Carboxylic acids

Carboxylic acids are a homologous series with the –COOH group: methanoic, ethanoic, propanoic and butanoic acid. They are made by oxidising alcohols, and vinegar is a solution of ethanoic acid. They are weak acids: they only partly ionise in water, but they still react with metals, bases and carbonates like other acids.

Key facts

  • Carboxylic acids contain –COOH: methanoic, ethanoic, propanoic and butanoic acid. They are weak acids and react with carbonates to give carbon dioxide.
  • Oxidising an alcohol gives the carboxylic acid with the same number of carbon atoms: ethanol gives ethanoic acid.
  • Vinegar is a dilute solution of ethanoic acid.

Worked examples

Example 1

Draw ethanol and ethanoic acid and name their functional groups.

  1. Ethanol has two carbons and an –OH group on the end: it is an alcohol. C2H5OH
  2. Ethanoic acid has two carbons; the second carbon has a double-bonded O and an –OH. Together they are –COOH. CH3COOH
  3. Oxidising ethanol turns its –CH2OH end into –COOH, which is how wine turns to vinegar.

Answer: ethanol: –OH (hydroxyl); ethanoic acid: –COOH (carboxyl)

Example 2

What is the name of chem HCOOH?

  1. It has 1 carbon atom, counting the carbon in –COOH.
  2. The –COOH group makes it a carboxylic acid: "-anoic acid".
  3. So it is methanoic acid.

Answer: methanoic acid

Example 3

What is the name of chem C2H5COOH?

  1. It has 3 carbon atoms, counting the carbon in –COOH.
  2. The –COOH group makes it a carboxylic acid: "-anoic acid".
  3. So it is propanoic acid.

Answer: propanoic acid

Example 4

Which gas is made when ethanoic acid reacts with sodium carbonate?

  1. Carboxylic acids react like other acids, but more slowly.
  2. With a carbonate they make a salt, water and carbon dioxide.

Answer: carbon dioxide

Example 5

A student adds sodium carbonate to ethanoic acid and to ethanol. What would be seen?

  1. Ethanoic acid is a carboxylic acid, a weak acid.
  2. Acids react with carbonates: fizzing as carbon dioxide is given off. acid + carbonate → salt + water + CO2
  3. Ethanol is not an acid, so nothing is seen.

Answer: fizzing (carbon dioxide) only with ethanoic acid

Example 6

Why is ethanoic acid called a weak acid?

  1. A weak acid only partly ionises in water.
  2. So it has fewer hydrogen ions than a strong acid of the same concentration, and a higher pH.

Answer: it only partly ionises in water