Carboxylic Acids (GCSE Chemistry)

Carboxylic Acids

Carboxylic Acids

• ‘COOH’ is the functional group of alcohols. Carboxylic acids have a functional group made up of a C=O bond and C-OH bond on the same carbon, which is simplified to COOH.

• Carboxylic acids can be named. As we have seen with alkanes,  alkenes and alcohols, carboxylic acids can also be named. The first four alcohols in the homologous series are called methanoic acid, ethanoic acid, propanoic acid, butanoic acid.

Representing Carboxylic Acids

• Carboxylic acids can be represented. We can represent carboxylic acids very easily, by using the number of carbons each molecule contains. Ethanoic acid can be represented as CH₃COOH.
• Carboxylic acids can be drawn out. We can also represent carboxylic acids using their displayed formulae. For example, the displayed formulae of ethanoic acid is shown below.

Reactions of Carboxylic Acids

• Carboxylic acids undergo various reactions. For exams, you need to know about the reactions of carboxylic acids with carbonates, water and alcohols.

Carboxylic Acids and Carbonates

• Carboxylic acids react with metal carbonates. Like other acids,  carboxylic acids can react with metal carbonates.
• Three products are formed. When carboxylic acids react with metal carbonates, salt, water and carbon dioxide are produced. The suffix ‘-anoate’ can be used to name the salts formed.

Propanoic acid + magnesium carbonate ⟶ magnesium propanoate + water + carbon dioxide

Carboxylic Acids and Water

• Carboxylic acids are soluble in water. When carboxylic acids are placed in water, they dissolve. This means that they are soluble.
• H⁺ ions are formed. When a carboxylic acid dissolves in water, they form free H⁺ ions. This means that they have ionised. The pH of the solution is decreased, making it weakly acidic.

Weak Acids

• Carboxylic acids ionise in water. As previously mentioned, carboxylic acids can ionise in water. They release H⁺ ions to form an acidic solution.
• The ionisation is only partial. The ionisation of carboxylic acids in water is only partial ionisation. This means that not all of the H⁺ ions are released. We use the reversible sign “ ⇌ “   to show that partial ionisation takes place. For example with ethanoic acid, the reaction is written as:

CH₃COOH(l)  ⇌   CH₃COO^–(aq)  +  H⁺(aq)

• Carboxylic acids are weak acids. Due to this partial ionisation, carboxylic acids are known as weak acids. Their pH is lower than a neutral solution such as water, but higher than a strong acid such as hydrochloric acid.

Carboxylic Acids and Alcohols

• Carboxylic acids react with alcohols. Carboxylic acids can react with alcohols in the presence of a catalyst. They react together to form an ester.
• Concentrated sulphuric acid can be a catalyst. In the reaction of carboxylic acids and alcohols, concentrated sulphuric acid can be the catalyst. As with all catalysts, the sulphuric acid will remain unchanged at the end of the reaction.

Vinegar

Vinegar is an aqueous solution containing ethanoic acid.

It is formed in the reaction between ethanol and oxygen. When a bottle of wine is opened, bacteria uses oxygen from the air to oxidise the ethanol in the wine. This is known as microbial oxidation.

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