Concentrated Sulfuric Acid (WJEC GCSE Chemistry)

• Concentrated sulfuric acid is a dehydrating agent
• Dehydrating agents are involved in dehydration reactions, which involve:
  • The removal of the elements of water, e.g. hydrogen and oxygen from sugar
  • The removal of water of crystallisation, e.g. from hydrated copper(II) sulfate

Removing the elements of water

• A common example of concentrated sulfuric acid as a dehydrating is its reaction with sugars, such as sucrose
• Sucrose has the chemical formula C₁₂H₂₂O₁₁ 
  • The ratio of hydrogen : oxygen atoms is 2 : 1, which is the same ratio found in water, H₂O
• Reacting concentrated sulfuric acid with sucrose removes the hydrogen and oxygen atoms, leaving carbon
• When concentrated sulfuric acid is added to sucrose, the sugar starts to caramelise and turn brown
  • This very quickly turns black as carbon is formed
• The reaction is very exothermic
  • The heat energy released makes some of the water evaporate, which can be seen as water vapour
  • Some of the water vapour also gets trapped inside the carbon, causing it to grow out of the container as a column

Concentrated sulfuric acid + sucrose

The evidence of a chemical reaction is turning brown then black, releasing water vapour and growing out of the container

Removing water of crystallisation

• Placing hydrated copper(II) sulfate in a container with concentrated sulfuric acid will produce anhydrous copper(II) sulfate
  • Due to its strength, the concentrated sulfuric acid is typically added dropwise
• This can be seen as the blue hydrated copper(II) sulfate forms white anhydrous copper(II) sulfate

Concentrated sulfuric acid + hydrated copper(II) sulfate

The evidence of a chemical reaction is a colour change from blue to white