Carboxylic AcidsOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is the carboxylic acid homologous series?
Carboxylic acids are a homologous series of compounds containing the functional group −COOH. The first three members are:
| Name | Formula |
|---|---|
| Methanoic acid | HCOOH |
| Ethanoic acid | CH3COOH |
| Propanoic acid | C2H5COOH |
Ethanoic acid is the most commonly studied member and is the main acid in vinegar.
Section 2
What are the properties of carboxylic acids?
Carboxylic acids:
- Dissolve in water to produce acidic solutions
- React with carbonates to produce carbon dioxide, a salt and water
- React with alcohols, in the presence of an acid catalyst, to produce esters
These reactions show that carboxylic acids behave chemically like other acids (e.g. hydrochloric acid), just more weakly.
Ethanoic acid reacting with sodium carbonate produces sodium ethanoate, water and carbon dioxide gas, which can be tested for with limewater.
Section 3
Why are carboxylic acids described as weak acids?
Carboxylic acids do not ionise completely when dissolved in water — only a small proportion of the molecules split into ions. This means that, at the same concentration, a carboxylic acid solution has:
- Fewer H+ ions in solution than a strong acid
- A higher pH than a strong acid of the same concentration (though it is still below 7, so still acidic)
A weak acid is not the same as a dilute acid — 'weak' refers to how completely the acid ionises, while 'dilute' refers to how much water it is mixed with. A concentrated weak acid can still have a higher pH than a very dilute strong acid.
Section 4
How do carboxylic acids react — writing equations?
Carboxylic acids react in similar ways to other acids. Balanced equations should be written using the correct formulae and state symbols:
- With carbonates: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2
- With alcohols (esterification): CH3COOH + C2H5OH → CH3COOC2H5 + H2O (needs an acid catalyst)
When writing these equations in an exam, always balance the equation and include state symbols where asked, and name the acid catalyst if the reaction is esterification.
Must Know
- Carboxylic acids contain the functional group −COOH; methanoic, ethanoic and propanoic acid are the first three members
- They dissolve in water to form acidic solutions
- They react with carbonates to produce carbon dioxide, a salt and water
- They react with alcohols, using an acid catalyst, to form esters
- Carboxylic acids are weak acids — they do not ionise completely in water
- A weak acid solution has a higher pH than a strong acid solution of the same concentration
That's the notes covered.
Carry on to the next subtopic.