Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid (CIE IGCSE Chemistry)

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