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

Spec point — 4.1.3 · Identify the products formed at the electrodes and describe the observations made during the electrolysis of: concentrated aqueous sodium chloride dilute sulfuric acid using inert electrodes made of platinum or carbon/ graphite

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 sulfuric 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

Exam Hint

• When a gas is produced during electrolysis, you should be able to give the appropriate gas test:

  • Oxygen - if a glowing splint is dipped into a sample of the gas, then the splint will relight
  • Hydrogen - if a lit splint is dipped into a sample of the gas, then a squeaky pop will be heard
  • Chlorine - if damp litmus paper is dipped into a sample of the gas, it will turn red and then bleach to a white colour