C4- electrolysis

• How do you set up a general electrolysis experiment?

  1. Place the positive and negative electrodes in a beaker containing a molten or dissolved ionic compound

  2. Connect both electrodes to a power supply with wires
• How could you investigate what happens when an aqueous solution of CuCl2 is electrolysed?

  1. Half fill a beaker with aqueous CuCl2

  2. Place a lid on the beaker and insert the electrodes into the solution through holes in the lid (electrodes must not touch)

  3. Connect the electrodes to a low voltage power supply

  4. Switch the power supply on to 4V

  5. Turn off the power after a few minutes and record any observations
• Cathode

  Metals or hydrogen
• What would you observe at each electrode when copper chloride solution is electrolysed?

  • Positive electrode: Bubbles of gas (chlorine)
  • Negative electrode: Electrode coated with a brown solid (copper)
• What would you observe at each electrode when sodium chloride solution is electrolysed?

  • Positive electrode: Bubbles of gas (chlorine)
  • Negative electrode: Bubbles of gas rapidly produced (hydrogen)
• The positive electrode (anode) must be regularly replaced
• Why must the positive electrode (anode) be regularly replaced?

  The positive electrode is made of carbon and will react with oxygen to produce carbon dioxide
• Write half equations for the reactions that occur at the electrodes when aqueous CuCl2 is electrolysed (higher only)

  1. Cathode: Cu2+ +2e- → Cu

  2. Anode: 2Cl2- → Cl2 +2e-
• Write half equations for the reactions that occur at the electrodes when aqueous NaCl is electrolysed (higher only)

  1. Cathode: 2H+ + 2e- → H2

  2. Anode: 2Cl- → Cl2 +2e-
• In the electrolysis of NaCl hydrogen is produced at the cathode

  Sodium is not produced because it is more reactive than hydrogen and remains in the solution
• How could you test that chlorine gas was produced at the anode?

  The gas produced will bleach damp litmus paper - it will turn white
• How could you test that hydrogen gas was produced at the anode?

  The gas produced will make a squeaky pop with a lighted splint
• How could you test that oxygen was produced at the anode?

  The gas produced will relight a glowing splint