What are the alcohols and carboxylic acids, and how do they react?
Alcohols and carboxylic acids: the alcohol and carboxylic acid homologous series, their functional groups, the reactions of alcohols including combustion and oxidation, and the reactions of carboxylic acids.
A focused answer to Edexcel GCSE Chemistry topic 9 (separate chemistry), covering the alcohol and carboxylic acid homologous series and their functional groups, how alcohols are made by fermentation and how they burn and are oxidised, and the reactions of carboxylic acids as weak acids including with carbonates.
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What this dot point is asking
Edexcel wants you (in separate chemistry) to describe the alcohol and carboxylic acid homologous series and their functional groups, recall how ethanol is made by fermentation, describe how alcohols burn and are oxidised, and describe the reactions of carboxylic acids as weak acids. The functional groups and the reactions of ethanol are the central marks.
The alcohols
A homologous series is a family of compounds with the same functional group and general formula, differing by , with similar chemical properties and a gradual trend in physical properties.
Making ethanol by fermentation
Ethanol is produced by the fermentation of sugars (such as glucose) using yeast, which provides enzymes:
Fermentation is carried out at a warm temperature (around ) in the absence of air (anaerobic), and stops when the ethanol concentration becomes too high for the yeast.
Reactions of alcohols
Alcohols undergo several characteristic reactions:
- Combustion. They burn in oxygen to give carbon dioxide and water, releasing energy: . This is why ethanol is used as a fuel.
- Dissolving in water. They are soluble and form neutral solutions (pH 7).
- Oxidation. They can be oxidised to carboxylic acids, either by microbial action in air (which is why wine left open turns sour) or by an oxidising agent. Ethanol is oxidised to ethanoic acid.
The carboxylic acids
Reactions of carboxylic acids
Carboxylic acids are weak acids: they only partially dissociate into ions in solution, so they have a higher pH than a strong acid of the same concentration. They react like other acids:
Try this
Q1. State the functional group of the alcohols and of the carboxylic acids. [2 marks]
- Cue. Alcohols ; carboxylic acids .
Q2. Name the substance and conditions used to make ethanol by fermentation. [2 marks]
- Cue. Yeast (enzymes) ferment sugar, warm and in the absence of air.
Q3. Describe what you would see when ethanoic acid is added to sodium carbonate, and name the gas. [2 marks]
- Cue. It fizzes; the gas is carbon dioxide.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksEthanol is a member of the alcohol homologous series. State the functional group of the alcohols, give the products when ethanol is completely burned in air, and describe what is produced when ethanol is oxidised.Show worked answer →
A 4-mark alcohol-reactions question.
The functional group of the alcohols is the hydroxyl group, (1 mark). When ethanol burns completely in air, it produces carbon dioxide and water: (1 mark for the products, 1 mark for a correct or balanced equation). When ethanol is oxidised (for example by an oxidising agent or by microbes in air), it forms ethanoic acid, a carboxylic acid (1 mark).
Markers reward the functional group, the combustion products, and oxidation to ethanoic acid.
Edexcel 20214 marksEthanoic acid is a weak carboxylic acid. State the functional group of the carboxylic acids, explain what is meant by a weak acid, and describe what you would observe when ethanoic acid is added to sodium carbonate.Show worked answer →
A 4-mark carboxylic-acid question.
The functional group of the carboxylic acids is (the carboxyl group) (1 mark). A weak acid is one that is only partially dissociated into ions in solution (1 mark). When ethanoic acid is added to sodium carbonate, it fizzes (1 mark), because the acid reacts with the carbonate to produce carbon dioxide gas (along with a salt and water) (1 mark).
Markers reward the group, partial dissociation for a weak acid, and the fizzing from carbon dioxide.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)