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

Forming salts from carboxylic acids

• Solutions of carboxylic acids are acidic

  • This means that they have a pH of less than 7

• Like other acids (e.g., hydrochloric acid), they take part in neutralisation reactions to form salts

Naming the salts

• The salts formed from carboxylic acids have a specific naming pattern

1. The first part of the salt's name comes from the metal, metal oxide, metal hydroxide or metal carbonate used in the reaction

   • For example, copper oxide would form a copper salt

2. The second part of the salt's name comes from the carboxylic acid

   • For example, ethanoic acid would form an ethanoate salt

   • The "-oic acid" ending is changed to "-anoate", as shown in the following table:

Table of salts

Reactions of carboxylic acids

• Carboxylic acids can react with:

  • Metals

  • Metal oxides

  • Metal hydroxides

  • Metal carbonates

Reaction with metals

• Carboxylic acids will react with metals above hydrogen in the Electrochemical Series

  • Metals below hydrogen, like copper, will not react

• The more reactive the metal (i.e., the higher it is in the series), the faster the reaction will be

• The reaction produces a salt and hydrogen gas

  • So, you would see fizzing (effervescence) as the hydrogen gas is produced

  • See Analytical Methods - Gas Tests for the test for hydrogen gas

• The general word equation is:

metal + carboxylic acid → metal alkanoate + hydrogen

• For example:

magnesium + ethanoic acid → magnesium ethanoate + hydrogen

Reaction with metal oxides

• Carboxylic acids react with metal oxides in a neutralisation reaction

• Metal oxides are examples of bases

  • Many metal oxides (like copper oxide) are bases but are not alkalis because they are insoluble in water

• The reaction produces a salt and water

  • So, you might observe the solid metal oxide "disappearing" as it reacts to form a soluble salt

• The general word equation is:

metal oxide + carboxylic acid → metal alkanoate + water

• For example:

copper(II) oxide + propanoic acid → copper(II) propanoate + water

Reaction with metal hydroxides

• Carboxylic acids are neutralised by metal hydroxides

• Soluble metal hydroxides are the most common examples of alkalis

  • Remember: an alkali is a soluble base that dissolves in water to form a solution with a pH above 7

• This is a classic neutralisation reaction, producing a salt and water

• The reaction is often very fast

• The general word equation is:

metal hydroxide + carboxylic acid → metal alkanoate + water

• For example:

sodium hydroxide + butanoic acid → sodium butanoate + water

Reaction with metal carbonates

• Carboxylic acids react with metal carbonates, which are a type of base

• This reaction produces three products:

  • A salt

  • Water

  • Carbon dioxide

• The production of carbon dioxide gas is seen as fizzing (effervescence)

  • This makes the reaction easy to identify and monitor

  • See Analytical Methods - Gas Tests for the test for carbon dioxide gas

• The general word equation is:

metal carbonate + carboxylic acid → metal alkanoate + water + carbon dioxide

• For example:

calcium carbonate + pentanoic acid → calcium pentanoate + water + carbon dioxide