Carboxylic Acids
Recap Carboxylic Acids for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) chemistry topic: carboxylic acids have the –COOH group, are weak acids, and include ethanoic acid (vinegar).
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Topic overview
Carboxylic acids contain the functional group –COOH, and their names end in -oic acid. This is a Triple-only topic.
The first four are methanoic acid, ethanoic acid, propanoic acid and butanoic acid. Ethanoic acid is the acid in vinegar.
They behave as typical acids but are weak. They dissolve in water to produce acidic solutions, and they react with carbonates to produce a salt, water and carbon dioxide. Because they only partially ionise in water, they have a higher pH than a strong acid of the same concentration.
Revision notes
Structure and naming
Carboxylic acids contain the –COOH functional group, and their names end in -oic acid.
The first four are methanoic, ethanoic, propanoic and butanoic acid. Ethanoic acid is the acid found in vinegar.
Reactions
They dissolve in water to produce acidic solutions.
They react with carbonates to produce a salt, water and carbon dioxide — the fizzing is a useful identification. They react with alcohols in the presence of an acid catalyst to produce esters.
Why they are weak acids
Carboxylic acids do not ionise completely in water.
Only a small proportion of molecules release hydrogen ions, so the hydrogen ion concentration is lower than for a strong acid of the same concentration, and the pH is therefore higher.
Key points
- Carboxylic acids contain the –COOH group.
- Their names end in -oic acid.
- Ethanoic acid is the acid in vinegar.
- They form acidic solutions in water.
- They react with carbonates to give carbon dioxide.
- They are weak acids because they partially ionise.
Worked examples
Example 1
Name the functional group present in all carboxylic acids. [1 mark]
Model answer
The –COOH group, also called the carboxyl group (1 mark).
Example 2
State the three products formed when a carboxylic acid reacts with a carbonate. [3 marks]
Model answer
A salt (1 mark), water (1 mark) and carbon dioxide (1 mark).
Example 3
Explain why ethanoic acid has a higher pH than hydrochloric acid of the same concentration. [3 marks]
Model answer
Ethanoic acid is a weak acid, so it only partially ionises in water (1 mark). It therefore produces fewer hydrogen ions than hydrochloric acid, which ionises completely (1 mark). A lower hydrogen ion concentration means a higher pH (1 mark).
Common mistakes
Saying carboxylic acids are strong acids.
They are weak, because they only partially ionise.
Forgetting carbon dioxide with carbonates.
Three products are formed: salt, water and carbon dioxide.
Writing the functional group as –OH.
That is an alcohol. Carboxylic acids have –COOH.
Naming ethanoic acid as ethanol.
Ethanol is the alcohol; ethanoic acid is the carboxylic acid.
Exam tips
- Learn –COOH and the -oic acid ending.
- Give all three products with carbonates.
- Explain weakness through partial ionisation.
- Remember this is a Triple-only topic.
Key terms
- Carboxylic acid
- A compound containing the –COOH functional group.
- Ethanoic acid
- The carboxylic acid found in vinegar.
- Weak acid
- An acid that only partially ionises in water.
- Ester
- The product of a carboxylic acid reacting with an alcohol.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.