Properties of Sulphuric Acid

Chemical Formula                    >H₂SO₄

Molar mass                               >98 g mol^-1

Melting point                             >10^oC

Boiling point                              >340^oC

Density                                       >1.83g cm^-3

Sulphuric acid is a dense, oily liquid once known as oil of vitriol. Pure sulphuric acid is almost twice as dense as water (1.98 g cm^-2). As water is added the density drops. Car batteries contain concentrated sulfuric acid. As the battery is discharged the concentration of the acid falls. By measuring the density of the acid the driver can check whether the battery is flat or not.

Action as an oxidising agent

It behaves as an oxidising agent only when hot and concentrated:

Cu + 2H₂SO₄ --> CuSO₄ + H₂O + SO₂

The sulphuric acid is reduced to sulphur dioxide.

Action as a dehydrating agent

Concentrated sulphuric acid has a great affinity for water. (It is important when diluting the concentrated acid to add the acid to water and NEVER water to acid.) The reaction is highly exothermic.

So great is its affinity for water that it can dehydrate compounds containing hydrogen and oxygen:

C₁₂H₂₂O₁₁ + nH₂SO₄ --> 12C + (11H₂O + nH₂SO₄)
sucrose            +  sulphuric     -->  Carbon +      (water + sulphuric acid)
                                 acid 

CH₃CH₂OH + nH₂SO₄ --> CH₂ = CH₂ + (H₂O + nH₂SO₄)
ethanol              + sulphuric      -->           ethene    +   (water + sulphuric acid)
                                 acid

It is used for drying gases, especially SO₂ and HCl, but cannot be used to dry a reducing gas such as H₂S or an alkaline gas such as NH₃.

Action as a dehydrating agent

The properties of acids are due to the hydrogen ions in solution. Concentrated sulphuric acid contains molecules, rather than ions. Since it contains very few hydrogen ions it does not react significantly with metals and can safely be stored in steel containers. A piece of magnesium ribbon does not dissolve in concentrated sulphuric acid.

Diluted with water, sulphuric acid behaves as a typical acid:

• it reacts with metals to form sulphates plus hydrogen gas
• it reacts with metal carbonates to form metal sulphates plus carbon dioxide plus water
• it neutralises bases to form sulphates plus water.

Industrial uses

• for making fertilisers such as superphosphate or ammonium sulfate
• In the refining of petroleum
• in the manufacture of various chemical compounds
• in lead-acid accumulators (batteries)
• for ‘pickling’ metal prior to electroplating.

In New Zealand almost all our sulphuric acid is used in the manufacture of fertilisers.