What makes a molecule a carboxylic acid, and how do these acids react?
Carboxylic acids: the carboxyl functional group, naming the straight-chain acids, their reactions with bases to form salts and water, and everyday examples such as vinegar.
An SQA National 5 Chemistry answer on carboxylic acids, covering the carboxyl functional group, naming the straight-chain acids with the -oic acid ending, their reactions with bases to form salts and water, and everyday examples such as ethanoic acid in vinegar.
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What this key area is asking
The SQA wants you to recognise the carboxyl functional group, name the straight-chain carboxylic acids using the -oic acid ending, describe their reactions with bases to form a salt and water, and give everyday examples such as ethanoic acid in vinegar. Carboxylic acids build directly on the alcohols and on the acids and bases key area.
The functional group
Naming the carboxylic acids
Carboxylic acids are acids
Because the carboxyl group releases hydrogen ions () in water, carboxylic acids behave like any acid:
- they turn universal indicator red or orange and have a pH below 7
- they react with reactive metals, metal oxides, metal hydroxides, alkalis and metal carbonates
Reactions to form salts
Worked example: naming a salt
How carboxylic acids link to alcohols
Carboxylic acids and alcohols are closely connected. When an alcohol is oxidised, it can be turned into the carboxylic acid with the same number of carbon atoms: ethanol (an alcohol) is oxidised to ethanoic acid (a carboxylic acid). This is the chemistry behind wine or cider turning sour, where the ethanol slowly reacts with oxygen in the air and becomes ethanoic acid. Because of this link, the two families are often taught and examined together, and you should be able to move between the alcohol name and the matching carboxylic acid name.
Carboxylic acids as a homologous series
Like the alkanes and the alcohols, carboxylic acids form a homologous series. Each member differs from the next by a unit, they all share the carboxyl group and therefore react in the same acidic way, and their physical properties change gradually down the series. This means that once you know how ethanoic acid behaves, you can predict the reactions of propanoic acid or butanoic acid, because the reactive carboxyl group is the same in every member.
Examples in context
Ethanoic acid is the acid in vinegar, giving its sharp taste and used in cooking, pickling and as a mild household cleaner because it dissolves limescale (a carbonate) by reacting with it. Methanoic acid is found in the sting of ants and nettles. Carboxylic acids also form when alcohols are oxidised, which is why wine left open turns sour as the ethanol becomes ethanoic acid, the same chemistry that turns cider into vinegar.
Try this
Q1. Name the functional group in all carboxylic acids. [1 mark]
- Cue. The carboxyl group, -COOH.
Q2. Name the carboxylic acid with one carbon atom. [1 mark]
- Cue. Methanoic acid.
Q3. Name the salt formed when ethanoic acid reacts with sodium hydroxide. [1 mark]
- Cue. Sodium ethanoate (plus water).
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 2019 style3 marksName the functional group present in all carboxylic acids, name the carboxylic acid with two carbon atoms, and name the everyday product in which it is found.Show worked answer →
Markers reward the functional group, the correct name and the everyday example.
The functional group in all carboxylic acids is the carboxyl group, written -COOH. It is the group that makes these compounds acidic.
The carboxylic acid with two carbon atoms is ethanoic acid; carboxylic acids are named from the matching alkane with the ending changed to -oic acid, so ethane becomes ethanoic acid.
Ethanoic acid is the acid found in vinegar, where it gives the sharp taste and sour smell.
SQA N5 2021 style3 marksEthanoic acid reacts with sodium hydroxide. Name the type of reaction, state the two products formed, and explain how this is similar to the reaction of a mineral acid such as hydrochloric acid with an alkali.Show worked answer →
A 3 mark answer needs the reaction type, the products, and the link to general neutralisation.
The reaction is neutralisation. Ethanoic acid reacts with the alkali sodium hydroxide to form a salt and water; the salt here is sodium ethanoate.
It is similar to a mineral acid reacting with an alkali because, like hydrochloric acid, ethanoic acid releases hydrogen ions in solution. These hydrogen ions react with the hydroxide ions from the alkali to form water, while the metal ion and the acid ion form the salt. The same salt-plus-water pattern applies to both kinds of acid.
Related dot points
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An SQA National 5 Chemistry answer on alcohols, covering the hydroxyl functional group, naming the straight-chain alcohols with the -ol ending, their properties such as solubility and flammability, and their uses as solvents and fuels.
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Sources & how we know this
- SQA National 5 Chemistry Course Specification — SQA (2019)