Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Chemistry): Revision Note

Electrolysis of aqueous sodium chloride & dilute sulfuric acid

Electrolysis of aqueous sodium chloride 

• Brine is a concentrated solution of aqueous sodium chloride

• It can be electrolysed using inert electrodes made from platinum or carbon / graphite

• The ions in brine are:

  • Na⁺ and Cl^– ions from the brine / aqueous sodium chloride 

  • H⁺ and OH^– ions from the water

• When electrolysed, it produces bubbles of gas at both electrodes

  • The gases chlorine and hydrogen are produced

  • Sodium hydroxide solution is the product remaining in the electrolysis chamber / container

• These substances all have important industrial uses:

  • Chlorine is used to make bleach

  • Hydrogen is used to make margarine

  • Sodium hydroxide is used to make soap and detergents 

Product at the negative electrode:

• The H⁺ ions are discharged at the cathode as they are less reactive than sodium ions

• The H⁺ ions gain electrons to form hydrogen gas 

Product at the positive electrode:

• The Cl^– ions are discharged at the anode 

• They lose electrons and chlorine gas forms

• The Na⁺ and OH^– ions remain behind and form the NaOH solution

Diagram showing the products of the electrolysis of aqueous sodium chloride

Electrolysis of dilute sulfuric acid 

• Dilute sulfuric acid can be electrolysed using inert electrodes made from platinum or carbon / graphite

• The ions in dilute sulfuric acid are:

  • H⁺ and SO₄^2– ions from the sufuric acid  

  • H⁺ and OH^– ions from the water

• When electrolysed, it produces bubbles of gas at both electrodes

  • The gases oxygen and hydrogen are produced

Electrolysing dilute sulfuric acid in a Hoffman voltameter shows that twice as much hydrogen is produced, compared to oxygen

Product at the negative electrode:

• H⁺ ions are attracted to the cathode 

• The H⁺ ions gain electrons to form hydrogen gas 

Product at the positive electrode:

• OH^- ions are attracted to the anode

• They lose electrons and form oxygen gas and water