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Carboxylic acids

The form a homologous series.

The general formula for carboxylic acid is CnH2nO2. The molecular formula is usually written with COOH functional group.

Example

Write the molecular formula of the carboxylic acid with 4 carbon atom.

General formula is CnH2nO2

n = 4

2n = 8

C4H8O2

removing COOH leaves C3H7 so the molecular formula may be written C3H7COOH.

The functional group in the carboxylic acids is the carboxyl group 鈥揅OOH. It is responsible for the carboxylic acids鈥 typical reactions. They are weak acids 鈥 vinegar is a dilute solution of ethanoic acid.

The first four carboxylic acids - higher tier

Acid table

Acid properties

The carboxylic acids have properties typical of acids. They:

  • dissolve in water to form acidic solutions with pH values less than 7
  • react with metals to form a salt and hydrogen
  • react with bases to form a salt and water
  • react with carbonates to form a salt, water and carbon dioxide

These properties are due to the 鈥揅OOH functional group.

Weak and strong acids (Higher tier only)

Carboxylic acids are weak acids because they only partially ionise in solution.

Their solutions do not contain many hydrogen ions compared to a solution of a strong acid at the same concentration. A weak acid鈥檚 pH will be higher than a strong acid鈥檚 pH at the same concentration. In a solution of strong acid, the molecules are fully ionised. In a weak acid, few of the molecules are ionised.