend it means the family.
Spec 4.29C-4.33C
What you need to know
- Recognise the alcohols and the O H group
- Describe how ethanol is made
- Describe how alcohols react
Active recall
Quick check
Answer each question before opening the answer.
What is the functional group of alcohols and the name of the first member?
The –OH group; the first alcohol is methanol.
Give two ways of making ethanol.
Fermentation of sugars by yeast, and hydration of ethene with steam.
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