Properties of Carboxylic Acids (SQA National 5 Chemistry): Revision Note

Miscibility of carboxylic acids

• The miscibility / solubility of a carboxylic acid in water changes as its carbon chain gets longer

Miscibility of small carboxylic acids

• Methanoic, ethanoic, propanoic and butanoic acid are miscible with water

• Their size and -COOH group allow them to mix completely with water

Miscibility of larger carboxylic acids

• From pentanoic acid onwards, the solubility decreases significantly as the carbon chain gets longer

• The long hydrocarbon part of the molecule does not mix well with water

• This effect becomes more significant as the chain grows

Solubility of carboxylic acids in water

Melting & boiling points of carboxylic acids

• As the size of carboxylic acid molecules increases, their melting and boiling points also increase

Melting points of carboxylic acids

• The melting points of carboxylic acids generally increase as the number of carbons in the chain increases

  • In general, a bigger molecule means a higher melting point

• The trend for melting points of carboxylic acids is less regular than boiling points

Boiling points of carboxylic acids

• The boiling points of carboxylic acids show a much clearer and more predictable trend:

  • The boiling points increase as the molecule gets bigger

Explaining the general trend

• The overall trend for both melting and boiling point is that they increase as the molecule gets bigger

• The melting and boiling points increase because:

  1. As carboxylic acid molecules get larger, the strength of the intermolecular forces increases

  2. More energy is needed to overcome these stronger forces to allow the substance to melt or boil

  3. Therefore, the melting and boiling points generally increase