Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid (Cambridge (CIE) O Level Chemistry): Revision Note

Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid

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

• When electrolysed, it produces bubbles of gas at both electrodes as chlorine and hydrogen are produced, leaving behind sodium hydroxide solution

• 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

Dilute sulfuric acid 

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

• Bubbles of gas are seen at both electrodes

• At the negative electrode:

  • H⁺ ions are attracted to the cathode, gain electrons and form hydrogen gas

• At the positive electrode:

  • OH^- ions are attracted to the anode, lose electrons and form oxygen gas and water

• Twice as much hydrogen as oxygen is produced, so the two gases collect in a 2:1 ratio by volume

Determining what gas is produced

• If the gas produced at the anode relights a glowing splint dipped into a sample of the gas then the gas is oxygen

Diagram showing the test for oxygen gas

• If the gas produced at the anode bleaches damp litmus paper then the gas is chlorine 

Diagram showing the test for chlorine gas

• If the gas produced at the cathode burns with a 'pop' when a sample is lit with a lighted splint then the gas is hydrogen

Diagram showing the test for hydrogen gas