Electrolysis of Aqueous solutions

• Electrolysis of aqueous solution


  Reactions that take place, products at electrodes, half-equations for cathode and anode reactions
• Electrolysis of molten compounds

  • Only need to consider two ions (positive and negative)
  • Positive ions move to cathode, gain electrons to form atoms
  • Negative ions move to anode, lose electrons to form atoms
• Electrolysis of aqueous solutions

  • Need to consider water molecules and their ions (H+ and OH-)
  • Positive ions (e.g. Cu2+) and H+ attracted to cathode
  • Oxygen gas produced at anode
• Electrolysis of copper sulfate solution

  1. Copper ions (Cu2+) and hydrogen ions (H+) attracted to cathode

  2. Copper ions reduced at cathode to form copper atoms

  3. Oxygen gas produced at anode
• Inert electrodes


  Electrodes that do not react with the chemicals being produced, e.g. platinum
• Half-equation at cathode


  Cu2+ + 2e- → Cu (reduction)
• Aqueous


  Dissolved in water
• Water molecules splitting


  1. Produce hydrogen ion H+

  2. Produce hydroxide ion OH-
• Sodium chloride


  Has sodium ion Na+ and chloride ion Cl-
• Electrolysis of sodium chloride solution

  1. Ions attracted to cathode: sodium ion Na+ and hydrogen ion H+

  2. Ions attracted to anode: chloride ion Cl- and hydroxide ion OH-
• Hydrogen is produced at the cathode if the metal is more reactive than hydrogen
• Sodium is more reactive than hydrogen
• Chlorine gas is produced at the anode if the aqueous solution contains halide ions such as chloride
• Reaction at cathode


  1. Hydrogen ion H+ gains 1 electron to form hydrogen atom

  2. Hydrogen atoms pair to form hydrogen molecule H2
• Reaction at anode


  1. Chloride ion Cl- loses 1 electron to form chlorine atom

  2. Chlorine atoms pair to form chlorine molecule Cl2
• Half equations can be written to show the reactions at the cathode and anode