Carboxylic Acids (Oxford AQA IGCSE Combined Science Double Award)

The Carboxylic Acid Functional Group

• Carboxylic acids are a homologous series of compounds that have the general formula of

C_nH_2n+1 COOH 

The general formula of a carboxylic acid

• Ethanoic acid has the formula CH₃COOH

• They differ by one -CH₂ in the molecular formulae from one member to the next

The first three carboxylic acids

Properties of Carboxylic Acids

• Carboxylic acids react with carbonates to form a salt, water and carbon dioxide

  • During this reaction fizzing is seen so can be used as a test to identify carbonate ions:

carboxylic acid + carbonate ⟶ salt + water + carbon dioxide

ethanoic acid + calcium carbonate ⟶ calcium ethanoate + water + carbon dioxide

2CH₃COOH + CaCO₃ ⟶ Ca(CH₃COO)₂ + H₂O + CO₂

• In the presence of a catalyst (usually concentrated sulfuric acid) carboxylic acids react with alcohols to produce esters

• Esters are compounds with the functional group R-COO-R

• Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate:

ethanoic acid + ethanol → ethyl ethanoate + water

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

• Carboxylic acids dissolve in water to produce acidic solutions and have pH values of between 3 and 7 which means they are classed as weak acids

• They do not fully dissociate when dissolved in water therefore form an equilibrium with their ions

  • Most carboxylic acid molecules remain un dissociated:

CH₃COOH ⇌ H⁺ + CH₃COO^–

• This means that at the same concentration as strong acids, carboxylic acids will have a higher pH