Carboxylic Acids (OCR A Level Chemistry A): Revision Note

Properties of Carboxylic Acids

• Carboxylic acid is the name given to the family of compounds that contain the carboxyl functional group, -COOH

• The general formula of a carboxylic acid is C_nH_2n+1COOH which can be shortened to just RCOOH

  • (In some countries, this family is also called alkanoic acid)

• The nomenclature of carboxylic acid follows the pattern alkan + oic acid, e.g. propanoic acid

• There is no need to use numbers in the name as the carboxyl group, COOH, is always on the number 1 carbon atom

Carboxylic Acids Examples Table

Weak Acids

• Carboxylic acids with fewer than six carbon atoms per molecule are water-soluble

  • This is because water molecules can hydrogen-bond with the functional group

• In aqueous solution they are only slightly ionised, to give low concentrations of hydronium ions and alkanoate ions (often called carboxylate ions)

  

   Carboxylic acids are weak acids that do not fully dissociate in water, the position of the equilibrium lies to the left

• This partial ionisation in solution means that carboxylic acids are weak acids

Reactions of Carboxylic Acids

• Carboxylic acids can form salts with metals, alkalis and carbonates.

• In the reaction with metal oxides, a metal salt and water are produced

  • For example in reaction with magnesium the salt magnesium ethanoate is formed:

2CH₃COOH (aq) + MgO (s) → (CH₃COO)₂Mg (aq) + H₂O (l) 

• In the reaction with alkalis a salt and water are formed in a neutralisation reaction

  • For example in reaction with potassium hydroxide the salt potassium ethanoate is formed:

CH₃COOH (aq) + KOH (aq) → CH₃COOK (aq) + H₂O (l)

• In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced

  • For example in reaction with potassium carbonate the salt potassium ethanoate is formed:

2CH₃COOH (aq) + K₂CO₃ (s) → 2CH₃COOK (aq) + H₂O (l) + CO₂ (g)