Edexcel IGCSE Chemistry · Spec 4.34C-4.37C
Carboxylic Acids
The carboxylic acids and their typical reactions.
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Carboxylic Acids
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Explained
Carboxylic acids, and what makes them weak
The carboxylic acids are a homologous series with the carboxyl group as their functional group, written COOH. It is a carbon with a double bond to one oxygen and a single bond to an O H.
Their names end in oic acid: methanoic acid, ethanoic acid, propanoic acid, butanoic acid. Ethanoic acid is the one in vinegar, and it is also called acetic acid, which mark schemes accept.
They behave like acids
In water they release hydrogen ions, which is what makes a solution acidic, and they do everything you would expect of an acid.
- With a metal: a salt and hydrogen.
- With a metal oxide or hydroxide: a salt and water.
- With a carbonate: a salt, water and carbon dioxide.
The salts are named after the acid with the ending changed to oate. Ethanoic acid gives ethanoates, so with sodium carbonate it gives sodium ethanoate.
Why they are weak
A weak acid is one that is only partially ionised in water. Most of the molecules stay whole, and only a small proportion release their hydrogen ion at any moment.
A strong acid such as hydrochloric acid ionises completely, so the same concentration produces far more hydrogen ions.
Note what weak does not mean. It is not the same as dilute. Concentration is how much acid there is in a given volume; strength is what proportion of it ionises. You can have a concentrated weak acid, and vinegar is roughly that.
The consequences are measurable. At the same concentration, a weak acid has a higher pH, conducts electricity less well, and reacts more slowly with a metal or a carbonate. Those three comparisons are what a question about strength is usually asking for.
Making them from alcohols
Oxidising an alcohol gives the corresponding carboxylic acid. Ethanol gives ethanoic acid.
This can be done with an oxidising agent such as acidified potassium dichromate, which turns from orange to green as it works. It also happens on its own when bacteria in the air act on ethanol, which is why an opened bottle of wine turns to vinegar.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks what you would see when ethanoic acid reacts with sodium carbonate. It credits bubbles, fizzing or effervescence, the solid disappearing or getting smaller, and the loss of the vinegar smell. Then it says: ignore gas produced, even if correct.
The examiner report on that paper explains the reasoning. The majority scored a mark for fizzing, fewer knew that the carbonate disappears, and no marks were given for saying that a gas is produced, because that is not an observation.
An observation is something you can see, hear or smell. A gas being produced is a conclusion drawn from seeing bubbles, and the question asked what you would observe.
On the ester reaction, a later mark scheme names the two reactants required, a carboxylic acid and an alcohol, and accepts acetic acid for ethanoic acid and butan-1-ol for butanol. It awards the ester link as one mark and the rest of the molecule with all bonds shown as a second, with the second depending on the first.
Esters
A carboxylic acid reacts with an alcohol, with an acid catalyst, to form an ester and water. Ethanoic acid and ethanol give ethyl ethanoate.
Esters have distinctive sweet or fruity smells and are used in perfumes and flavourings. Mark schemes credit distinctive, fruity or sweet and ignore nice, so choose the descriptive word rather than the approving one.
Spec 4.34C-4.37C
What you need to know
- Recognise carboxylic acids and the C O O H group
- Describe their reactions as weak acids
- Explain how they're made from alcohols
Active recall
Quick check
Answer each question before opening the answer.
What is the functional group of carboxylic acids?
The –COOH group.
What is formed when a carboxylic acid reacts with a carbonate?
A salt, water and carbon dioxide.
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