Topic 9 - Sulphur dioxide and sulphuric acid

SULPHUR DIOXIDE, SO2

• This is the most important oxide of sulphur, and is formed when sulphur burns. In industry it is obtained when sulphide metal ores such as zinc sulphide are roasted in air.
• Chemical properties of sulphur dioxide:
  • It's an acidic oxide - reacts with aklais to form a salt.
  • It's a reducing agent - will remove oxygen from other compounds.
  • It has a bleaching action - used in paper-making industry.
  • It kills bacteria - used as a food preservative.

A chemical test for sulphur dioxide

A solution of potassium dichromate(VI) is acidified with dilute sulphuric acid. This mixture is bright orange. A piece of filter paper dipped into this and exposed to sulphur dioxide gas will turn green due to the formation of chromium sulphate.

Another chemical test for sulphur dioxide

A solution of potassium manganate(VII) is acidified with sulphuric acid. This is deep purple. A paper dipped in this will be decolourised by sulphur dioxide gas.

SULPHURIC ACID, H2S04

• The process is called the ‘contact process’. The starting material is sulphur dioxide, obtained from burning sulphur in air, or as a by-product from roasting metal ores.
  • The sulphur dioxide is mixed with more air, dried and freed from dust.
  • The…