Chapter 6: Electrolysis

Question 1

Introduction to electrolysis

Answer 1

• Electrolysis breaks down a substance using electricity
• Ionic compounds can only be electrolysed when they are molten or dissolved in water. This is because their ions are then free to move and carry their charge to the electrodes
• In electrolysis, positive ions move to the cathode (negative electrode), while negative ions move to the anode (positive electrode)

Question 2

Changes at the electrodes

Answer 2

• In electrolysis electrons move to the oppositely charged electrode
• At the negative electrode (cathode) positive ions gain electrons, so are reduced
• At the positive electrode (anode) negative ions lose their extra electrons so are oxidised
• When electrolysis happens in aqueous solution, the less reactive element, either hydrogen or the metal, is usually produced at the cathode. At the anode, you get either:

  o	Oxygen gas given off, from discharged   hydroxide ions produce from water
  
  o	Or a halogen produced if the electrolyte is a solution of a halide

Question 3

The extraction of aluminium

Answer 3

• Aluminium oxide, from the ore bauxite, is electrolysed in the extraction of aluminium metal.
• The aluminium metal is mixed with molten cryolite to lower its melting point, reducing the energy needed to extract the aluminium
• Aluminium forms at the cathode and oxygen forms at the anode
• The carbon anodes are replaced regularly and they gradually burn away as the oxygen reacts with the hot carbon anodes, forming carbon dioxide gas

Question 4

Electrolysis with aqueous solutions

Answer 4

• When you electrolyse sodium chloride solution, you get three products – chlorine gas and hydrogen gas given off at the electrodes, plus sodium hydroxide solution left in solution
• Hydrogen is produced at the cathode and H+(aq) ions are discharged from solution in preference to Na+(aq) ions
• Chlorine is produced at the anode as Cl-(aq) ions are discharged from solution in preference to OH-(aq) ions